that includes showing a fuel tank is equivalent to a conventional
unheated aluminum wing tank or 3 percent, whichever is greater. For
purposes of this final rule, a conventional unheated aluminum wing tank
is a conventional aluminum structure, integral tank of a subsonic
transport airplane wing, with minimal heating from airplane systems or
other fuel tanks and cooled by ambient airflow during flight. Heat
sources that have the potential for significantly increasing the
flammability exposure of a fuel tank would preclude the tank from being
considered ``unheated.'' Examples of such heat sources that may have
this effect are heat exchangers, adjacent heated fuel tanks, transfer
of fuel from a warmer tank, and adjacent air conditioning equipment.
Thermal anti-ice systems and thermal anti-ice blankets typically do not
significantly increase flammability of fuel tanks.
e. Tanks Located Within the Fuselage Contour
    Boeing disagreed with the distinction in proposed Sec.  25.981
between tanks located within the fuselage contour that are normally
emptied and other tanks. Boeing suggested that main tanks and tanks not
partially within the fuselage do not represent all the tanks with low
flammability exposure and acceptable safety records. Boeing stated that
on the other hand it is possible to design a main or wing tank with
exceptional heat sources and/or minimal cooling. It is also possible to
design a normally emptied tank that is partially within the contour of
the fuselage which is low flammability (3 percent or less).
    Bombardier did not understand the justification for introducing a
maximum

3 percent fuel tank flammability exposure for wing tanks with a portion
of the tank located within the fuselage. Bombardier stated that there
is an inconsistency in requiring wing tanks to have flammability
exposure of between 2 percent and 5 percent, while requiring fuselage
tanks to be below 3 percent. Bombardier concluded that keeping all
tanks below a 7 percent flammability exposure level should be
considered acceptable, and recommended that tanks with less than 7
percent flammability exposure not be required to have FRM.
    The distinction in flammability exposures in the rule between tanks
located within the fuselage contour that are normally emptied and other
tanks was made because the former generally have an increased risk of
explosion. The location within the fuselage typically results in little
or no cooling of the tank and, in some cases, actually heats the tank.
Tanks that are normally emptied operate much of the time empty.
Therefore, components that could be potential ignition sources are
exposed to the tank ullage. We agree with Boeing on the possibility
that fuel tanks located in the wing can be high flammability if the
tank is heated or does not cool due to tank design features. However,
the rule limits fuel tank flammability in these tanks to 3 percent or
equivalent to a conventional unheated aluminum wing tank, addressing
that risk.
    For fuel tanks located outside the fuselage contour, Sec.  25.981,
as amended by this final rule, retains the flammability limits 3
percent or equivalent to a conventional unheated aluminum wing tank.
Only if any portion of the fuel tank is located within the fuselage
contour, and if the tank is normally emptied, is it required to meet
the 3 percent average and 3 percent warm day requirement. If an
applicant chooses to locate a portion of a main fuel tank inside the
fuselage, the rule requires that the fuel tank meet the same standard
as a main fuel tank located solely outside of the fuselage contour
(i.e., 3 percent or equivalent to a conventional unheated aluminum wing
tank wing).
    Since existing airplane types with main fuel tanks that go from the
wing into the fuselage are not normally emptied, FRM or IMM is required
for these tanks only if the tank flammability exposure exceeds 7
percent (proposed Sec.  25.1815 (now Sec.  26.33)). For future designs
using similar architecture, these types of designs would need to show
that the main tank that extends into the fuselage meets the standard of
equivalent to a conventional unheated aluminum wing tank or 3 percent.
f. Compliance Demonstration
    Boeing, Airbus, and BAE requested that applicants be allowed to use
design review to determine that an aluminum fuel tank is equivalent to
the low flammability standard fuel tank as defined by ARAC. This would
be in lieu of a detailed Monte Carlo based flammability analysis. The
BAE stated that performing a cumbersome and expensive Monte Carlo
analysis for metallic wing tanks of conventional design is unnecessary
and adds no value. For other types of tanks, or wing tanks with a
substantial heat input, BAE believes the use of alternative analytical
methods may be appropriate and suggested a qualitative assessment of
the design and the installation should be adequate to determine whether
a given tank has a low flammability exposure. Finally, BAE recommended
a simple set of objective criteria be allowed for establishing fuel
tank flammability in these tanks.
    Boeing requested that we:
     Revise proposed Sec.  25.981(b) to allow a simplified
flammability analysis for fuel tanks shown by design review to be a
Conventional Unheated Aluminum Wing Tank.
     Delete proposed Sec.  25.981(b)(1) and (b)(2), which
reference Appendixes N and M for the flammability analysis methodology
and flammability exposure criteria, respectively.
     Revise the definition of conventional unheated aluminum
wing tanks to consider allowing some minimal heat sources (i.e.,
hydraulic systems) and significant cooling which results in low
flammability exposure and a satisfactory level of safety.
    We agree with the commenters' assertion that a simplified
qualitative flammability analysis for conventional unheated aluminum
wing tanks is appropriate and have modified Appendix N to permit this.
Our intent is to limit the quantitative analysis for aluminum wing
tanks with unique or unconventional designs that are heated or designed
such that minimal cooling occurs. For example, a quantitative
flammability analysis would be necessary for a wing tank that has a
relatively small surface area, thereby minimizing surface cooling
effects, a composite tank or a tank that has equipment inducing heat
into the fuel tank greater than a small amount.
    We have also added guidance to AC 25.981-2 that describes how to
conduct a qualitative analysis to establish equivalency to a
conventional unheated aluminum wing tank. This guidance provides
examples of allowable heat sources and cooling characteristics for a
fuel tank to be considered a ``conventional unheated aluminum wing
tank,'' so that the safety standard established by the ARAC definition
for a conventional unheated aluminum wing tank is maintained. For
compliance with Sec.  25.981(d), the guidance also includes a
discussion of how Critical Design Configuration Control Limitations
(CDCCL) would need to be developed to define any critical features of
the fuel tank design needed to limit the flammability to that of a
conventional unheated aluminum wing tank.
    As for Boeing's specific changes to Sec.  25.981, we do not agree
that Sec.  25.981(b)(1) and (b)(2) should be deleted because Appendix N
provides necessary definitions and methods for establishing Fleet
Average Flammability Exposure and Appendix M establishes performance
standards for FRM. These appendices, and the references to them in
Sec.  25.981(b)(1) and (b)(2), are necessary to achieve the safety
objectives of this rulemaking. We have not adopted Boeing's suggestion
to modify the definition of ``Equivalent Conventional Unheated Aluminum
Wing.'' However, we do agree with the comment to allow some minimal
heating of tanks such as that from a hydraulic heat exchanger that does
minimal heating. We have revised the term ``Conventional Unheated
Aluminum Wing'' used in Sec.  25.981 to ``Conventional Unheated
Aluminum Wing Tank'' to clarify that the flammability of the fuel tank
is the standard. Since some minimal degree of heating typically occurs
in many of these tanks, this change recognizes that such minimal
heating is permissible.
g. Heat Sources Located in or Near Fuel Tanks
    Transport Canada and the UK Air Safety Group suggested we prohibit
the placement of heat sources within or near fuel tanks. Transport
Canada questioned why we would allow such an undesirable design
practice to continue. The UK Air Safety Group contended the NPRM failed
to address the contribution of high fuel tank temperature to fuel tank
explosions. The commenter noted that the Boeing 737 and 747 have air
conditioning units that raise the fuel tanks' temperature well above
the outside ambient temperature because these units are located beneath
the center fuel tanks.
    We agree with the commenters' underlying concern about controlling
fuel tank temperature. While locating heat sources in or near fuel
tanks increases the tanks' flammability, specifically prohibiting this
design

practice may not be the most efficient and effective way to address the
problem. This rule is performance-based and is seeking innovative
design solutions which could permit locating heat sources near or in
fuel tanks. For example, designers may wish to develop an FRM based
upon managing the fuel tank temperature by transferring heat between
tanks. These designs may provide flammability exposures well below that
of a tank that complied with the proposal made by the commenters. Risk
is directly proportionate to the flammability exposure of a tank.
Therefore, we have developed a flammability performance standard that
is independent of the design details of a tank installation.
h. Effects of Systems Failures on Flammability
    The CAPA requested that we ensure the effects of any system
failures that might increase the fuel tank flammability above the
acceptable limit be considered and properly evaluated prior to issuing
the final rule.
    The flammability analysis required by Sec.  25.981 includes a
requirement to show that flammability exposure does not exceed minimum
levels. It also requires that the overall flammability exposure
analysis includes consideration of system failures when demonstrating
that the FRM meets the reliability requirements of this rule. In
addition, the analysis required by Sec.  25.981(d) that determines the
CDCCL and airworthiness limitations includes consideration of possible
critical design features that must be maintained and may not be altered
to assure the flammability limits are achieved. We have provided
additional guidance and clarification in AC 25.981-2 regarding
reliability assessments and establishing CDCCL and airworthiness
limitations for FRM and IMM. Accordingly, we believe the commenter's
concerns are already addressed by the proposed language, and no change
was made to the final rule.
i. Move Flammability Exposure Method to Advisory Circular
    The EASA, Transport Canada, Boeing, and Bombardier commented that
the Monte Carlo method should not be defined in the rule as the method
for determining fuel tank flammability. Instead, it would be more
appropriately included in advisory material.
    We do not agree with these commenters. The Monte Carlo method is
specified in the rule to ensure standardization of the methodology for
determining fuel tank flammability across all airplane models so a
uniform level of safety is achieved. Advisory circulars (ACs) provide
guidance for methods, procedures, or practices that are acceptable to
us for complying with regulations. ACs are only one means of
demonstrating compliance, and we cannot require their use. Specifying
Monte Carlo analysis in an AC could result in numerous methodologies
and input parameters being used to determine flammability exposure, and
we believe that this could result in differing flammability exposures
in the fleet that may allow some fuel tanks to have greater
flammability than intended by the rule. To ensure that all DAHs reach
comparable conclusions from their assessments, it is necessary to
require that they use the same methodology. This can only be
accomplished through the rulemaking process.
    However, to accommodate minor revisions that would not appreciably
affect analytical results, we have included a provision in Appendix
N25.1(c) permitting use of alternative methods if approved by the FAA.
This is similar to the flexibility provided in Sec.  25.853 for
alternative test methods to those defined in Appendix F of part 25.
3. Flammability Exposure Requirements for Current Airplane Designs
    Proposed Sec.  25.1821 (now Sec.  26.39) contains the fuel tank
flammability safety requirements for newly produced airplanes.
Paragraph (b) sets forth the criteria that, when met by any fuel tank,
requires that fuel tank to have an FRM or IMM meeting the new
requirements of Sec.  25.981. Paragraph (c) contains the requirements
for all other fuel tanks that exceed a Fleet Average Flammability
Exposure of 7 percent.
a. Same Standards for New and Current Airplane Designs
    Boeing asked that we revise proposed Sec.  25.1821(b) to state
``any fuel tank not shown by design review to be a Conventional
Unheated Aluminum Wing Tank, must meet the requirements of Sec.  25.981
in effect on [effective date of final rule].'' In conjunction with this
change, paragraph (c) would be deleted. Boeing stated that new
production airplanes should meet the same requirements as new airplane
designs, since the criteria for tanks at risk should be a function of
heating and cooling, not whether the fuel tank is normally emptied and
located partially within the fuselage.
    We do not agree with Boeing. As discussed earlier, tanks that are
normally emptied and located at least partially within the fuselage are
generally more susceptible to explosion because of both increased
ullage and operating at higher temperatures. We have determined that
the 7 percent flammability exposure limit recommended by ARAC is an
adequate standard to determine which fuel tanks in newly produced
airplanes need an FRM or IMM. If the fleet average flammability
exposure is above 7 percent for fuel tanks normally emptied and located
within the fuselage contour, these fuel tanks will be required to be
flammable no more than 3 percent on average and 3 percent for warm day
operations. We expect that the vast majority of large transport
category airplanes will have a fleet average flammability exposure
above 7 percent for these specific fuel tanks and will be required to
comply with Sec.  25.981 for production airplanes affected by the DAH
requirement.
    Other tanks on newly produced airplanes also may not exceed the 7
percent flammability exposure limit, but the final rule would allow
reduction to that level by various methods of FRM described in AC
25.981-2 that would not necessarily require the added complexity and
cost of a nitrogen inerting based FRM. We believe this requirement is
sufficient to provide an acceptable level of safety for current
production airplanes because these tanks have significantly lower risk
of fuel tank explosions, as demonstrated by their service history.
Therefore, we do not believe the safety improvements from redesign of
these tanks to meet the new requirements of Sec.  25.981 are sufficient
to justify the resulting costs.
b. 7 Percent Exposure Flammability Questioned
    In the NPRM, we stated that fuel tanks that have a flammability
exposure higher than 7 percent are unduly dangerous. American Trans Air
commented that this statement is arbitrary, based on flawed analysis,
and cannot be supported. Bombardier expressed its opinion that the NPRM
and its supporting data did not adequately substantiate the declared 7
percent exposure. Although Bombardier considered that achieving 7
percent exposure is feasible with reasonable design precautions,
Bombardier stated that this is not an acceptable reason for creating a
standard. Bombardier also quoted information shared among the airline
industry and authorities that heated tanks may vary between 8 percent
to as high as 40 percent in flammability exposure.
    Boeing did not agree with the proposed flammability requirements
for newly produced airplanes, because fuel tanks other than those
located within

the fuselage contour that are normally emptied would be allowed to have
flammability of up to 7 percent. Boeing commented that this
flammability is more than twice that of what is allowed for similar
tanks in new designs. Boeing noted that the first ARAC determination
that 7 percent flammability exposure is acceptable was based on the
original coarse ARAC flammability analysis which determined that
unheated tanks had a flammability level of approximately 5 percent. Two
percent was added for potential variation resulting in the 7 percent
proposal. Boeing pointed out that the Monte Carlo analysis has been
significantly refined since the first ARAC report, and the estimated
flammability exposure of 5 percent (7 percent with potential variation)
has been reduced to be in the range of 3 percent (4 percent with
potential variation) or less for the same fuel tanks.
    We have determined that the 7 percent or less fleet average
flammability exposure recommended by ARAC is an adequate value that can
be used to identify those airplane models that need to be retrofitted
with an FRM or IMM. The fuel tank flammability limits established for
newly produced airplanes (subject to the production cut-in
requirements) are the same as those for retrofit of the existing fleet
(proposed Sec.  25.1815 (now Sec.  26.33)). We determined this
flammability exposure achieves the desired safety benefits, since
currently produced airplanes generally have conventional unheated
aluminum wing tanks, the tanks ARAC determined to have adequate safety
level, with flammability exposures below 7 percent.
    We agree with Boeing that newly produced airplanes should not be
allowed to have fuel tank flammability that is twice that of new
designs, and this is not what we intended. The intent of this rule is
to apply its safety improvements to the fuel tanks that have been shown
to have an increased risk of explosion, not to require modifications to
conventional unheated aluminum wing tanks, or other fuel tanks that
have significantly lower flammability. Data we have available for
currently produced airplanes indicate the flammability of tanks located
outside the fuselage contour have flammability below 7 percent and
further reduction in flammability exposure as recommended by Boeing
would add significant cost to the rule, since a number of fuel tanks
would be required to have an FRM or IMM to meet the suggested
flammability values of 3 to 4 percent.
    Recognizing that, based on the applicability criteria of proposed
Sec.  25.1821(a) (now Sec.  26.39), this section only applies to
current production Boeing models. We have revised paragraph (a) to
specifically identify those models. As discussed previously, we have
also added a requirement to the operational rules that operators must
meet these requirements for any airplane subject to this rule that is
produced more than two years after the effective date.
4. Continued Airworthiness and Safety Improvements
a. 7 Percent Standard Should Apply to All Tanks
    Boeing requested that Sec.  25.1815(c)(1) be modified to state
that, for fuel tanks with flammability exposure exceeding 7 percent
that require an FRM, ``a means must be provided to reduce the fuel tank
flammability exposure to meet the criteria of Appendix M of this
part.'' In addition, Boeing recommended that we delete Sec.
25.1815(c)(1)(i) and (ii). Boeing stated that any fuel tank that has
significant heat loads, regardless of the location on the airplane,
should meet the requirements of Appendix M if an FRM is selected as the
design modification.
    We do not concur with Boeing's comment that the flammability
requirements of Appendix M should apply to any fuel tank that exceeds 7
percent average flammability. As discussed previously, the reason we
are adopting more stringent requirements for fuel tanks that are
normally emptied and located within the fuselage contour is that those
tanks both have higher flammability exposure and are more likely to
have ullage exposed to ignition sources. For other fuel tanks where the
fleet average flammability exposure exceeds 7 percent, the requirements
of Appendix M apply with the exception that the flammability
requirements of M25.1(a) and (b) are replaced by the requirement that
fleet average flammability exposure must not exceed 7 percent. We
believe this is acceptable for these tanks on existing airplanes. Since
most of these tanks are not ``normally emptied,'' the risk that
flammable vapors will be exposed to ignition sources is generally much
lower.
b. Compliance Planning
    Airbus requested that the compliance planning requirements
contained in Sec.  25.1815 be removed because they are unnecessary.
Airbus believes the only important compliance date is the final date
for DAHs to submit the data and documents necessary to support operator
compliance. Airbus commented that the compliance plan requirements in
Sec. Sec.  25.1815(g), (h) and (i) add constraints on the manufacturer
with no safety benefit. Airbus stated these documents should not be
subject to a requirement with respect to the DAH documentation delivery
date. However, if the delivery dates for these documents are mandated,
Airbus requested that they be expressed in the format of a duration
tied to the date of approval of the previous submittal.
    Boeing recommended we remove the Sec.  25.1815(g)(3) requirement to
identify deviations to methods of compliance identified in FAA advisory
material, because the proposed means of compliance should not be
compared to other means. Instead, they should be evaluated on their own
merits.
    While we understand the commenters' concerns, these documents will
provide assurance that the required flammability exposure analyses and,
if applicable, proposed design changes, are being addressed in a timely
fashion. As stated in the NPRM, the resolution of fuel tank safety
issues needs to be handled in a ``uniform and expeditious'' manner.
Providing compliance times based on the dates of our previous approvals
would result in various compliance times, depending upon whether DAHs'
submissions are acceptable. It would have the undesirable effect of
providing more time for those manufacturers submitting deficient
documents.
    Compliance planning will promote communication between the affected
manufacturer and us. It will also provide sufficient time to discuss
any concerns with respect to how the affected manufacturer proposes to
analyze fleet average flammability exposure or certify design changes.
Compliance planning will also help to ensure that the affected
manufacturer is able to meet the required compliance times of the rule
for accomplishing the submittal of the flammability exposure analysis,
design changes, and service instructions, if applicable (proposed Sec.
25.1815 (now Sec.  26.33) and proposed Sec.  25.1817 (now Sec.
26.35)). We intend to closely monitor compliance status and take
appropriate action, if necessary.
    However, we do acknowledge that some provisions of proposed Sec.
25.1815(g), (h) and (i) could be removed without adversely affecting
our ability to facilitate TC holder compliance. Specifically, proposed
paragraph (g)(3) would require TC holders to identify intended means of
compliance that differ from those described in FAA advisory materials.

While this is still a desirable element of any compliance plan, we now
believe that an explicit requirement is unnecessary and it is not
included in the final rule. As with normal type certification planning,
we expect that TC holders will identify differences and fully discuss
them with the FAA Oversight Office early in the compliance period to
ensure that these differences will ultimately not jeopardize full and
timely compliance. Because we believe that timely review and approval
is beneficial and will save both DAH and FAA resources, the advisory
material will recommend that if the DAH proposes a compliance means
differing from that described in the advisory material, the DAH should
provide a detailed explanation of how it will demonstrate compliance
with this section. The FAA Oversight Office will evaluate these
differences on their merits, and not by comparison with FAA advisory
material.
    Similarly, proposed Sec.  25.1815(i) contains provisions that would
have authorized the FAA Oversight Office to identify deficiencies in a
compliance plan, or the TC holder's implementation of the plan, and
require specified corrective actions to remedy those deficiencies.
While we anticipate that this process will still occur in the event of
potential non-compliance, we have concluded that it is unnecessary to
adopt explicit requirements to correct deficiencies and have removed
them from the final rule. Ultimately, TC holders are responsible for
submitting compliant FRM or IMM by the date specified. This section
retains the requirements to submit a compliance plan and to implement
the approved plan. If the FAA Oversight Office determines that the TC
holder is at risk of not submitting compliant FRM or IMM by the
compliance date because of deficiencies in either the compliance plan
or the TC holder's implementation of the plan, the FAA Oversight Office
will document the deficiencies and request TC holder corrective action.
Failure to implement proper corrective action under these
circumstances, while not constituting a separate violation, will be
considered in determining appropriate enforcement action if the TC
holder ultimately fails to meet the requirements of this section.
    Finally, we realized that the rule text could more clearly state
our intent to allow DAHs flexibility to modify their approved plan if
necessary. Accordingly, we changed proposed Sec.  25.1815 (now Sec.
26.33(i)) to read: ``Each affected type certificate holder must
implement the compliance plans, or later revisions, * * *''
c. Changes to Type Certificates Affecting Flammability
    Proposed Sec.  25.1817 (now Sec.  26.35) addressed changes to TCs
that could affect fuel tank flammability. This section proposed to
require that a flammability exposure analysis be accomplished in
accordance with Appendix N for all affected fuel tanks installed under
an STC, amended TC, or field approval within 12 months after the
effective date of the final rule. An impact assessment that identifies
any features of the design change that compromise any CDCCL applicable
to any airplane with high flammability tanks for which CDCCL are
required must also be submitted to the FAA Oversight Office. This
section also proposed a requirement to develop service instructions to
correct designs that compromise airworthiness limitations, defined by
the TC holder under proposed Sec.  25.1815 (now Sec.  26.33), within 48
months after the final rule's effective date.
    Airbus proposed we restrict the application of any proposed changes
to Sec.  25.981 to new TCs and significant design changes (i.e., new
fuel tanks). For minor design changes such as relocating a fuel level
sensor or a small increase in tank capacity, the TC holder should only
be required to show no degradation in the flammability under the
criteria proposed by Sec.  25.1815. Airbus stated that the cross-
reference between what is in the preamble and Sec.  25.1815, and what
is required by Sec.  25.1817, is misleading.
    We agree with Airbus, and have revised proposed Sec.  25.1817 (now
Sec.  26.35) to require compliance with the new Sec.  25.981 only for
new fuel tanks. Other design changes that increase capacity of existing
fuel tanks must comply with Sec.  26.33. Design changes that affect the
flammability exposure of existing tanks equipped with FRM or IMM must
comply with CDCCLs for those tanks. This will ensure that these design
changes do not degrade the level of safety required by this rule.
d. Combine Sec. Sec.  25.1815 and 25.1817
    Boeing requested that we combine proposed Sec. Sec.  25.1815 and
25.1817 into one section. We do not agree with this suggestion, since
it would not achieve the goals of this rulemaking. As proposed,
Sec. Sec.  25.1815 (now Sec.  26.33) and 25.1817 (now Sec.  26.35)
would apply to different entities. Section 25.1815 (now Sec.  26.33)
would apply to TC holders of transport category airplanes, and Sec.
25.1817 (now Sec.  26.35) to auxiliary tank STC holders and future
applicants for design changes. The STC holders have distinctly
different compliance dates because information such as CDCCL developed
by the DAHs under proposed Sec.  25.1815 (now Sec.  26.33) is needed
before the STC holders can comply with proposed Sec.  25.1817 (now
Sec.  26.35). Separate sections provide a clear statement of the
requirements for each situation so affected persons can more easily
understand what is needed to comply with the rules applicable to them.
Therefore, the final rule retains the language as proposed with no
change.
e. Pending Type Certification Projects
    Proposed Sec.  25.1819 contains the requirements for pending TC
projects. As proposed, this section contains different requirements for
those transport category airplanes based on whether the application was
made before or on/after June 6, 2001 (the effective date of Amendment
25-102). Boeing requested that this section be deleted because it saw
no reason to differentiate among designs based on the date of
application.
    We partially agree with Boeing and have revised this section. In
the final rule, any pending certification projects that have not
received type certification by the effective date of this rule will be
required to meet the requirements of Sec.  25.981, as amended by this
rule. Since there are no longer any ongoing TC projects where the
application was received prior to June 6, 2001, there is no reason for
this distinction and we have removed proposed Sec.  25.1819(c).
However, we have received applications for type certification projects
after June 6, 2001, that are still pending (e.g., the Boeing 787 and
Airbus A350), and we have determined that a specific requirement in
Sec.  25.1819 is needed to address these projects. We do not believe
this section should be completely deleted, as requested, because these
projects (and future design changes to these airplanes), would not
otherwise be required to comply with Sec.  25.981, as amended by this
final rule. The change to the rule will maintain the requirement that
pending projects meet the same flammability standards as required for
new type certificates and that applicants develop CDCCL as proposed in
the NPRM.
f. Type Certificates Applied for on or After June 6, 2001
    Proposed Sec.  25.1819(d) (now Sec.  26.37(b)) requires that if an
application for type certification was made on or after June 6, 2001,
the requirements of Sec.  25.981 of this rule apply. Section 25.981
requires, in part, that the fleet average flammability exposure of a
fuel

tank not exceed 3 percent or that of a conventional unheated aluminum
wing tank.
    Airbus objected to the setting of a 3 percent flammability limit
for all fuel tanks for a pending type certification, if the application
was made on or after June 6, 2001. Airbus agreed that a 3 percent
flammability limit could be considered as an acceptable goal when FRM
is used. However, for fuel tanks that have a base flammability exposure
less than 7 percent, there should not be a requirement to impose FRM,
and the existing minimization of heat sources should be considered
adequate. If initial flammability is between 3 and 7 percent, the
safety benefit to reduce it to 3 percent through the use of FRM is not
justified, when considering the introduction of new failure conditions,
and operational and ownership costs of an FRM.
    Airbus apparently misunderstood the effect of the proposed
requirements of Sec.  25.1819 (now Sec.  26.37) for TCs for which
application was made on or after June 6, 2001. The following is
provided to clarify the requirements of the rule and address the
concern expressed by Airbus. The flammability requirements for an
airplane for which application was made on or after June 6, 2001, would
include Sec.  25.981 at Amendment 25-102 for all tanks except normally
emptied tanks located within the fuselage contour. As stated earlier in
this preamble, the rule text has been changed to clarify that the
flammability exposure is equivalent to a conventional unheated aluminum
wing tank or 3 percent, at the applicant's option. This flammability
exposure is unchanged from Amendment 25-102, which would not have
permitted a flammability exposure of 7 percent. This rule adds a new
requirement for fuel tanks located within the fuselage contour that are
normally emptied. Normally emptied tanks located within the fuselage
must meet the 3 percent average and the 3 percent warm day flammability
limits defined in Appendix M, which is the same flammability
requirement being applied to these types of fuel tanks on existing
airplanes.
g. Design Change to Add a Normally Emptied or Auxiliary Fuel Tank
    As proposed, Sec.  25.1819(e) would require that any future design
change to a TC for which the application is pending when this rule is
adopted and that--
     Adds an auxiliary fuel tank, or
     Adds a fuel tank designed to be normally emptied, or
     Increases fuel tank capacity, or
     May increase the flammability exposure of an existing fuel
tank must meet the requirements of Sec.  25.981, as amended by this
rule. Boeing asked that this paragraph be deleted because it is
specifically for ``pending'' type certification projects and, by
definition, there is no existing type certificate to change. If the
intent of proposed Sec.  25.1819 (now Sec.  26.37) is to define
requirements for projects in work at the time of the final rule, then
Boeing suggested there is no need for this section. Any change after
the new production compliance date would have to meet the new
production requirements (Sec.  25.1821).
    Proposed Sec.  25.1819(e) specifically targets potential future
changes to certain long-term, pending type certification programs.
Under proposed Sec.  25.1819(c), these programs would not be required
to comply with Sec.  25.981, as amended by this rule. Our intent was
that, although the original TC would not have to comply with the
current requirements, any later changes would have to comply. Since we
issued the NPRM, all of these projects have been certified, so there
are no pending projects for which this paragraph is needed. Therefore,
we have removed it from the final rule.

E. Flammability Exposure Requirements for Airplane Operators

    The proposed operating rules would prohibit the operation of
certain transport category airplanes operated under parts 91, 121, 125,
and 129 beyond specified compliance dates, unless the operator of those
airplanes has incorporated approved IMM, FRM or FIMM modifications and
associated airworthiness limitations for the affected fuel tanks. The
proposed rules would not apply to airplanes used only in all-cargo or
part 135 operations. Finally, the proposed operating rules would also
create new subparts that pertain to the support of continued
airworthiness and safety improvements.
1. General Comments About Applicability to Existing Airplanes
    Airbus, AEA and AAPA believe the retrofit requirement is not cost
effective. Our analysis showed that the benefit/cost ratio of the
production cut-in and retrofit requirements are similar. This was our
rationale for adopting the combined approach of production cut-in and
retrofit. However, these commenters believe the 7 percent discount rate
used in our cost/benefit analysis is too high and is responsible for
the determination that cost/benefit ratios are similar between the
production cut-in and retrofit. We infer from their comments that they
believe that 3 percent is a more realistic number and supports their
contention that retrofit is not justified. The commenters note that an
EASA analysis concluded that the retrofit was not justified. A major
concern was that the bulk of the retrofit costs (present value terms)
will be incurred in about 1/3 of the time (7 years) required for the
forward fit costs (22 years). They believe that the cash outlay to
retrofit in such a short time, coupled with the small safety benefit,
is not justified when compared with the cost/benefit of the production
cut-in. They also stated that the high cost of the retrofit over such a
short period would place financial stress on an industry that is
already financially constrained. In contrast, the cost of production
incorporation of FRM in new airplanes will be borne by airlines that
are prepared to accept the cost of new airplanes with the FRM included
in the ``sticker price.''
    Except as discussed previously regarding the exclusion of part 91
operations, we continue to believe that a retrofit requirement is
justified. As discussed in the NPRM and earlier in this preamble, the
risk of fuel tank explosions on the current fleet of airplanes with
high flammability tanks is still significant because, despite our
efforts to eliminate ignition sources, they continue to occur. At the
same time, we have made a number of changes to the proposed
requirements to reduce their cost and improve their cost-effectiveness.
As discussed later in this preamble, the final regulatory evaluation
(FRE) has been revised to include the benefits of preventing lost
revenue to the industry as a whole if another fuel tank explosion were
to occur. When these benefits are included, variations in the discount
rate do not alter the conclusion that this rule is reasonably cost-
effective.
    The compliance time for the retrofit requirement allows for
incorporation of design modifications over a seven-year period.
Operators can spread the costs over this time period. We have also
included a provision in the operational rules (discussed later) that
allows operators an extension of up to one year after the 50 percent
and 100 percent retrofit deadlines for full fleet incorporation of the
design modifications if the operator includes requirements in their
operations specifications to use ground conditioned air when available.
For 50 percent of an operator's fleet, this would allow retrofit to be
completed by September 21, 2015 rather than September 19, 2014.
Similarly, for 100 percent of an operator's fleet, this would allow
retrofit to be completed by

September 19, 2018 rather than September 19, 2017. This provision
provides a reduction in the costs to operators because it allows an
additional year to install an FRM or IMM. We also adjusted the
applicability of the rule so that older airplanes that were produced
prior to 1992, which will be nearing the end of their useful life in
passenger service, will not be subject to the phase-in-requirement of
the rule. The DAH-supported design modifications will only be required
on airplanes with significant remaining useful life in passenger
service so the benefits of the rule are optimized.
    As for the comments on the standard discount rate, the rate that is
mandated by the Office of Management and Budget when conducting
regulatory evaluations is 7 percent. The Initial Regulatory Evaluation
included a sensitivity study where variations in the discount rate
(using 3 and 7 percent) were considered. Variations in the discount
rate affect both the cost and the benefits of the rulemaking. Thus,
using a discount rate of 3 percent (as they recommend) increases the
benefits of the rulemaking, because the value of averted future
accidents would also have a higher present value.
2. Authority to Operate With an Inoperative FRM, IMM or FIMM
    In the NPRM, we requested public comment on the proposal to allow
the current Flight Operations Evaluation Board (FOEB) process to
establish the Master Minimum Equipment List (MMEL) interval for the FRM
or IMM rather than requiring a specific maximum fixed time interval
that the FRM can be inoperative. Airbus, Boeing, ATA, AEA and British
Airways supported the rule as proposed and generally agreed the FOEB is
the appropriate vehicle to establish the approved MMEL interval for
inoperative FRM. In contrast, Smith's Aero commented that FRM must be
considered a flight critical system, without MMEL relief for the
performance of the system to meet the overall intended safety level
stated by the FAA in the NPRM. Finally, Frontier asked how long an
airplane could be operated with an inoperative FRM system.
    As stated in the NPRM, the intent of the rule is to provide an
additional layer of protection from having a fuel tank explosion if an
ignition source occurs inside a fuel tank. While the FRM system is
needed to maintain the safety of a fleet of airplanes, it is not
considered flight critical for every flight, since the ignition
prevention means required by Sec.  25.981 requires robust fail-safe
features that provide an adequate level of safety during short periods
of time when the FRM is inoperative under the MMEL (no greater than 1.8
percent of the operating time). We agree with the commenters that ``FRM
designers'' should make the design goals for the MMEL relief intervals
available and notify the FOEB of their recommendation. The allowable
MMEL interval is design dependent and cannot be defined by us until a
design is presented and the interval is justified by the system
reliability analysis and the FOEB.
    Frontier also asked whether en route weather conditions would be a
factor with the MEL. At this time, en route weather conditions are not
part of the consideration for operation under the operator's MEL. This
is one of the considerations in the Monte Carlo assessment, so
operation under an operator's MEL during warm days would not be an
additional consideration for the MMEL.
3. Availability of Spare Parts
    Frontier asked if we had given proper consideration to the fact
that there will most likely be an initial spare parts shortage. The
compliance time for fleet-wide retrofit of FRM or IMM is nine years
after the effective date of this final rule, with 50 percent compliance
required within 6 years. Therefore, the manufacturers of components
should have the capability to produce needed spares and no shortage of
parts is anticipated. We have not included a consideration of parts
shortages when establishing the MMEL interval.
4. Requirement That Center Fuel Tank Be Inert Before First Flight of
the Day
    Frontier requested information on whether the final rule would
require that the center fuel tank be inert before the first flight of
the day and, if so, if the Auxiliary Power Unit is inoperative, could
the inerting system then be inoperative until after main engine start.
The final rule does not directly address the operational details of the
FRM. These will be determined based on the DAH's design and any
operating limitations that may be necessary to meet the performance
standards of this final rule.

F. Appendix M--FRM Specifications

    Appendix M to part 25 contains detailed specifications for all FRMs
if they are used to meet the flammability exposure limitations. These
specifications are designed to ensure the performance and reliability
of FRMs. We received several comments on Appendix M and have made
changes to the rule based on some of them.
1. Fleet Average Flammability Exposure Level
    Paragraph M25.1(a) requires that the Fleet Average Flammability
Exposure of each fuel tank may not exceed 3 percent of the Flammability
Exposure Evaluation Time. As discussed previously, as a portion of this
3 percent, if flammability reduction means (FRM) are used, each of the
following time periods cannot exceed 1.8 percent of the FEET: (1) When
any FRM is operational but the fuel tank is not inert and the tank is
flammable; and (2) when any FRM is inoperative and the tank is
flammable. Boeing requested a change to this paragraph to clarify that,
for both the operational and inoperative requirements, only time
periods when the fuel tank is in a flammable state are counted toward
each 1.8 percent flammability exposure limit.
    We agree that the method of determining these times needs
clarification and we have revised paragraph M25.1(a) as requested by
Boeing.
2. Inclusion of Ground and Takeoff/Climb Phases of Flight
    Paragraph M25.1(b) requires that ground, takeoff and climb phases
of flight be included in the fuel tank fleet average flammability
exposure analysis. Boeing asked that paragraph M25.1(b) be reworded to
exclude a specific reference to the takeoff flight phase. Boeing's
justification was that there is no benefit in conducting a separate
flammability analysis for the takeoff phase of flight since it is a
very short duration. Boeing recommended the takeoff phase be included
with the climb phase of flight. Boeing also suggested the rule clarify
that the transition from ground to climb phase for this analysis occurs
at weight off wheels.
    We agree with Boeing and have revised paragraph M25.1(b) in the
final rule to remove consideration of the takeoff phase of flight as a
separate requirement. These two phases are now required to be
considered in combination using the term ``takeoff/climb'' phase. In
addition, we added a sentence to paragraph M25.1(b)(2) stating that the
transition from ground to takeoff/climb phase for this analysis occurs
at weight off wheels.
3. Clarification of Sea Level Ground Ambient Temperature
    Paragraph M25.1(b)(1) requires that the fuel tank fleet average
flammability

exposure analysis, as defined in Appendix N, ``must use the subset of
flights starting with a sea level ground ambient temperature of
80[deg]F. (standard day plus 21[deg]F. atmosphere) or more, from the
flammability exposure analysis done for overall performance.'' An
individual commenter requested that we define the term ``more'' in this
statement. We agree that this requirement needs clarification and, in
the final rule, paragraph M25.1(b)(1), we replaced the word ``more''
with the word ``above.'' We also replaced the word ``starting'' with
``that begin.''
4. Deletion of Proposed Paragraph M25.2 (Showing Compliance)
    Paragraph M25.2 establishes the means for showing compliance with
fuel tank flammability requirements. Boeing requested the contents of
paragraph M25.2 be moved to Advisory Circular 25.981-2A as it defines a
method of compliance and, as such, should be located in an AC.
    As discussed previously, ACs provide guidance for methods,
procedures, or practices that are acceptable to us for complying with
regulations. ACs are only one means of demonstrating compliance, and we
cannot require their use. The compliance means in paragraph M25.2 is
regulatory in nature to ensure that applicants are providing the data
necessary to validate the parameters used in their calculations for
fuel tank fleet average flammability exposure (as required by paragraph
M25.1), and to substantiate that their system meets these requirements
during normal airplane operations for any combination of airplane
configuration (as required by paragraph M25.2(b)). We have made no
change as a result of this comment.
5. Deletion of ``Fuel Type'' From List of Requirements in Proposed
Paragraph M25.2(b)
    Boeing also requested that paragraph M25.2(b) be revised to remove
``fuel type'' from the list of requirements and add ``or other relevant
airplane system configuration'' to it. Boeing stated the items listed
in paragraph M25.2(b) affect the performance of a FRM system that is
supplied by engine bleed air, and fuel type does not affect bleed
system pressure. We agree with Boeing and have revised this paragraph
in the final rule.
6. Latent Failures
    Paragraph M25.3(a) requires that reliability indications be
provided to identify latent failures of the FRM. These indications are
needed to ensure appropriate actions can be taken to maintain the FRM's
reliability. An individual commenter asked that we define what is meant
by ``reliability indications'' in paragraph M25.3.
    In this context, reliability indications are normally computer
messages or lights that identify whether components are functioning
properly. Reliability indications are likely to be needed for the FRM
to meet the reliability requirements in the rule. The type of
indications needed will depend on the design and the outcome of the
reliability analysis. If a nitrogen inerting FRM were to be developed
with no indication of system failures, the system would have
significant exposure to long-term operation with latent failures.
Maintenance indications would likely be needed so that the minimum
reliability of the system could meet the rule.
    Boeing requested that paragraph M25.3 be deleted or modified to
remove the term ``latent.'' This would be consistent with the special
conditions issued for the Boeing 737 and 747 flammability reduction
systems. In addition, the term ``latent'' would not be applicable if an
indication is provided. An individual commenter agreed, stating that
latent failures are not detectable and, hence, cannot be indicated.
Embraer commented that both paragraphs M25.3(a) and (b) should be
deleted because a literal interpretation would require any latent
failure to be detected and indicated. This contradicts the NPRM's
preamble, which states that the designer is allowed to make a trade-off
between system failure probability and failure detection/ annunciation
to show compliance with the system performance requirements. In
addition, Embraer maintained that paragraph M25.3(a) is already
addressed and should not be repeated here because the requirement for
failure detection is inherent in the flammability exposure requirement
and in the 1.8 percent limit on system failure contribution to
flammability exposure.
    On a related topic, Airbus and Embraer commented that the proposed
rule is too restrictive and mandates an excessive amount of indication
and monitoring. Airbus indicated that the proposed text appears to
assume the adoption of an active system to reduce flammability and this
may not necessarily be appropriate if a passive system were to be used.
Some means of verifying that the passive means is fully functional
could be required, but it may be inherent in the design and therefore,
no specific action would be required except to ensure that other
airplane modifications do not adversely affect the fuel tank
flammability.
    The FAA agrees with these commenters and has modified paragraph
M25.3(a) in the final rule.
    This change makes it clear that the intent of the rule is to
require only those indications needed to assure any FRM meets the
minimum reliability requirements of the rule. The preamble to the NPRM
provided a detailed explanation of the intent of these requirements.
The need for indications is determined from the system reliability
assessment that requires a minimum reliability for any FRM. The type of
indications that may be needed to meet the reliability requirements
depends upon the details of the design and the outcome of the system
reliability analysis. Various design methods may be used to make sure
an FRM meets the reliability and performance requirements in this rule.
For example, if an FRM based upon nitrogen inerting is developed and no
indication of system failures is provided, the system would have
significant exposure to long-term operation with latent failures.
Maintenance indications would likely be needed so that the minimum
reliability of the system could meet the rule. Other designs may use
active or passive cooling means for flammability reduction. For these
systems, the level of indication required would depend upon the
reliability of the cooling system components.
    The need for FRM indications and the frequency of checking system
performance (maintenance intervals) must be determined based on the
results of the FRM fuel tank fleet average flammability exposure
analysis. The determination of a proper maintenance interval and
procedure will follow completion of the certification testing and the
reliability analysis used to establish the system complies with the
performance requirements.
7. Identification of Airworthiness Limitations
    Paragraph M25.4(a) requires that if FRM is used to comply with
paragraph M25.1, airworthiness limitations must be identified for all
maintenance or inspection tasks required to identify failures of
components within the FRM that are needed to meet paragraph M25.1.
Boeing requested that paragraph M25.4(a) be modified to require only
airworthiness limitations be identified for ``significant'' maintenance
or inspection tasks. Boeing stated that it is overly restrictive to
require that all maintenance tasks be identified as airworthiness
limitations. It argued that applicants should be granted the
flexibility to identify significant tasks as

airworthiness limitations and other non-significant tasks as
maintenance significant items.
    We agree with Boeing that we should not require that all
maintenance tasks for FRM be identified as airworthiness limitations.
Airworthiness limitations for the FRM system are only required for
those FRM components that, in the event of failure, would affect the
ability of the fuel tank to meet the Fleet Average Flammability
Exposure specified in paragraph M25.1. We regard any task that is
necessary to meet this objective as ``significant.'' We recognize that
manufacturers are also required to provide other maintenance
information for the FRM as part of the instructions for continued
airworthiness required by Sec.  25.1529.
8. Catastrophic Failure Modes
    EASA noted that Appendix M significantly differs from the
harmonized special conditions it used for certifying FRM on some
specific airplane models. EASA asked that we explicitly state that
catastrophic results must not occur from any single failure or
combination of failures not shown to be extremely improbable (for the
FRM system) as required in the noted special conditions. We agree that
possible catastrophic failure modes of the FRM must be shown to meet
the requested standard. However, we do not agree that EASA's change is
needed since the regulatory intent is already addressed by other
regulations that apply to FRM. For example, the general requirements of
Sec.  25.901 that apply to all Subpart E regulations apply to an FRM
certificated to meet Sec.  25.981 and Appendix M. Therefore, we did not
make any change to Appendix M based on EASA's comment.
9. Reliability Reporting
    Paragraph M25.5 requires the applicant to demonstrate an effective
means to ensure collection of FRM reliability data and to provide a
report to the FAA. We requested comments on the proposal to require
DAHs to submit a quarterly report on FRM reliability for 5 years. We
consider these reports necessary to determine whether the predicted
reliability for these systems is accurate, and to enable us to initiate
necessary corrective actions if they are not. We intend for DAHs to
gather the needed data from operators using existing reporting systems
that are currently used for airplane maintenance, reliability, and
warranty claims. The operators would provide this information through
existing or new business arrangements between the DAHs and the
operators.
    The AEA and ATA questioned this reliability reporting process. They
stated the current reporting systems may not be equipped to accommodate
this new data requirement without additional burden and cost. Airbus
also stated the reporting requirement is unclear and without sufficient
detail to enable them to fully comment. The AEA and Airbus also contend
that the reporting requirement places operators in a position of having
an obligation to report this information to the DAHs where such an
obligation did not previously exist. They suggested that we not rely on
technicalities and recognize the new obligation being imposed on the
operators. Finally, Transport Canada commented that the rule appears to
require extensive data collecting and reporting and requested more
details be provided regarding what this data will be used for.
    The purpose of collecting reliability data is to ensure that
failures of the system are reviewed and corrected. In this manner,
system reliability is enhanced and FRM malfunctions will become very
infrequent. The reporting requirement will also provide data necessary
to validate that the reliability of the FRM achieved in service meets
the values used in the fleet average flammability exposure and
reliability analyses so that the actual flammability reduction in
service airplanes will achieve the safety goals of this rulemaking.
    The reliability reporting requirements in paragraph M25.5 would not
add an additional burden or cost to the operators. We also continue to
believe that this rule does not directly impose reporting requirements
on operators. These reporting requirements are placed upon the DAH, not
the operator. The NPRM and proposed AC 25.981-2B provided a description
of the level of complexity that was intended in the quarterly reporting
requirements. Furthermore, they do not specify that a new reporting
system be created. The current reporting system could be used to gather
the data and it could then be provided to the DAHs through normal
business agreements. The DAH is required to make arrangements to
collect sufficient data and provide a report to us. Reporting would be
necessary only for a representative sampling of airplanes, as
determined by the manufacturer in its compliance plan. Airlines
routinely collect and store reliability data from airplane systems for
a variety of reasons, such as engine and airplane system reliability
data collected for Extended Twin Operations, warranty claims and
maintenance planning, and in many cases they report these data to DAHs.
    Therefore, DAHs should be able to readily obtain these data through
normal business practices. As a practical matter, DAHs will be
monitoring the performance of these systems, just as they monitor other
systems, both for warranty and liability reasons. Operators will be
providing this information to DAHs as normal business practice to
obtain DAH support in correcting any problems that occur. Our
expectation is that the DAHs' compliance plans will simply state that
DAHs will compile this information into periodic reports (which they
would normally do for their own use anyway) and provide them to the
FAA. No change has been made to the final rule as a result of these
comments.
    Bombardier requested that paragraph M25.5(b) be revised to allow
non-U.S. manufacturers to submit their reports to their national
authorities rather than the FAA. While we acknowledge that submitting a
report to a foreign manufacturer's national authority might simplify
the paperwork exchange, at this time other authorities have not agreed
to harmonize with this rule. Therefore, there are no corresponding
regulations that would require the submittal of reliability reports to
these authorities or to ensure that we will see these reports. We have
revised the requirement to allow for FAA approval of alternative
reporting procedures, which would include reporting to other
authorities with harmonized requirements. The rule also provides that,
after the first five years of reporting, if the demonstrated
reliability of the FRM meets and will continue to meet the reliability
requirements in paragraph M25.1 (not to exceed 1.8 percent of the
FEET), other reliability tracking methods could be proposed to us for
approval, or possibly reporting could be eliminated.
    Boeing requested that M25.5(b) be revised to allow the applicant to
suggest alternative methods of reporting and submit the report to us on
a yearly basis instead of a quarterly basis. It asserted that a one-
year reporting requirement will allow for more statistically
significant data to be collected for new systems. We agree that a
quarterly requirement may be unduly burdensome, but we believe that a
yearly requirement is too long to enable us to initiate timely
corrective action to address reliability problems. Therefore, we have
modified paragraph M25.5(b) in the final rule to extend the reporting
to once every 6 months for the first five years after service
introduction of the FRM. This reporting period should

allow adequate time to gather data to establish the performance of the
FRM and for any needed corrective actions to be taken if the
performance of the FRM falls below minimum levels.
    Boeing also requested changes be made to allow applicants that have
established reporting methods to suggest these as alternative methods
of meeting the reporting requirements. We believe the current wording
allows the DAH the latitude to develop a reporting system and request
FAA approval based upon their business arrangements with operators so
long as the reporting system provides sufficient data to the FAA to
determine the reliability of the FRM. Allowing the use of alternative
reporting methods could lead to disparate reports among manufacturers,
making FAA oversight difficult.

G. Appendix N--Fuel Tank Flammability Exposure and Reliability Analysis

1. General
    Appendix N to part 25 provides the requirements for conducting the
analyses for fleet average fuel tank flammability exposure required to
meet Sec.  25.981(b) and Appendix M and to comply with part 26
requirements. Appendix N contains the method for calculating overall
and warm day fuel tank flammability exposure values needed to show that
the affected airplane's tanks comply with the proposed limitations on
flammability exposure.
2. Definitions
    Paragraph N25.2 provides specific definitions associated with
flammability and analysis terminology used in Appendix N. We received
comments requesting clarification on five of these definitions:
    a. Ullage: Boeing suggested this definition should ensure that all
of the ullage space is considered (not just the fuel volume), and we
agree. In the final rule, this definition has been revised to clarify
that the total ullage space must be considered.
    b. Flammability Exposure Evaluation Time (FEET): An individual
commenter wanted to understand when the evaluation time begins and ends
for airplanes using ground conditioned air with the auxiliary power
unit (APU)/ground power unit (GPU) operating or electrical power that
is connected to the airplane. The evaluation time would begin as soon
as the airplane is prepared for flight, regardless of whether an APU or
electrical ground power is used. The time would end as soon as the
airplane has landed and passengers and crew have disembarked and
payload has been unloaded. In passenger operations where numerous
flights may occur each day, this definition would result in all the
time between flights also being part of the FEET. The only exception
would be the time at the end of the last flight of the day to the point
in the next morning when the airplane is being readied for flight. This
is consistent with the definition for FEET given in paragraph N25.2(b).
    c. Bulk Average Fuel Temperature: An individual commenter suggested
the definition include the means for determining ``bulk average fuel
temperature.'' As we stated in the preamble to the NPRM, the
determination of whether the ullage in the fuel tank is flammable is
based on the temperature of the fuel in the tank or compartment of
interest. This is derived from a fuel tank thermal model, the
atmospheric pressure in the tank, and the properties of the fuel. The
thermal model is comprised of temperature data acquired from various
locations within the fuel tank. In order to express the fuel
temperature of the tank as a whole in the fuel tank fleet flammability
exposure analysis, a weighted average by volume should be calculated at
each point in time since the temperature may vary across the tank or
compartments of the tank depending upon the volume of that area. We
will provide additional guidance on how to determine Bulk Average Fuel
Temperature in AC 25.981-2A.
    d. Flash Point: An individual commenter asked what the term
``heated sample'' meant in this definition. The standardized methods
for determining flash point are ASTM D 56 and ASTM 3828. Both methods
place a sample of fuel in a closed cup and heat it at a constant rate.
A small flame is introduced into the cup, and the lowest temperature at
which ignition is observed is referred to as the flash point. The
heated sample is the fuel that is placed in the closed cup when
conducting this test.
    e. Inerting: An individual commenter requested that fuel removal
from the ullage mixture be included as an acceptable inerting method.
We do not agree with this request. The definition of inerting is based
upon oxygen concentration, not fuel content of the ullage. The Monte
Carlo method uses the bulk fuel temperature to determine fuel tank
flammability, and does not consider transport effects or tank
ventilation. However, if an applicant wishes to consider methods for
removing fuel from the ullage mixture, it could request a finding of
equivalent safety under the provisions of Sec.  21.21. To be
equivalent, such a method would have to be shown to provide at least
the same level of safety as an FRM meeting the performance requirements
of Appendix M.
3. Input Parameters
    Paragraph N25.3(c) provides the parameters that are specific to a
particular airplane model under evaluation that must be provided as
inputs to the Monte Carlo analysis. Boeing had two comments on these
parameters.
    First, Boeing requested we add a new parameter to paragraph
N25.3(c) for airplane utilization. This parameter would require the
applicant to provide data supporting the number of flights per day and
the number of hours per flight from existing fleet data. Boeing stated
that this information is necessary to determine when to apply the
diurnal effect that is required by paragraph N25.4(c) based upon the
number of flights per day. The number of hours per flight will also
provide validation of the mean hours per flight generated by the Monte
Carlo analysis.
    We agree with Boeing's comment and the final rule includes a new
paragraph N25.3(c)(7) for airplane utilization that addresses this
comment. Boeing's second comment was a request that the statement ``or
for the section of the tank having the highest flammability exposure''
be removed from paragraph N25.3(c)(5). As proposed, paragraph
N25.3(c)(5) requires that, for any fuel tank that is subdivided by
baffles or compartments, the bulk average fuel temperature inputs must
be provided either for each section of the tank or for the section of
the tank having the highest flammability exposure. Boeing stated that
every region in a fuel tank should be considered in order to establish
the total flammability exposure of the tank. If the bulk temperature
input only consisted of a section of the fuel tank having the highest
flammability exposure, Boeing argued that the total flammability of the
tank would not be accurately accounted for because the analysis would
not consider regions that were less flammable.
    Any fuel tank that is compartmentalized or subdivided into sections
by baffles is ``flammable'' under the definition for Appendix N
(N25.2(c)) when the bulk average fuel temperature within any section of
the tank that is not inert is within the flammable range for the fuel
type being used. We agree with Boeing that the clause ``or for the
section for the tank having the highest flammability exposure'' in
paragraph N25(c)(3) causes confusion, and we

have revised paragraph N25.3(c)(5) as requested.
    We are providing guidance in AC 25.981-2 on the need to conduct the
flammability analysis for each bay or compartment and then sum the time
any portion of the tank is flammable in the flammability analysis.
4. Verification of ``Flash Point Temperature''
    An individual commenter requested verification of the flash point
temperature (120 [deg]F) that is used in Table 1 of Appendix N. We have
defined in Table 1 of Appendix N a ``mean fuel flash point
temperature'' based upon worldwide survey data that was collected from
1998 through 1999. The Monte Carlo analysis varies the flash point
based upon the distribution of possible flash point temperatures for
the fuel, similar to what would be expected for a fleet of airplanes
where fuels from various refineries and locations are used.

H. Critical Design Configuration Control Limitations (CDCCLs)

    Past experience has shown that critical features of airplane
designs have inadvertently been changed when maintenance actions or
alterations to airplanes have been made. For example, critical wiring
that was intended to be separated from other wiring to prevent possible
unsafe conditions has been modified so new or rerouted wiring was co-
routed with the critical wires. These instances revealed the need for
airplane designers to identify safety critical features, in this case
wiring separations, and for these features to be marked so that
maintenance personnel are aware of the critical features.
    We proposed adding fuel tank flammability related design features
to the existing fuel tank ignition source CDCCL requirements in Sec.
25.981(d) (formerly paragraph (b)). This section requires CDCCL,
inspections, or other procedures as necessary, to prevent increasing
the flammability exposure of tanks above that permitted by the amended
Sec.  25.981(b) and to prevent degradation of the performance and
reliability of any means provided for compliance with paragraphs
25.981(a), (b) or (c). We also proposed adding fuel tank flammability
to the existing requirements to place visible means of identifying
critical features of the design in areas of the airplane where
foreseeable maintenance actions, repairs or alterations could
compromise the CDCCL. Similar provisions were proposed in Sec.
25.1815(e) for existing type certificates.
1. Remove Requirement
    Boeing, Embraer and Bombardier requested that we remove the
requirement to establish CDCCLs to prevent the increase of flammability
in the fuel tanks and to prevent degradation of the performance and
reliability of the FRM. They stated that it is not practical or
effective to try to control flammability through the use of CDCCLs.
Instead, they argued that the certification process should be used to
establish the design's flammability exposure. Bombardier also pointed
out that the type certification data sheet is the appropriate means to
capture limitations (e.g., fuel type, fuel temperature) that would
affect flammability.
    The intent of the CDCCL requirement is to define the critical
features of the design that could be unintentionally altered in a way
that could cause a reduction in fuel tank safety. In the case of IMM or
FRM, maintenance or alterations to the airplane could significantly
affect fuel tank flammability and the performance of these systems.
Since the heating or cooling rate of a fuel tank could be a critical
feature, placing a heat exchanger or other heat source in or near the
tank or changing the cooling rate by transferring warm fuel to the tank
are examples of changes that could result in a significant increase in
fuel tank flammability.
    The commenters did not provide any substantiating information as to
why they believe it is not practical or effective to use CDCCLs to
control fuel tank flammability. Our experience with applying the CDCCL
concept to fuel tank ignition sources has shown it to be both practical
and effective. Locating this information on the TC data sheet, as
suggested by Bombardier, would not provide the information to
individuals, such as maintenance personnel, who could be responsible
for inadvertently changing the system. Accordingly, we do not believe
this suggestion would be effective. In contrast, as airworthiness
limitations, CDCCLs are clearly defined as maintenance requirements
that are routinely complied with by maintenance personnel and that are
enforceable under the operational rules (e.g., Sec.  91.403(c)). The
intent of applying the CDCCL concept to FRM and IMM is to provide a
common location within the maintenance instructions where information
on fuel tank safety related critical features are located. Therefore,
we have retained the requirement in Sec.  25.981(d) to identify CDCCLs
for FRM and IMM.
    On a related issue, paragraph (h) of each of the proposed
operational rules would have required operators to comply with the
CDCCLs. In the NPRM, we inadvertently omitted reference to Sec.  25.981
as one of the sources of requirements for these CDCCLs. Therefore, we
have added these references to the final rule. This change is simply
clarifying, since operators are required to comply with airworthiness
limitations under existing regulations.
2. Clarification on Responsibility for Later Modifications
    As proposed, Sec.  25.1817(d) (now Sec.  26.35(d)) would require
that modifications made to an airplane comply with any CDCCL applicable
to that airplane. The AEA questioned whether this paragraph would
require the TC holder or STC applicant applying for a design change to
achieve a flammability exposure level equal to or better than that
existing on the unmodified airplanes, or if the TC holder or STC
applicant will be held to the flammability exposure limits specified in
the rule.
    The proposed requirement for TC holders to develop CDCCL is
contained in proposed Sec.  25.1815(e) (now Sec.  26.33(d)). It would
require CDCCL ``to prevent increasing the flammability exposure of the
tanks above that permitted under this section and to prevent
degradation of the performance of any means provided under paragraph
(c)(1) or (c)(2) \23\ of this section.'' The AEA has identified an
ambiguity and potential conflict in this quoted provision.
Specifically, if a TC holder develops FRM whose performance exceeds
that required by proposed Sec.  25.1815(c)(1), it is not clear whether
the CDCCL would have to maintain the flammability exposure provided by
the FRM or whether the rule would allow an increase in flammability
exposure up to that permitted (i.e., 3 percent or equivalent to a
conventional unheated aluminum wing tank, along with the ``warm day''
requirement).
---------------------------------------------------------------------------

    \23\ Paragraphs (c)(1) and (c)(2) provide for FRM and IMM,
respectively.
---------------------------------------------------------------------------

    To eliminate this ambiguity, we have deleted the reference to
paragraph (c)(1) in the quoted provision. This revision has the effect
of requiring CDCCL for FRM that allow increasing flammability up to
that permitted by the rule, but retains the requirement that
degradation of performance of IMM is not permitted. Since IMM may be
installed on high flammability tanks, degradation of IMM could have
serious safety consequences and would not be consistent with the intent
of the rule.

    We note that TC holders may be inclined to develop overly stringent
CDCCL for FRM that could potentially make it impossible for holders of
auxiliary fuel tank STCs to meet them. This would force operators to
deactivate these tanks. This over-stringency would not be consistent
with this rule's intent, which is to minimize the burden on operators,
consistent with achieving the safety objectives of this rule. This
issue is discussed in more detail in AC 25.981-2B.
    Proposed Sec.  25.981(d) contained the same ambiguity by requiring
CDCCL to prevent degradation of performance and reliability of any
means provided according to paragraph (b) of that section (FRM). We
have made a similar change to paragraph (d) to allow degradation of FRM
as long as the airplane still meets the standard required by paragraph
(b).
3. Limit CDCCLs to Fuel Tanks That Require FRM or IMM
    Boeing requested that proposed Sec.  25.1815(e) (now Sec.
26.33(e)) be modified to only require CDCCLs that are necessary to
prevent the increase of fuel tank flammability for fuel tanks that
require an FRM or IMM. Boeing stated that development of CDCCLs for
other fuel tanks is not practical, nor is there history to show that
changes to the fuel tanks of airplanes in service significantly
increase flammability in the tanks. Boeing also requested that the
requirement to make critical features of the design visibly
identifiable only apply to areas where it is practical to do so.
    For existing designs subject to proposed Sec.  25.1815(e) (now
Sec.  26.33(e)), we agree with Boeing, and have limited the
applicability of the requirement to develop CDCCL to those tanks for
which FRM or IMM are required. We recognize that there are many
existing modifications that may affect the flammability exposure of
existing fuel tanks. We agree with Boeing that, for main tanks and
other tanks not incorporating FRM or IMM, it is impractical to impose
CDCCLs on these tanks that may result in significant compliance
problems for affected operators. For tanks equipped with FRM or IMM,
however, we believe CDCCLs are necessary to prevent degradation of
these systems below acceptable levels of performance.
    We also agree with Boeing that, in many instances, it may not
always be practical to mark critical features relating to controlling
fuel tank flammability and the proposed rule should be modified to
allow the applicant to justify why markings are not needed. We have
modified the next to last sentence in Sec.  26.33(e) accordingly.
    This change will allow acceptance of designs without markings when
the applicant can show that such markings would be impracticable. We
intend for applicants to identify any CDCCL that are required and to
provide justification for why the marking would be impracticable. Like
all CDCCLs, these would still be documented as airworthiness
limitations in the instructions for continued airworthiness.
4. STC Holders May Not Have Data to Comply
    The AEA and Airbus challenged our statement in the NPRM that
operators have access to information that may be needed by STC and
field approval holders to perform flammability and impact assessments.
The commenters noted that such information is highly proprietary and is
rarely provided to operators. AEA added that contractual agreements to
obtain TC holder information are difficult, if not impossible, to
obtain.
    For many years, the FAA and other regulatory authorities (including
EASA) have routinely required manufacturers to make available
information that they consider proprietary when we determine providing
this information is necessary for aviation safety. For example, most
ADs reference information that would otherwise be proprietary in the
form of service bulletins, which manufacturers are required to make
available to operators. Similarly, Sec.  21.50 requires manufacturers
to make available instructions for continued airworthiness, which
manufacturers would also typically consider proprietary.
    In existing Sec.  25.981(b), we required DAHs to define and make
available CDCCL to prevent the unintended creation of ignition sources
as a result of maintenance or airplane modifications. In proposed Sec.
25.981(e), we required the identification of critical features of a
design that cannot be altered without consideration of the effects on
safety. As discussed previously in this section, the final rule
includes a new requirement for CDCCLs affecting fuel tank flammability.
    Some of the data that STC and field approval holders may need are
already normally provided to operators in the airplane flight manual,
including fuel management information and airplane climb rates. For
other necessary data, such as fuel tank thermal characteristics, we
believe that the market will promote business agreements where TC
holders will make their data available to customers willing to pay for
the data. Airbus or other TC holders may make a business decision not
to support their customers and provide these data. In these cases, it
may be necessary for the operator or STC applicant to acquire the data
from other sources. Another option is for applicants to provide a Monte
Carlo analysis based on conservative inputs for parameters where no
data are available. For example, an applicant could provide thermal
characteristics data that are conservative so that detailed testing and
confirmation of data from flight testing of an airplane would not be
required. Finally, if these approaches are not practical, the
information needed to conduct the Monte Carlo analysis could be
obtained from in-service airplanes.\24\
---------------------------------------------------------------------------

    \24\ Most of the STCs that could be affected by this rulemaking
are auxiliary fuel tanks that use pressurized air to transfer fuel.
In these cases, the inputs needed for the Monte Carlo assessment are
simplified because the fuel tank pressure is controlled to provide
fuel transfer, and the temperature changes of the fuel tank are
limited because the fuel tank is located in the cargo compartment.
---------------------------------------------------------------------------

I. Methods of Mitigating the Likelihood of a Fuel Tank Explosion

1. Alternatives to Inerting
    In the IRE, we selected the use of onboard nitrogen inerting to
assess the costs of reducing fuel tank flammability. By doing this,
several commenters thought we were mandating fuel tank inerting as the
only acceptable means of compliance. ATA and Bombardier commented that
the proposal is not a performance-based rule, since it ``effectively
prescribes the use of fuel tank inerting.'' ATA also stated that they
were not aware of any existing or emerging FRM or IMM that would meet
the proposed performance-based requirements other than inerting.
Frontier Airlines questioned why we focused on FRM and IMM as methods
of compliance when the FAA concluded that other solutions were better
and more practical.
    This rule does not mandate fuel tank inerting as the only
acceptable means of compliance. Rather, it establishes performance-
based requirements that allow applicants to choose the FRM or IMM that
best suits their particular airplane design, so long as it meets the
performance requirements of this final rule. While the Initial
Regulatory Evaluation is based upon the use of inerting, this
technology was chosen because it is considered the most cost-

effective based upon extensive review by industry experts on the ARAC.
    Technology now provides a variety of commercially feasible methods
to accomplish the vital safety objectives addressed by this rule.
Advisory Circular 25.981-2 discusses a number of technologies other
than fuel tank inerting that can be used for demonstrating compliance.
For example, many auxiliary tank manufacturers are considering
pressurizing the fuel tanks to reduce flammability, and many military
airplanes use IMM consisting of polyurethane foam. One recent applicant
has proposed FRM incorporating pressurization of the fuel tanks and a
fuel recirculation system that circulates fuel to the outboard wing to
cool the fuel. Therefore, we believe that other technologies are
available.
    ATA commented that we should consider convening an industry study
group to re-examine the potential of higher flash point fuel as a
possible alternative method for reducing flammability and overall
airplane level risk. ATA noted that refineries may now be capable of
producing higher flash point fuels in the near term in sufficient
quantity for commercial aviation use. In addition, Boeing advised ATA
that a 10 [deg]F elevation in the flash point standard for Jet A could
effect a reduction in flammability exposure rates approximately
equivalent to the proposed FRM. While ATA acknowledged the likelihood
is not high that this approach would provide a more cost-effective
solution than FRM, particularly in the long term, it deserves
reconsideration. The UK Air Safety Group, through one of its members,
agreed with ATA and suggested the use of higher flash point fuels (such
as JP-5) should be investigated as a possible solution.
    While we welcome the potential for using various forms of FRM, we
do not believe delaying implementation of the rule is in the public's
interest. The FAA and industry participated in ARAC activities that
provided economic analysis of existing technologies, including inerting
and mandatory use of higher flash point fuels. At that time, inerting
was found to be a more cost-effective means of showing compliance with
the performance-based FRM rule. In contrast, as shown in the ARAC
report,\25\ using higher flash point fuels was not the most practical
means of achieving the desired safety level because of the higher cost
of these fuels.
---------------------------------------------------------------------------

    \25\ Document Number FAA-22997-7 in the docket for this
rulemaking.
---------------------------------------------------------------------------

    If technology and refining capabilities have advanced to the point
where higher flash point fuels are available in quantity at a
competitive cost, the industry may use that means to show compliance,
and this means is discussed in the proposed AC 25.981-2. Flammability
assessments with a specified minimum fuel flash point, in conjunction
with airplane flight manual limitations requiring use of such fuel,
could be used as a means of compliance with this rule. Since the rule
is performance-based and does not mandate any particular solution,
industry may find innovative ways to show compliance to standards.
2. Inerting Systems Could Create Ignition Sources
    Transport Canada expressed concern that adding inerting systems to
fuel tanks may create ignition sources and result in additional heating
of in-fuselage tanks. It argued the solution may inadvertently increase
flammability exposure. Transport Canada recommended the FRM be designed
to ensure its reliable operation and minimal maintenance. The UK Air
Safety Group, through one of its members, also expressed this concern.
The commenter suggested that inerting systems could actually compromise
the fuel tank system, that insulation could impede inspections of
equipment and structure, and that ventilation could cause performance
penalties.
    We acknowledge the commenters' concerns that installing FRM could
introduce negative safety consequences. However, these potential
consequences do not outweigh the safety benefits of flammability
reduction. As with all safety equipment, the FRM must comply with the
existing applicable airworthiness standards that are intended to
prevent system failures from having a negative safety impact. In
addition, we have introduced new requirements in this rule to address
the possible negative safety impact of using an onboard nitrogen
inerting system. Compliance with these combined requirements should
produce systems that are reliable, maintainable, and meet the
flammability requirements of this rule.
3. Instruments to Monitor Inerting Systems
    ATEXA recommended that when a nitrogen dilution system is used, the
airplane should be equipped with instruments to verify that the system
is functioning as expected. These instruments should record data
continuously so the pilot can control the oxygen concentration in the
tanks within prescribed limits on the ground, before take-off, and at
landing. This data should also be recorded in the flight data recorder
so that, should another accident happen, the cause/origin could be
identified.
    As we stated before, this rule is performance based and allows
designers the ability to be innovative. The need for indications and
controls is design dependent, and the blanket requirement recommended
by ATEXA could be overly stringent. DAHs may choose to provide flight
crew indications of FRM status, or they may propose an automated FRM
with built-in test to verify proper operation. It would be
inappropriate for the rule to mandate specific design features.
    As for the suggestion to record data, adding additional parameters
to the FDR would be cost-prohibitive. Furthermore, we do not consider
this necessary because the functioning of any FRM or IMM would likely
not have any direct bearing on determining the cause of an accident.
The flammability exposure of the fuel tank is not actually an indicator
that a tank has exploded and the determination that a fuel tank
explosion caused an accident could be made using physical evidence.
    In a related comment, the Shaw Aerospace team (Shaw) commented that
failure monitoring of system operation is inadequate. As proposed, the
system relies totally on the built-in test to detect when the tanks are
not inert due to a failure rather than direct measurement of the fuel
tank oxygen concentration to determine if the tank is flammable. Shaw
cited factors such as oxygen evolution from the fuel as the airplane
climbs and local areas of high oxygen in the tanks because of lack of
adequate nitrogen distribution as sources of flammability that will not
be detected by monitoring the performance of the FRM, rather than
measuring the oxygen concentration in the tank. Shaw stated that if the
oxygen concentration in the fuel tank ullage is not monitored and
periodically sampled, it would be difficult to prove the effectiveness
of the system.
    From the Shaw team's comments, we infer that Shaw believes the
monitoring requirements should be modified to require ullage sampling
to ensure that the tank remains non-flammable. We do not agree that a
change to the proposed regulation is needed. Compliance methods are
discussed in AC 25.981-2. Applicants may choose to measure fuel tank
oxygen concentration directly or infer the concentration through system
performance capability and monitoring. Appendix M25.2 requires that
localized higher concentrations of oxygen that

might result from inadequate distribution of nitrogen, as well as the
possible effects of oxygen evolution from the fuel, be addressed in the
compliance demonstration.
4. Risk of Nitrogen Asphyxiation
    If fuel tank inerting is used to reduce the flammability exposure
of a fuel tank, several commenters noted that the introduction of
nitrogen enriched air within the fuel tank, and possibly in
compartments adjacent to the tank, could create additional risk because
of the lack of oxygen in these areas. They believe the risk to
maintenance personnel from nitrogen asphyxiation may exceed any safety
benefit that fuel tank inerting may provide. To support their position,
these commenters cited the Fuel Tank Inerting Harmonization Working
Group's (FTIHWG) 2002 Final Report (24-81 lives could be lost between
2005-2020 due to asphyxiation while servicing transport airplanes) and
other industrial accident data showing that oxygen depleted atmospheres
account for significant loss of life. The commenters are concerned that
we have failed to consider this potential loss of life that will result
from this rule.
    We acknowledge that special precautions are needed for worker entry
into confined spaces where fuel vapors or nitrogen enriched air may be
present. The standard practice of U.S. industry today is to comply with
existing Occupational Safety and Health Administration (OSHA)
requirements. These requirements have resulted in ventilating fuel
tanks with air and measuring the oxygen concentration before entry into
a fuel tank. In addition, persons entering a fuel tank must wear
respirators as well as oxygen monitors to alert them should the oxygen
concentration be insufficient.
    The introduction of nitrogen into a fuel tank does not change the
existing requirements for personnel to enter a fuel tank. No new
training or changes to fuel tank entry procedures should be needed as a
result of this rule. Since there are already specific OSHA requirements
for fuel tanks that would prevent any fatalities, any loss of life
would be due to non-compliance with OSHA regulations, not this
rulemaking. Despite these existing OSHA requirements and the
protections they afford, we have added new requirements for markings to
notify workers at all access points and areas of the airplane where
lack of oxygen could be a hazard. For these reasons, we have not
included costs for loss of life due to asphyxiation in the final
regulatory evaluation for this rulemaking.
    We are also not persuaded by the commenters' reference to the
FTIHWG 2002 Final Report. The predicted number of fatalities in that
report is based upon application of data from every possible cause of
nitrogen asphyxiation that is included in data collected between 1980
and 1989 by the U.S. National Institute of Occupational Safety and
Health. The data quotes a total number of fatalities for all causes,
including cases such as bottled nitrogen being hooked up to oxygen
systems at a nursing home. This bulletin is not based upon data that
can easily be applied to the aviation industry and does not provide any
data that could be used to predict a rate of fatalities for the
specific circumstances relating to airplane fuel tank safety. In
addition, we do not think it is appropriate to extrapolate the data
from the bulletin without taking into account existing OSHA
requirements used in the aviation industry or that the placards
required by this rule will heighten awareness to the risks associated
with entering fuel tanks.
5. Warning Placards
    This rule attempts to reduce the risk of nitrogen asphyxiation by
requiring markings on the access doors and panels to the fuel tanks
with FRMs, and to any other enclosed areas that could contain hazardous
atmosphere. These markings will warn maintenance personnel of the
possible presence of a potentially hazardous atmosphere. Bombardier
commented that the use of placards and the exact wording proposed is
too prescriptive. Bombardier recommended the rule require a general
warning, with guidance defining methods of compliance placed in the
corresponding AC 25.981-2.
    The requirement for placards is based upon methods used throughout
aviation and other industries where safety warnings are needed to
protect workers from possible harm. Locating the requirements in the
regulation rather than in advisory material provides appropriate level
of regulatory review of this safety critical information and will
result in standardizing the means of warning maintenance personnel.
Applicants may apply for a finding of equivalent safety should they
wish to propose an alternative means of achieving the level of safety
provided by the placard requirement in the rule.
6. Definition of ``Inert''
    A fuel tank is considered inert when the bulk average oxygen
concentration within each compartment of the tank is 12 percent or less
from sea level up to 10,000 feet altitude, then linearly increasing
from 12 percent at 10,000 feet to 14.5 percent at 40,000 feet altitude,
and extrapolated linearly above that altitude.
    Several commenters, including Airbus, AAPA, AEA and Blaze Tech,
questioned whether an allowable oxygen concentration of 12 percent
would inert a fuel tank. They pointed to comments in an FAA research
document stating that ``(f)urther experiments to examine the trend of
peak pressure rise as a function of both altitude and oxygen
concentration are needed.'' The commenters stated that this is an
indication that the 12 percent oxygen concentration limit would not
prevent the ignition of fuel vapors from rupturing an airplane fuel
tank and that further work is necessary before accepting the 12 percent
value. American Trans Air and ATEXA noted that the chemical process
industry, as quoted by the French National Institute for Research and
Security (INRS, 2004), uses a safety factor of 0.5 for industrial
volumes on non-homogenous fuels, and operators must strive to maintain
a maximum oxygen content of 5 percent for inerting purposes. Based on
this, American Trans Air and ATEXA stated that the 12 percent limit
would not be safe.
    In 1997, we initiated research activity to determine a maximum
oxygen concentration level at which civilian transport category
airplane fuel tanks would be inert from ignition sources resulting from
airplane system failures and malfunctions. Our testing determined that
a maximum value of 12 percent was adequate at sea level. The 12 percent
value was initially based on the limited energy sources associated with
an electrical arc or thermal sparks that could be generated by airplane
system failures and lightning on typical transport airplanes and was
not intended to include events such as explosives or hostile fire.\26\
As a result of this research, we learned that the quantity of nitrogen
needed to inert commercial airplane fuel tanks was less than previously
believed. An effective FRM can now be smaller and less complex than
earlier systems that were designed to meet the more stringent military
standards intended to prevent ignition from high energy battle damage.
---------------------------------------------------------------------------

    \26\ These test results are available on our Web site: http://
www.fire.tc.faa.gov/pdf/tn02-79.pdf as FAA Technical Note ``Limiting
Oxygen Concentrations Required to Inert Jet Fuel Vapors Existing at
Reduced Fuel Tank Pressures,'' report number DOT/FAA/AR-TN02/79.
---------------------------------------------------------------------------

    The 12 percent value is further substantiated by the results of
live fire testing conducted by China Lake Naval Weapons Center that
showed a 12 percent oxygen concentration prevents

ignition, even when high energy incendiary rounds were used that had
ignition energies well in excess of any source anticipated to occur on
a commercial airplane. These data show that 12 percent oxygen
concentration for commercial airplanes achieves a comparable level of
protection against catastrophic fuel tank explosions as the traditional
9 percent value used by the military for combat airplanes. The
suggestion that the oxygen concentration should be limited to 5 percent
is impractical for commercial airplanes since a significantly larger
flammability reduction system would be needed and, based upon these
test results, there would be no appreciable improvement in airplane
safety.
    Finally, the quoted FAA comment that additional testing is needed
was taken out of context. The recommendation for additional testing
referred to conditions when the oxygen concentration was between 1 to
1.5 percent greater than the limit of 12 percent. Testing at these
higher oxygen concentration values was not extensive since the focus of
the testing was to establish the limiting oxygen concentration where
ignition was not possible. Our report's suggestion that additional
experiments are needed was not an indication that the 12 percent limit
was inadequate--quite the opposite. In fact, the next sentence of the
report confirms the importance of the study's validation of the 12
percent limit: ``The results contained in this report should be useful
in the design, sizing, and optimization of future airplanes inerting
systems and add to the overall knowledge base of jet fuel flammability
characteristics.'' \27\
---------------------------------------------------------------------------

    \27\ Document FAA-22997-14, Executive Summary.
---------------------------------------------------------------------------

7. Use of Carbon Dioxide
    An individual commenter stated that inerting a fuel tank with
carbon dioxide may introduce new concerns because of the solubility of
this gas in fuel and the possible effects on fuel system operation.
This commenter also wanted to know what the acceptable level of oxygen
would be to consider the fuel tank ullage inert when this gas was used.
    We acknowledge the use of carbon dioxide for inerting may require
special considerations for fuel feed system performance. The subject of
inerting with carbon dioxide is addressed in AC 25.981-2 and we have
revised it to highlight these concerns. As for the commenter's specific
question about oxygen concentration in the fuel tank, the acceptable
level of oxygen is the same as if nitrogen is used.
8. Environmental Impact of FRM
    The UK Air Safety Group, Phyre Tech and one individual questioned
the environmental impact of using FRM to displace air and fuel vapor
from the fuel tanks into the surrounding environment. These commenters
expressed concern about increased hydrocarbon emissions into the
atmosphere.
    The IRE did not include an environmental assessment or analysis
because we determined the environmental impact of a FRM or IMM to be
negligible. Their installation will not affect the amount of fuel
vapors and hydrocarbon emissions that are discharged from fuel tanks
during refueling. Currently, fuel tank designs vent fuel vapors and
hydrocarbon emissions into the atmosphere when air is exhausted from
the fuel tanks during refueling and flight. Data from recent flight
tests of a Boeing 737 equipped with a nitrogen-based FRM showed that
installation of FRM and related design changes actually reduce the
amount of hydrocarbons vented from the tanks during flight.\28\ In
those test flights, the data indicated that pressure differences from
one wing tip to the other wing tip, where the two airplane fuel tank
vent outlets are located, resulted in cross flow of air through the
fuel tanks including the center wing tank for the original vent
configuration. This occurred often in flight and periodically on the
ground when any crosswinds were present. As a result, fuel vapors were
exhausted from the fuel tanks into the atmosphere. Any air that entered
the fuel tank diluted the nitrogen concentration in the tank such that
the fuel tank vent outlets needed to be modified to prevent cross flow
of air through the vent system. Modification of the vent system
resulted in reduced hydrocarbon discharge to the atmosphere.
---------------------------------------------------------------------------

    \28\ Data from flight testing on the Boeing 737 (DOT/FAA/AR-01/
63, ``Ground and Flight Testing of a Boeing 737 Center Wing Fuel
Tank Inerted With Nitrogen-Enriched Air,'' dated August 2001).
---------------------------------------------------------------------------

9. Current FRMs Fail To Meet Requirements
    Transport Canada noted that an FRM must meet not only the
requirements in this rule, but also the relevant other sections within
part 25, in particular Sec.  25.1309. Transport Canada stated that
current FRM designs would not meet Sec.  25.1309 because of a lack of
system redundancy, a lack of appropriate system performance monitoring
and indication, and the allowance of MMEL relief.
    We do not agree that existing FRM systems do not meet all the
relevant sections of part 25, including Sec.  25.1309. We approved the
FRM systems for the Boeing 747-400 and 737NG series airplanes in August
2005, and December 2006, respectively, as showing compliance with all
the applicable part 25 regulations. This approval was validated by EASA
shortly thereafter. While the commenter is correct that these systems
lack redundancy, and limited dispatch with the systems inoperative is
allowed under the MMEL, these systems are supplementary safety systems
that are intended to work in combination with the ignition prevention
features required by Sec.  25.981 to prevent future fuel tank
explosions.
10. FRM Based on Immature Technology
    Airbus had numerous objections regarding our description of the
prototype hybrid onboard inert gas generation system (OBIGGS) that was
tested on an Airbus A320 in 2003. Airbus objected to the OBIGGS being
called a ``prototype.'' Instead, Airbus would characterize the OBIGGS
as ``laboratory demonstration equipment.'' Airbus (and AEA) commented
that the OBIGGS was not in an advanced state of development and would
require extensive development before it reached a level of maturity
suitable for certification and operation. Airbus also stated that we
have not identified to Airbus an existing regulation that would require
Airbus to develop an FRM, and Airbus is not committed to any such
development program. British Airways also expressed concerns that the
proposed systems have not been fully tested or developed and operators
may find themselves required to install a system that is not yet fully
certified.
    We acknowledge that the development and certification of a
production and retrofit FRM would require significant engineering and
development. While the FRM equipment (i.e., FAA-developed prototype
OBIGGS) installed and flown on an Airbus airplane had not been
certified, an FRM system similar in concept was designed, tested, and
certified on Boeing 737 and 747 series airplanes within two years of
the Airbus demonstration flights. This certification demonstrates that
the technology is mature, and that our proposed two-year compliance is
reasonable and achievable. The harmonized certification requirements
for the Boeing 737 and 747 FRM, which were nearly identical to those
proposed in the NPRM, were published as Special Conditions in 2005 for
public comment.

This provided the public, including Airbus, with detailed information
needed to develop an FRM. In addition, much of the hardware and
components needed for an FRM have been developed by aerospace
manufacturers and this developmental work should reduce the time needed
for Airbus to develop a system.
    During development of the NPRM, Airbus provided us with a cost
analysis for an FRM that included the cost of engineering, components
and operation of the system. We trust that the cost information was
based upon initial engineering assessments of FRM and contact with
component vendors. We concur with Airbus that, prior to this final
rule, there was no regulation that would require a flammability
reduction means to be developed and installed. However, since the NPRM
was published, two Boeing 737 and two Boeing 747 airplanes have been
delivered with operational FRM based upon nitrogen inerting technology.
These systems have performed very well and provide an indication that
the technology is mature for application to commercial aviation. In
addition, in its March 5, 2007, letter, Airbus confirmed information it
shared with FAA in November 2006, that Airbus is proceeding with the
development of an FRM (Docket No. 22997-149).

J. Compliance Dates

    The Families of TWA Flight 800 Association, Inc., as well as
several members of the public, commented that the compliance times are
too long and should be shortened. While we understand the commenters'
frustration with the proposed compliance times, the schedules chosen
are based on the industry's ability to respond to this rule. Each DAH,
operator, and after-market modifier will have to follow a series of
steps to make appropriate assessments and develop designs and
installation plans. Designing FRM for each affected airplane model will
require engineering resources; allowing less than 24 months for
developing the design changes is not practical and could result in
unintended reduction in airplane safety because of increased likelihood
of design errors. Accelerating the retrofit schedule could
significantly increase the cost of the program due to the need to
introduce FRM into operators' fleets during lengthy out-of-sequence
maintenance visits. We believe that the schedules chosen correctly
balance the risk of a fuel tank explosion during the compliance period
with the industry implementation capability.
1. Part 26 Design Approval Holder Compliance Dates
a. Submitting the Flammability Exposure Analysis
    Boeing requested that proposed Sec.  25.1815(b)(1) (now Sec.
26.33(b)(1)) be revised to remove the compliance time (i.e., 150 days
after the effective date of the rule) for TC holders to submit the
flammability exposure analysis for affected airplane fuel tanks. Boeing
stated that a large amount of test data is required to develop the
analysis and, as such, a compliance time of 150 days would be
inadequate. They believe this requirement is primarily for program
planning purposes and that the compliance time in Table 1 of proposed
Sec.  25.1815(d) is appropriate for that purpose.
    Embraer and Bombardier similarly commented that the 150-day
compliance time for submitting the flammability analysis is inadequate.
The basis for their comment was that validation of fuel tank thermal
models will require developing new flammability tools and flight
testing, which will require additional time. Embraer proposed a 24-
month compliance time, and Bombardier proposed a 12-month compliance
time.
    We believe the proposed compliance time is adequate. It will ensure
that the flammability exposure analyses are completed for every
affected fuel tank in a timeframe we consider acceptable because of the
reduced amount of work required for conventional unheated aluminum wing
tanks. These analyses will determine if FRM is required for a given
fuel tank, and the timeliness of completing the analysis is needed to
meet the design and implementation schedule. As discussed earlier, we
have revised proposed Sec.  25.1815(b)(2) (now Sec.  26.33(b)(2)(i)) of
the final rule to allow TC holders to avoid performing the flammability
analysis for particular tanks by stating in their compliance plans that
they will treat the tank as high flammability and develop FRM or IMM,
as required. In addition, no flammability analysis will likely be
required to determine the flammability of the center wing tanks of
Boeing and Airbus models, since we have determined from their comments
that these models exceed the 7 percent limit. We have also
significantly reduced the complexity of fuel tank thermal analyses that
will be required by the industry because we modified the analysis
requirements to allow a qualitative flammability assessment for
conventional unheated aluminum wing tanks. No flight testing would be
needed to gather data for conventional unheated aluminum wing tanks.
    For the remaining tanks for which a flammability assessment is
needed, the DAHs have been aware of the need to address fuel tank
flammability and have conducted testing of airplanes to develop fuel
tank thermal models. Therefore, additional time should not be needed to
develop fuel tank thermal modeling for the majority of fuel tanks in
the fleet. We believe 150 days is sufficient to complete the required
analyses, and have made no change to the compliance time in the final
rule.
b. Submitting a Compliance Plan for Developing Design Changes and
Service Instructions
    Under proposed Sec.  25.1815(h), each holder of an existing TC
would need to submit to the FAA Oversight Office a compliance plan for
developing design changes and service instructions within 210 days of
the effective date of the rule, which equals 60 days after the
compliance date for submitting the flammability analysis. Embraer and
Bombardier claimed developing a compliance plan within 60 days of
submitting the flammability analysis was impractical. They based their
objections on the fact that Boeing and Airbus, who are specifically
cited in the NPRM, were already preparing for compliance prior to
publication of the NPRM. They claimed that those DAHs not cited in the
NPRM are not doing advanced preparation and will need extra time.
    While Airbus acknowledged that 210 days is a reasonable timeframe,
Airbus was concerned about how this timeframe would accommodate delays
caused by our review. For example, if the TC holder delivers a
flammability analysis which indicates a value under 7 percent, and,
after review, the FAA identifies failings resulting in a value above 7
percent, the TC holder would then have significantly less time to draw
up any potential compliance plan. Airbus stated that, in such cases, it
could be unreasonable for us to require the TC holder to comply within
210 days. Therefore, Airbus suggested that we consider removing the
fixed time period of 210 days and allow 60 days after the FAA and TC
holder have agreed that the correct result is greater than 7 percent.
It noted the requirements on operators of such airplanes should also be
adjusted by a similar time.
    We do not agree with this suggestion. Airbus provided comments to
the NPRM that its airplane models have HCWT with flammability that
ranges between 9 and 16 percent. Boeing has

previously provided a statement to the FAA in response to SFAR 88
evaluations that all of its airplane models with HCWT are above the 7
percent value that determines when an FRM or IMM is needed. Based upon
this information we have determined that all Boeing and Airbus models
specifically listed in proposed Sec.  25.1815 (now Sec.  26.33) have
center wing fuel tanks that will require an FRM or IMM. Since the
analysis needed to determine whether the affected tanks would require
an FRM or IMM is already completed, Airbus and Boeing can begin
developing compliance plans for design changes immediately after
publication of this final rule. Similarly, if Embraer and Bombardier
believe their tanks may be high flammability, they should also begin
developing compliance plans for design changes immediately after
publication of this final rule.
c. Service Instruction Submittal Dates
    Airbus and Boeing recommended that the compliance dates for each
airplane model shown in Sec.  25.1815(d), Table 1, be replaced by a
specific time period for all airplanes in the table. Boeing suggested
the same two-year compliance period be applied to all affected models
to allow adequate time to complete design development, validation and
certification of flammability reduction systems, and development and
validation of service bulletins. Boeing stated that this two-year
period would provide the required timing for airline coordination and
parts procurement flow time needed to support the beginning of the
retrofit period. Airbus suggested 36 months is required to develop the
system design and that an additional 6 months should be provided to
allow for an in-service evaluation of the FRM so that any problems with
the design could be identified and corrected before implementation into
the fleet by the operating rules. Embraer requested a compliance time
of 48 months to develop the design change. Cathay similarly commented
that, while Boeing is making advanced preparations, Airbus is not.
Cathay also requested that the compliance time be extended to support a
more ``realistic'' FRM development schedule. Cathay also commented that
the FAA states ``the proposed compliance date is based on the premise
that the NPRM was to be issued in 2005.'' The new compliance dates need
to be revised to reflect delays in issuing the final rule. Bombardier
felt that 24 months for the design changes should only commence once
the authorities have accepted the design change plan.
    We agree with the commenters that a fixed time for all airplane
models should be established. We have determined that a 24-month
compliance time for DAH development of the IMM or FRM is adequate for
each of the DAHs to complete the task. Since we have determined from
the comments that the Airbus and Boeing models listed in Table 1 in the
NPRM require FRM or IMM, no flammability analysis is needed before
design development begins. The full 24-month time can, therefore, be
used by Airbus and Boeing to develop the design and service
instructions for our approval.
    In addition, Airbus and Boeing have had significant notification of
this rulemaking. In February 17, 2004, we made a public announcement of
our plans to develop and publish a proposal to require both retrofit
and production incorporation of FRM or IMM. The NPRM was issued in
November, 2005, and the rulemaking processing time has provided
extensive time to develop designs as well as work with suppliers to
discuss cost and schedule issues. Special conditions for the Boeing 737
and 747 were published by the FAA and EASA that provided performance
standards for FRM in 2005. Many of the components in nitrogen based FRM
systems are similar or identical to components used in military
applications or pneumatic systems on commercial airplanes. The air
separation modules used in these systems are based on technology
currently used extensively in other industries. Therefore, we believe
Airbus's request to increase the development and certification time
from 24 months to 42 months, and Embraer's request for 48 months, are
excessive, and we are confident that 24 months provides adequate time
for design and service instruction development. Extending this
compliance time would delay the operators' installation of these
important safety improvements. Therefore, we have not revised the final
rule as requested.
2. Operator Fleet Retrofit Compliance Dates
    In proposed Sec. Sec.  91.1509, 121.1117, 125.509 and 129.117, we
included a Table 1 that contained the interim and final compliance
dates for operators to complete the installations of IMM, FRM or FIMM
required by those sections. Table 1 proposed unique compliance dates
for those affected Boeing and Airbus models with high flammability fuel
tanks. These dates were selected based upon the availability of service
instructions and the risk associated with each airplane model.
a. Removal of Unique Compliance Dates for Affected Airplane Models
    Boeing stated that, assuming the FAA concludes that retrofit is
justified, the compliance time should be 7 years from the date that
service instructions are available for all airplane models. Boeing
maintained there is no justification for requiring unique compliance
times tied to airplane models and recommended deleting Table 1.
    We agree and have removed Table 1 from the final rule. This table
has been replaced with a standardized compliance date for all affected
airplanes. As explained below, the new compliance time for all models
is 9 years from the effective date of this rule. We did not link the
operators' compliance time to our approval of the service instructions
because the length of time it will take us to approve the submission
will depend upon the quality of the submission. While the compliance
planning provisions are intended to ensure that the submissions are
approvable, whether they have that effect is within the control of the
DAHs.
b. Increase Compliance Times From 7 to 10 Years
    The ATA asked that the compliance times be increased from 7 to 10
years after manufacturers develop the necessary design changes. ATA
argued that the accident rate is such that there is little risk of
catastrophic in-flight fuel tank explosion during that period. A 10-
year compliance time would allow all operators to incorporate the FRM
in heavy maintenance visits instead of only 85 percent of them.
    We partially agree with ATA. As discussed previously, we are
providing a compliance time of 24 months for all affected manufacturers
to develop necessary design changes. We have adjusted the compliance
times in the operational rules to allow 6 years after the effective
date for compliance by 50 percent of an operator's fleet, and 9 years
for full implementation, i.e., we are retaining the compliance time of
7 years after the design changes are developed. The compliance period
of 7 years for operators to incorporate the design modifications into
each fleet was selected to allow the vast majority of the FRM or IMM to
be incorporated during airplane heavy checks and to achieve the safety
level expected by the public.
    Nevertheless, as ATA noted, 15 percent of the airplanes may need to
incorporate FRM at a time other than during a heavy check. To address
this concern and reduce the costs of this rule, we have revised the
operational requirements of parts 121 and 129 to

allow a one-year extension for retrofit if the operator elects to use
ground conditioned air for all airplanes with high flammability tanks
(i.e., Boeing and Airbus models) for ``actual gate times'' exceeding 30
minutes when ground air is available at the gate and operational and
the ambient temperature exceeds 60 degrees F. This approach responds to
requests for more time to retrofit while providing compensating risk
reduction by use of ground conditioned air, which reduces flammability
for airplanes on the ground. We are not including this extension
provision in part 125, because these airplanes are typically not parked
at gates where ground conditioned air is available. Also, these
operators typically only operate one or very few airplanes subject to
this rule, so they will not encounter the difficulties that ATA
identified in scheduling large fleets of airplanes for modifications.
    For purposes of this provision, ``actual gate time'' is time when
the airplane is parked at a gate for servicing and passenger egress and
ingress. If scheduled gate time is 30 minutes or less, but departure is
delayed so that airplane is parked for more than 30 minutes, use of
ground air is required for any period longer than 30 minutes. This
ensures that heating of tanks (and resulting increased flammability) is
limited. ``Available'' means installed at the gate. ``Operational''
means working, so that an operator is not in violation simply because
ground conditioned air is out of service for maintenance. Ambient
temperature is the official temperature at the airport as provided by
the U.S. National Weather Service or worldwide METAR \29\ weather
report system. This provision requires revision of operator's
operations specifications and relevant manuals to ensure that the
commitment to use of ground air is fully implemented and enforceable.
In the near future we will be issuing guidance on compliance with the
conditions for this extension.
---------------------------------------------------------------------------

    \29\ METAR (from the French, ``message d'observation
m[eacute]t[eacute]orologique r[eacute]guli[egrave]re pour
l'aviation,'') is a format for reporting weather information. METAR
means ``aviation routine weather report'' and is predominantly used
by pilots in fulfillment of a part of a pre-flight weather briefing,
and by meteorologists, who use aggregated METAR information to
assist in weather forecasting.
    METAR reports usually come from airports. Typically, reports are
generated once an hour; however, if conditions change significantly,
they may be updated in special reports called SPECI's. Some reports
are encoded by an Automated Surface Observing System located at
airports, military bases and other sites. Some locations still use
augmented observations, which are recorded by digital sensors and
encoded via software, but are reviewed by certified weather
observers or forecasters prior to being transmitted. Observations
may also be taken by trained observers or forecasters who manually
observe and encode their observations prior to their being
transmitted. Source: Wikipedia, August 2007.
---------------------------------------------------------------------------

c. Interim Compliance Dates
    We proposed interim compliance dates for operators to incorporate
any FRM or IMM into 50 percent of their affected high flammability
airplanes within their fleet. Boeing requested we revise Sec. Sec.
91.1509(d)(1), 121.1117(d)(1), 125.509(d)(1), and 129.117(d)(1) to
state:
    ``IMM, FRM or FIMM, if required by Sec. Sec.  25.1815, 25.1817, or
25.1819 of this chapter, that are approved by the FAA Oversight Office,
are installed in at least 50 percent of the operator's fleet within 4
years from the date service instructions are available. This does not
apply for certificate holders with only one airplane in the fleet.''
    Boeing stated that newly delivered airplanes should be included in
the operator's ``fleet'' for purposes of Table 1. Boeing also commented
that Table 1 should not be split by individual airplane model, but
should include all airplanes in a given operator's current fleet. The
recommended revision to 50 percent of the operator's fleet should also
specify if this is 50 percent of their fleet operating on the
compliance date, 50 percent of their fleet that is operating at the
beginning of the compliance period, or 50 percent of their fleet that
will be operating at the end of the compliance period.
    We agree that additional clarification is needed on the definition
of ``50 percent of fleet.'' We intended that the 50 percent figure be
based on all airplanes that are required to be modified under this rule
and that are being operated by an operator 6 years after the effective
date of this rule. Any airplanes transferred or purchased with high
flammability fuel tanks, would be included in the operator's ``fleet.''
Since newly delivered airplanes are not required to be modified, they
are not included as part of the 50 percent of the fleet to meet this
requirement.

K. Cost/Benefit Analysis

    As noted in the Regulatory Evaluation Summary, specific comments on
the quantitative costs and benefits estimates are more completely
discussed in the FRE. In this section, we only address general economic
issues that were addressed by the comments.
1. Security Benefits
    In the NPRM, we noted that the potential benefits from preventing
terrorist-initiated accidents were excluded from consideration in both
the ARAC reports and the IRE. While the proposed FRM requirements were
not primarily intended to address terrorist-initiated explosions, we
invited public comment on possible additional security benefits that
inerting fuel tanks may provide. In response to this request, we
received several comments, including the following:
     The NTSB and several individuals supported including
benefits from prevented consequences of terrorist action in the FRE and
suggested we should complete a cost/benefit analysis of inerting all
fuel tanks to address terrorist threats. The NTSB noted that, although
not intended for missile defense or entirely effective as such,
flammability reduction systems could mitigate the results of shrapnel
entering fuel tanks during a terrorist act. Therefore, the NTSB
recommended that the cost-benefit analysis for the final rule should
include estimates of potential missile attacks on airplanes. In
addition, these commenters also supported including possible benefits
from preventing terrorist actions caused by bombs exploding in the
airplane.
     CAPA stated that the United States is at a heightened risk
of terrorist attacks. CAPA noted the aviation industry affects nearly 9
percent of the U.S. Gross Domestic Product, and suggested that
terrorists will undoubtedly seek ways to attack the aviation
infrastructure. CAPA recommended that we should complete a cost benefit
analysis of inerting all fuel tanks and make recommendations to the
Department of Homeland Security and aviation industry.
     NATCA commented that there would be an adverse effect on
the public's confidence in flying if another fuel tank explosion
occurred.
     Airbus and AEA stated that, in theory, there may be some
benefit to improving security by installing FRM on airplanes. However,
they noted that we have no basis for estimating the amount of that
benefit and they do not believe it to be substantial.
     ATA and FedEx objected to the FAA's including the Avianca
727 accident in its justification of this rule. They stated that this
accident, which resulted from a small bomb placed above the center wing
fuel tank on the previous flight, would not have been prevented by the
requirements of this rule.
    Based upon the comments received and our review of historical
evidence, we have not quantified any potential benefits from an FRM
system preventing a fuel tank explosion caused by a terrorist missile
or an on-board bomb.