[Federal Register Volume 74, Number 84 (Monday, May 4, 2009)]
[Proposed Rules]
[Pages 20431-20443]
[FR Doc No: E9-10162]

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DEPARTMENT OF TRANSPORTATION

Federal Aviation Administration

14 CFR Part 39

[Docket No. FAA-2007-27862; Directorate Identifier 2007-CE-036-AD]
RIN 2120-AA64

Airworthiness Directives; Thrush Aircraft, Inc. (Type Certificate
Previously Held by Quality Aerospace, Inc. and Ayres Corporation) Model
600 S2D and S2R (S-2R) Series Airplanes

AGENCY: Federal Aviation Administration (FAA), Department of
Transportation (DOT).

ACTION: Notice of proposed rulemaking (NPRM).

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SUMMARY: We propose to supersede Airworthiness Directive (AD) 2006-07-
15, which applies to Thrush Aircraft, Inc. Model 600 S2D and S2R (S-2R)
series airplanes (type certificate previously held by Quality
Aerospace, Inc. and Ayres Corporation). AD 2006-07-15 currently
requires repetitive inspections of the \1/4\-inch and \5/16\-inch bolt
hole areas on the wing front lower spar caps for fatigue cracking;
replacement or repair any wing front lower spar cap where fatigue
cracks are found; and reporting of any fatigue cracks found to the FAA.
AD 2006-07-15 also puts the affected airplanes into groups for
compliance time and applicability purposes. Since we issued AD 2006-07-
15, FAA analysis reveals that inspections are not detecting all
existing cracks and shows the incidences of undetected cracks will
increase as the airplanes age. Consequently, this proposed AD would
retain the actions of AD 2006-07-15 and impose a life limit on the wing
front lower spar caps that requires replacement of the wing front lower
spar caps when the life limit is reached. This proposed AD would also
change the requirements and applicability of the groups discussed above
and remove the ultrasonic inspection method. We are proposing this AD
to prevent wing front lower spar cap failure caused by undetected
fatigue cracks. Such failure could result in loss of a wing in flight.

DATES: We must receive comments on this proposed AD by July 6, 2009.

ADDRESSES: Use one of the following addresses to comment on this
proposed AD:
     Federal eRulemaking Portal: Go to http://www.regulations.gov. Follow the instructions for submitting comments.
     Fax:<PHONE_NUMBER>.
     Mail: U.S. Department of Transportation, Docket
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New
Jersey Avenue, SE., Washington, DC 20590.
     Hand Delivery: U.S. Department of Transportation, Docket
Operations, M-30, West Building Ground Floor, Room W12-140, 1200 New
Jersey Avenue, SE., Washington, DC 20590, between 9 a.m. and 5 p.m.,
Monday through Friday, except Federal holidays.
    For service information identified in this proposed AD, contact
Thrush Aircraft, Inc., 300 Old Pretoria Road, P.O. Box 3149, Albany,
Georgia 31706-3149. The service information is also available on the
Internet at www.thrushaircraft.comm.
    For Further Information, Contact One of the Following:

--Cindy Lorenzen, Aerospace Engineer, ACE-115A, Atlanta Aircraft
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite 450,
Atlanta, Georgia 30349; telephone:<PHONE_NUMBER>; facsimile: (770)
703-6097; e-mail<EMAIL_ADDRESS>or
--Keith Noles, Aerospace Engineer, ACE-117A, Atlanta Aircraft
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite 450,
Atlanta, Georgia 30349; telephone:<PHONE_NUMBER>; facsimile: (770)
703-6097; e-mail<EMAIL_ADDRESS>
SUPPLEMENTARY INFORMATION:

Comments Invited

    We invite you to send any written relevant data, views, or
arguments regarding this proposed AD. Send your comments to an address
listed under the ADDRESSES section. Include the docket number, ``FAA-
2007-27862; Directorate Identifier 2007-CE-036-AD'' at the beginning of
your comments. We specifically invite comments on the overall
regulatory, economic, environmental, and energy aspects of the proposed
AD. We will consider all

comments received by the closing date and may amend the proposed AD in
light of those comments.
    We will post all comments we receive, without change, to http://www.regulations.gov, including any personal information you provide. We
will also post a report summarizing each substantive verbal contact we
receive concerning this proposed AD.

Discussion

History of AD Actions

    An accident in which the wing on a Thrush S2R series airplane
separated from the airplane in flight prompted us to issue AD 97-13-11.
The following presents the sequential AD history on this subject to
date:
     AD 97-13-11, Amendment 39-10071 (62 FR 36978, July 10,
1997), required (until superseded by AD 97-17-03) inspecting certain
areas of the wing front lower spar caps for fatigue cracks, replacing
any wing front lower spar cap where fatigue cracks were found, and
reporting any fatigue cracks to the FAA.
     AD 97-17-03, Amendment 39-10195 (62 FR 43926, August 18,
1997), superseded AD 97-13-11. AD 97-17-03 corrected a model
designation and retained the actions of AD 97-13-11.
     AD 2000-11-16, Amendment 39-11764 (65 FR 36055, June 7,
2000), superseded AD 97-17-03. AD 2000-11-16 changed the inspections
required in AD 97-17-03 to repetitive, added airplanes to the
Applicability section, changed the initial compliance time for all
airplanes, and arranged the affected airplanes into six groups based on
usage and configuration.
     AD 2003-07-01, Amendment 39-13097 (68 FR 15653, April 1,
2003), superseded AD 2000-11-16. AD 2003-07-01 added airplanes
manufactured with a similar design to the Applicability section and
added an additional repair option.
     AD 2006-07-15, Amendment 39-14542 (71 FR 16691, April 4,
2006), superseded AD 2003-07-01. AD 2006-07-15 increased the inspection
frequency of Groups 1, 2, 3, and 6 airplanes and lowered the initial
inspection time of Group 2 airplanes based on analysis of crack report
data compiled from the previous ADs.

Events That Initiated This Proposed AD

    All of the ADs listed above required submitting reports to the FAA
anytime a fatigue crack was found on a wing front lower spar cap.
Recent analysis of the data from those reports and other historical and
statistical data indicate the current inspections are not completely
addressing the unsafe condition.
    Specifically, the data indicate a risk that some airplanes in the
Thrush fleet may currently have undetected fatigue cracks in the steel
spar cap using the existing inspection program. Airplanes with cracks
in the wing front lower spar caps are unable to meet ultimate strength
requirements, which could lead to a wing failure. As the incidences of
cracking increase, which has occurred in the Thrush airplanes, the
chance of an existing crack not being detected during an inspection
increases.

FAA Analysis

    The FAA used a risk-based probability analysis to determine the
risk of fatigue cracks occurring in the wing front lower spar cap on
Model 600 S2D and S2R (S-2R) series airplanes. This analysis indicates
the risk to the pilot and the public is too great to allow the
continuation of the repetitive inspections as the only method to ensure
the safety of these airplanes. The actions in this proposed AD are
necessary to assure the continued airworthiness of these airplanes.
    We analyzed data obtained from reports of 117 fatigue cracks found
on the wing front lower spar caps on these airplanes since 1997. The
analysis of the crack reports led to our determination to consider
imposing a life limit on the wing front lower spar caps. We have
confidence in the accuracy of the reports submitted by the owner/
operators, Airframe and Powerplant (A&P) mechanics, and Level 2 and 3
non-destructive inspectors. Anyone with documented evidence of owner/
operators, inspectors, or A&P mechanics on behalf of the owner
submitting inaccurate crack reports or not submitting crack reports to
the FAA should send that evidence to their local FAA Flight Standards
District Office.
    We have a documented occurrence of a fatigue crack that went
undetected for at least two inspection cycles. The crack grew until the
wing front lower spar cap was completely severed, which is considered a
failure even though the wing stayed attached to the airplane. The ``big
butterfly'' plate and the lower splice plate, part numbers (P/Ns)
20211-09 and 20211-11 respectively, installed on this airplane as an
optional modification helped keep the wing together; however, the
plates are not designed to carry all of the possible flight loads in
the event a spar cap is severed.
    Installing stronger ``big butterfly'' plates is beneficial because
it reduces stress in the wing front lower spar caps. The reduced stress
slows the crack growth rate in the spar cap. This slower crack growth
rate in airplanes equipped with ``big butterfly'' plates allows for
less frequent inspections. Even though P/Ns 20211-09 and 20211-11
reduce stress in the wing front lower spar caps and slow the crack
growth rate, the plates will not handle all possible flight loads once
the spar cap is severed. Any known cracks must still be repaired.
    Thrush Aircraft, Inc. has developed Custom Kit No. CK-AG-41,
Revision A, dated March 8, 2007. This kit includes parts and procedures
for replacing both wing front lower spar caps with new wing front lower
spar caps, P/Ns 20207-15 and 20207-16, new inboard spar webs and
doublers, and new, thicker ``big butterfly'' plate and lower splice
plate, P/Ns 94418-5 and 94418-7 respectively.
    Airplanes that have Custom Kit No. CK-AG-41, Revision A, installed
in its entirety will have lower stresses in the spar cap, which will
delay the initiation of fatigue cracks and slow the fatigue crack
growth rate allowing for less frequent inspections. A life limit would
remain the same even after Custom Kit No. CK-AG-41, Revision A, is
installed in its entirety. If additional fatigue testing and analysis
is completed on this configuration in the future, a life limit may be
adjusted.
    Our analysis showed the wing front lower spar caps will all crack
due to fatigue. In determining the maximum time allowed for life
limits, we gave consideration to the following:
     Reliability of the significant amount of crack data on the
Thrush fleet;
     Existence of the on-going inspection program for the wing
front lower spar caps; and
     Allowance of credit for time the airplanes operated with
lower horsepower radial engines and were later modified by installing a
turbine engine, a higher horsepower radial engine, or larger hopper.
    We could not consider the following when determining life limits:
     Individual airplanes operated at lower weights; and
     Individual airplanes operated at lower G loads.
    To consider these factors, individual airplanes would need to have
recorded data for every flight since the wings were installed showing
the weight and recorded Gs throughout each flight, along with fatigue
analysis and tests using this data.
    In addition, we could not consider the effect of the following
modifications when determining life limits:
     Kaplan splice blocks installed;
     ``Big butterfly'' plates and lower splice plates
installed;

     Winglets installed; or
     Cold work process on the bolt holes performed.
    We do not have service information to calculate the effect of these
modifications, and accurate fatigue test data or fatigue analysis data
supported by tests has not been provided to us for these
configurations. If we receive accurate fatigue substantiation data for
airplanes with these modifications, we may allow changes to life limits
by an alternative method of compliance.
    There is evidence of sharp, uneven edges on the spar cap bolt holes
that resulted from the manufacturing process in Group 5 airplanes. Five
fatigue cracks have been reported on Group 5 airplanes, and our
analysis concludes fatigue cracks will occur on all these airplanes.
Premature fatigue cracks begin when there is a crack starter, such as
an uneven edge. At this time, there is no rework method to address the
condition of these wing front lower spar caps with uneven bolt hole
edges. Once the original wing front lower spar caps are replaced, a
higher life limit for wing front lower spar caps without uneven bolt
hole edges may be used.
    Initial compliance times for replacement of the wing front lower
spar caps would be based on risk analysis that allows for compliance
scheduling. For any of the affected airplanes that may exceed any life
limits, the compliance time range would be based on total hours time-
in-service (TIS), which would address those high-usage airplanes first.
Graduated compliance times would help alleviate grounding of airplanes
due to the limited supply of wing front lower spar caps, while still
addressing the increased risk for high-usage airplanes.

Long-Term Continued Operational Safety

    Repeated loads and the resulting stresses in the metal lead to
fatigue. Over time, these stresses cause the metal to wear out and
cracks will form in these airplanes even when operated within the
approved limitations and envelope. Higher stresses in the wing front
lower spar cap, caused by pulling excessive Gs and/or operating over
the design weight of the airplane, will accelerate metal fatigue. Metal
will also fatigue more quickly when operated in a wet or corrosive
environment, which exists when dispensing agricultural chemicals or
dropping fire retardants or water.
    Any type of inspection method may be affected by the reliability of
the equipment used, the inspection procedure used, the environment in
which the inspection is done, the quality of the calibration reference
standard used, and various human factors, such as the knowledge, skill,
experience, and dexterity of the inspector. Because of all these
variables, most inspection results, while very good, are not always
100-percent accurate. Over time, the probability of failing to detect a
crack increases due to these variables, which increases the risk to the
safety of these airplanes.
    Studies of the factors leading to inspection inaccuracy and their
effect on a variety of inspection methods, including magnetic particle
inspections and eddy current inspections, have been done by the
National Aeronautics and Space Administration (since 1973 for the Space
Shuttle design), the United States Air Force, and the FAA. These
studies show variability in inspection results that are inherent to any
measurement process.
    We received a report of cracks not being detected in the Thrush
wing front lower spar cap using the ultrasonic method because of the
configuration of the joint. Our records indicate that ultrasonic
inspections are no longer being used in the field. This inspection
method should be removed. If ultrasonic inspections are no longer
allowed for these inspections, the availability of inspection
facilities should not be affected because the two inspection facilities
certified for ultrasonic inspections are also certified for eddy
current inspections.
    As wing front lower spar caps accumulate hours TIS beyond the time
when cracks have been found on other products of the same type design,
the likelihood of fatigue cracks occurring in these wing front lower
spar caps increases. Many of the affected airplanes have wing front
lower spar caps that have been in service well past the number of hours
TIS when cracks have been appearing on wing front lower spar caps in
other products of the same type design. FAA statistical analysis of the
crack data indicates the risk of a wing failure occurring is becoming
very high for these airplanes.

Reclassification of Airplane Groups

    A recent review of the manufacturer's build record data shows some
airplanes were placed in incorrect Groups and one airplane was
inadvertently left out in the previous ADs. Our review shows that Model
S2R-T34 airplanes, serial numbers (S/Ns) T34-147 through T34-167, were
built with wing front lower spar caps identical to Group 2 airplanes;
these airplanes should be reclassified from Group 1 to Group 2. Model
S2R-G10 airplane, S/N G10-137, is currently included in Group 4
airplanes but was built identical to Group 2; this airplane should be
reclassified into Group 2. We inadvertently omitted Model S2R-T34
airplane, S/N T34-170, from AD 2006-07-15; that airplane should be
included in Group 2. We inadvertently listed Model S2R-T34 airplane, S/
N T34-225, in both Group 2 and Group 4 airplanes in AD 2006-07-15; it
should be in Group 2 only. Model S2R-G1 airplane, S/Ns G1-107, G1-108,
G1-109; Model S2R-G10 airplane, S/Ns G10-139 and G10-142; and Model
S2R-T34 airplanes, S/Ns T34-236, T34-237, and T34-238, were built
identical to Group 5; these airplanes should be in Group 5. No
airplanes were built to the configuration previously identified as
Group 4; Group 4 should be removed.

Relevant Service Information

    The following service information was included in AD 2006-07-15 and
will be included in this proposed AD:

--Ayres Corporation Service Bulletin No. SB-AG-39, dated September 17,
1996;
--Ayres Corporation Custom Kit No. CK-AG-29, dated December 23, 1997;
and
--Quality Aerospace, Inc. Custom Kit No. CK-AG-30, dated December 6,
2001.

    The new service information for this proposed AD is Thrush
Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8,
2007.

FAA's Determination and Requirements of the Proposed AD

    We are proposing this AD because we evaluated all information and
determined the unsafe condition described previously is likely to exist
or develop on other products of the same type design. This proposed AD
would supersede AD 2006-07-15 with a new AD that would:
     Retain the actions of AD 2006-07-15;
     Add life limits for the wing front lower spar caps;
     Lower the initial and repetitive inspection times for
Group 5 airplanes;
     Correct some airplane Group classifications;
     Add an airplane to the Applicability section; and
     Remove the use of ultrasonic inspection methods.
    The initial compliance time for all airplanes would be at least an
additional 500 hours TIS after the effective date of the proposed AD
for replacement of the wing front lower spar caps. Calculated from
actual flight hour data from 285 S2R series airplanes, 500 hours TIS
equates to the average yearly operational

time. The proposed compliance schedule should give owner/operators
enough time to schedule the replacement of the wing front lower spar
caps.
    Although not required in this proposed AD, we recommend installing
``big butterfly'' and lower splice plates, P/Ns 20211-09 and P/N 20211-
11, or Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, since
they increase the strength of the wing beyond the minimum safety
standards.
    This proposed AD would require you to use the service information
described previously to perform these actions.

Costs of Compliance

    We estimate that this proposed AD would affect 808 airplanes in the
U.S. registry, including those airplanes affected by AD 2006-07-15.
    We estimate the following costs to do each proposed inspection:

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                                                                        Total cost per       Total cost on U.S.
                 Labor cost                        Parts cost              airplane              operators
----------------------------------------------------------------------------------------------------------------
3 work-hours x $80 = $240..................                  $525                   $765               $618,120
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    We estimate the following costs to do cold work of bolt holes for
the repair that may be required based on the results of the proposed
inspection. We have no way of determining the number of airplanes that
may need such repair:

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                                                                                Total cost per
                            Labor cost                                    Parts cost              airplane
----------------------------------------------------------------------------------------------------------------
1 work-hour x $80 = $80...........................................                  $100                   $180
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    We estimate the following costs to do any reaming of outer holes to
\5/16\-inch diameter for the repair that may be required based on the
results of the proposed inspection. We have no way of determining the
number of airplanes that may need such repair:

------------------------------------------------------------------------
                                                       Total cost per
          Labor cost                Parts cost            airplane
------------------------------------------------------------------------
1 work-hour x $80 = $80.......  None.............                   $80
------------------------------------------------------------------------

    We estimate the following costs to do any drilling and reaming of
outer holes and adding three holes to install a Kaplan splice block for
the repair that may be required based on the results of the proposed
inspection. We have no way of determining the number of airplanes that
may need such modification:

------------------------------------------------------------------------
                                                       Total cost per
          Labor cost                Parts cost            airplane
------------------------------------------------------------------------
65 work-hours x $80 = $5,200..  $4,400 for splice               $10,200
                                 block and $600
                                 for hardware.
------------------------------------------------------------------------

    We estimate the following costs to do the proposed optional
installation of Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision
A, dated March 8, 2007. This kit may be used to do any necessary wing
front lower spar cap replacement that would be required based on the
results of the proposed inspection or that would be required based on
reaching the proposed life limit:

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                                                                                Total cost per
                            Labor cost                                    Parts cost              airplane
----------------------------------------------------------------------------------------------------------------
300 work-hours x $80 = $24,000....................................               $40,000                $64,000
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    We estimate the following costs to do any necessary wing front
lower spar cap replacement that would be required based on the results
of the proposed inspection or by the wing front lower spar cap reaching
the proposed life limit:

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  Labor cost per wing front lower spar    Parts cost per wing                                Total cost on U.S.
                  cap                     front lower spar cap   Total cost per airplane         operators
----------------------------------------------------------------------------------------------------------------
200 work-hours x $80 = $16,000.........                $8,000   Each spar cap replacement           $38,784,000
                                                                 = $24,000.
                                                                Two spar caps per
                                                                 airplane = $48,000.
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Authority for This Rulemaking

    Title 49 of the United States Code specifies the FAA's authority to
issue rules on aviation safety. Subtitle I, Section 106, describes the
authority of the FAA Administrator. Subtitle VII, Aviation Programs,
describes in more detail the scope of the Agency's authority.
    We are issuing this rulemaking under the authority described in
Subtitle VII, Part A, Subpart III, Section 44701, ``General
requirements.'' Under that section, Congress charges the FAA with
promoting safe flight of civil aircraft in air commerce by prescribing
regulations for practices, methods, and procedures the Administrator
finds necessary for safety in air commerce. This regulation is within
the scope of that authority

because it addresses an unsafe condition that is likely to exist or
develop on products identified in this rulemaking action.

Initial Regulatory Flexibility Analysis

Introduction and Purpose of This Analysis

    The Regulatory Flexibility Act of 1980 (Pub. L. 96-354) (RFA)
establishes ``as a principle of regulatory issuance that agencies shall
endeavor, consistent with the objectives of the rule and of applicable
statutes, to fit regulatory and informational requirements to the scale
of the businesses, organizations, and governmental jurisdictions
subject to regulation.'' To achieve this principle, the RFA requires
agencies to solicit and consider flexible regulatory proposals and to
explain the rationale for their actions to assure that such proposals
are seriously considered.'' The RFA covers a wide-range of small
entities, including small businesses, not-for-profit organizations, and
small governmental jurisdictions.
    Unless the FAA can certify that a proposed rule will not have a
significant economic impact on a substantial number of small entities,
the FAA is required to prepare an initial regulatory flexibility
analysis (IRFA) as described in Sec.  603 of the RFA. Such an analysis
must include (1) a description of the reasons for the agency's action;
(2) a statement regarding the objectives and legal basis for the
proposed rule; (3) an estimate of the number of small entities that
will be affected by the proposed rule; (4) a description of the
projected recordkeeping, reporting, and other compliance costs; (5) a
statement regarding any potential duplication, overlap, or conflict
with all other relevant rules; and (6) a description of any significant
alternatives that may minimize the significant economic impact of the
proposed rule on small entities. Based on the following analysis, the
FAA concludes that this proposed rule will have a significant economic
impact on a substantial number of small entities.

Reasons Action by the FAA Is Being Considered

    A series of ADs, beginning in 1997 and culminating in AD 2006-07-15
in 2006, addressed the issue of fatigue cracking of the wing front
lower spar caps in Thrush Aircraft, Inc. (Thrush) Model 600 S2D and S2R
(S-2R) series airplanes (type certificate previously held by Quality
Aerospace, Inc. and Ayres Corporation). This type of fatigue cracking,
if not addressed, could result in catastrophic wing failure. The
original 1997 AD was issued after an accident on an S2R series airplane
in which the wing separated from the airplane in flight. Requirements
of inspection and possible replacement were changed in 2000 to
repetitive inspections and possible replacement. In 2006, the
inspection rate was doubled after a completely severed spar cap was
found on one of the affected airplanes and the FAA noted that it was
working with Thrush to develop a future terminating action. Analysis
indicated that an undetected crack had existed during the previous two
repetitive inspections of that spar cap.
    Subsequent FAA analysis has shown that spar cap fatigue cracking
has increased as the fleet has aged, and will continue to increase.
Consequently, the incidences of undetected cracks will increase,
increasing the probability of catastrophic wing failure. The FAA has
concluded that repetitive inspections, as required since the 2000 AD,
are insufficient by themselves to ensure the safety of these airplanes
and, accordingly, in this proposed AD the FAA proposes spar cap life
limits to address this safety issue.

Objectives of, and Legal Basis for, the Proposed Rule

    The FAA is issuing this rulemaking under the authority set forth in
49 U.S.C. 44701(a)(5), which mandates the Administrator prescribe
regulations for practices, methods, and procedures necessary for safety
in air commerce. This regulation is within the scope of that authority
because it addresses an unsafe condition that is likely to exist or
develop on the airplanes identified in this AD.

Description of the Small Entities That the Proposed Rule Will Apply and
an Estimate of Their Number

    This proposed rule would potentially affect 808 U.S. registered and
operated Thrush Model 600S2D and S2R (S-2R) series airplanes.\1\ In
conducting this analysis, the FAA reviewed data from the FAA Registry
(Registry) to determine how many of the affected Thrush airplanes are
registered and operated by small entities. The Registry indicates that
these 808 airplanes are owned by 546 separate entities in agricultural
aviation. Although the Registry does not record financial or business
data about the registered owners of aircraft, and such data for these
entities are not readily available elsewhere, it appears that most, if
not all, of the 546 entities are engaged in crop dusting, spraying, and
seeding operations. These activities are classified in North American
Industry Classification System (NAICS) industry, NAICS 115112--Soil
Preparation, Planting, and Cultivating (including Crop Dusting, Crop
Spraying). The concentration of these entities in a single NAICS
industry reflects the specialized nature of agricultural airplanes with
restricted airworthiness certificates. Furthermore, several of these
entities were classified in the Standard Industrial Classification
(SIC) equivalent of NAICS 115112 by http://www.manta.com. Although a
few of these entities may also be engaged in firefighting, which is
classified in NAICS 115310--Support Activities for Forestry (including
Forest Fire Suppression), the FAA is unable to identify any of these
entities as being principally engaged in firefighting. The Small
Business Administration (SBA) small business classification for NAICS
115112 is $6.5 million in business receipts, and $16.5 million in
business receipts for NAICS 115310. Only one entity in this sample
appears to have business receipts over $6.5 million, and no entity has
business receipts in excess of $16.5 million. Using the total number of
airplanes owned as a size criterion, the FAA selected a sample of 41 of
the largest affected entities, and found median sales shown by http://www.manta.com to be just $250,000 annually. Firms in agricultural
aviation appear to be inherently of small size. Accordingly, the FAA
estimates that 545 small entities will be affected by this proposed
rule.
---------------------------------------------------------------------------

    \1\ FAA Registry, http://www.faa.gov/licenses_certificates/aircraft_certification/aircraft_registry/releasable_aircraft_download. Data downloaded on 4/14/08.
---------------------------------------------------------------------------

Description of the Projected Reporting, Recordkeeping, and Other
Compliance Requirements of the Proposed AD

    The proposed AD does not impose any additional reporting or
recordkeeping requirements beyond those required by the 2006 AD. The
proposed rule would retain the requirements of AD 2006-07-15 and impose
a life-limit on the wing front lower spar caps, which would require
operators of affected airplanes to replace the wing front lower spar
caps when the life-limit is reached.
    The estimated compliance cost varies widely by airplane submodel;
from a cost of zero for the more than 200 older airplanes that we
estimate will retire \2\ before the life-limit on their wing front
lower spar caps is reached, to a cost of $320,000 (5 replacements at
$64,000 per

replacement) for two airplanes. Individual airplane compliance costs
will likely result in costs to the small entities that own these
airplanes. The exact cost will vary, depending on the number of
affected Thrush airplanes owned by the entity and the specific
compliance cost for each airplane. The ownership table below shows the
variation in the number of owners with particular numbers of airplanes.
The table shows that almost 75% of the 546 individual owners have only
one affected airplane, and more than 90% of owners have no more than
two affected airplanes. The average (mean) number of affected airplanes
held is 1.48, while the median number held is just 1.00, so the median
airplane cost is equivalent to the median owner cost.
---------------------------------------------------------------------------

    \2\ As fully analyzed in the ``Cost of Compliance'' section of
this proposed rule, the FAA estimates that the airplanes affected by
this proposed rule retire at age 40.

                   Number of Thrush AD Owners Having Particular Numbers of Affected Airplanes
----------------------------------------------------------------------------------------------------------------
                                                 Number of affected
                                                  airplanes held by     Number of owners        Cumulative %
                                                    single owner
----------------------------------------------------------------------------------------------------------------
                                                                   1                   406                  74.4
                                                                   2                    86                  90.1
                                                                   3                    26                  94.9
                                                                   4                    13                  97.3
                                                                   5                     7                  98.5
                                                                   6                     2                  98.9
                                                                   7                     2                  99.3
                                                                   8                     1                  99.5
                                                                   9                     2                  99.8
                                                                  13                     1                 100.0
----------------------------------------------------------------------------------------------------------------
Total.........................................                   808                   546
Mean..........................................                  1.48
Median........................................                  1.00
----------------------------------------------------------------------------------------------------------------
Source: FAA Registry. Data downloaded on 4/18/08.

    In the ``Cost of Compliance'' section of this proposed AD, the FAA
estimates total cost (undiscounted) to be $37.1 million and the present
value cost to be $25.2 million. The FAA estimates that 545 of the 546
airplanes affected by this proposed AD are small firms, and, in fact,
98.8% of the proposed AD's estimated cost is attributed to small
entities. The following documents and analyzes the impact of this cost
on the substantial number of small firms identified in this proposed
AD.

Economic Impact on Small Entities

    Because the Registry does not collect financial or business data on
these entities, and such data is not readily available elsewhere, the
FAA also used Census Bureau size distribution data to assess the
economic impact on small firms. The FAA used data from the 2002 Census
since this is the latest Census for which size distribution by business
receipts is readily available. These data are available in a special
Census compilation for the SBA.\3\ The FAA used the data for NAICS
115112--Soil Preparation, Planting, and Cultivating (including Crop
Dusting, Crop Spraying), but did not use the data for NAICS 115310--
Support Activities for Forestry (including Forest Fire Suppression)
since, as noted above, a very high percentage of the affected small
firms, if not all, meet the classification standard of NAICS 115112.
Moreover, the size distribution of NAICS 115310 appears to be similar
to that of NAICS 115112. The concentration of the affected airplanes in
one NAICS industry, noted above, makes the use of Census data feasible
and appropriate.
---------------------------------------------------------------------------

    \3\ Small Business Administration, Office of Advocacy. http://www.sba.gov/advo/research/us_rec02.txt.
---------------------------------------------------------------------------

    The relevant Census data are provided in the table below:

       2002 Census Data for NAICS 115112--Soil Preparation, Planting, and Cultivating (Including Crop Dusting, Crop Spraying)--Small Size Classes
--------------------------------------------------------------------------------------------------------------------------------------------------------
              Measure                    Total        $0-$99,999    $100,000-$499,999  $500,000-$999,0000  $1,000,000-$4,999,999  $5,000,000-$10,000,000
--------------------------------------------------------------------------------------------------------------------------------------------------------
Firms.............................            2336             509               992                412                   394                     29
Percentage of firms...............  ..............           21.8%             42.5%              17.6%                 16.9%                   1.2%
Upper bound percentile............  ..............           21.8%             64.3%              81.9%                 98.8%                 100.0%
Est. Receipts ($000)..............      $1,531,004         $25,681          $257,447           $286,462              $772,401               $189,013
Receipts/Firm ($).................        $655,396         $50,454          $259,523           $695,296            $1,960,409             $6,517,690
--------------------------------------------------------------------------------------------------------------------------------------------------------
Source: ``Firms'' and ``Est. Receipts'' from Small Business Administration, Office of Advocacy. http://www.sba.gov/advo/research/us_rec02.txt.

    The table shows the number of firm and business receipt data for
the five smallest size classes of NAICS 115112 that encompass the size
range of the firms affected by this proposed AD. In the ``Percentage of
firms'' row, for each size class, the FAA calculates that class's
number of firms as a percentage of the total number of firms in the
five size classes. Cumulating this percentage from the smallest to
largest size class establishes the ``Upper bound percentile''--the
cumulated percentage of firms of business receipt size ranging up to
the upper bound of the size class.

The proposed AD's cost for the firms at the upper bound percentiles is
then estimated as the corresponding percentiles in the estimated firm-
level compliance cost data. In order to assess the economic impact of
the proposed AD, these costs are calculated as a percentage of the
Census data upper bounds. For example, the upper bound percentile for
the 100-500 thousand dollar size class is 64.3%, so the NAICS 115112
firms at that percentile are estimated to have $500,000 business
receipts of $500,000. As shown in the table below, the FAA then
determined the estimated compliance cost of firms at the same
percentile in the compliance cost data to be $61,754. The FAA assumes
these firms are the same so the percentage cost impact (Proposed AD
Cost/Firm Size) is 12.4%. This procedure assumes the size distribution
of the 808 firms affected by the proposed AD have a distribution
similar to the overall distribution of the small firms in NAICS 115112.
It also assumes there is a perfect rank correlation between the size of
the affected firms and the firms' compliance cost. While the latter
assumption is certainly not the case, any deviation from such perfect
correlation can only increase the impact of the proposed AD because
smaller firms will have larger costs. Accordingly, the FAA's
determination that the proposed AD will have a significant impact on a
substantial number of small entities is unaffected.

                                   Economic Impact of Thrush AD on Small Firms
----------------------------------------------------------------------------------------------------------------
                                                             Estimated firm
                                                              size (Census    Proposed AD Cost/    Cumulative
        Proposed AD cost to firm           Firm percentile   Bureau receipts      Firm Size      number of firms
                                                              upper bound)        (percent)
----------------------------------------------------------------------------------------------------------------
$0......................................            21.8th          $100,000               0.0             119.2
$61,754.................................            64.3rd           500,000              12.4             351.5
$91,335.................................            81.9th         1,000,000               9.1             447.9
$273,734................................            98.8th         5,000,000               5.5             540.2
----------------------------------------------------------------------------------------------------------------

    The above table shows a zero-cost impact on a firm at the 21.8th
percentile. This result reflects the estimate in the ``Cost of
Compliance'' section of this proposed AD that more than 200 older
airplanes will retire before their spar cap life-limits are reached. As
already mentioned, the proposed AD cost for a firm at the 64.3rd
percentile is $61,754, which as a percentage of estimated firm size
(size class upper bound) is 12.4% of annual business receipts. This
impact declines to 9.1% for a firm at the 81.9th percentile and to 5.5%
for a firm at the 98.8th percentile. As a result, the overall pattern
is zero impact for the smallest of the small firms, owners of the
oldest airplanes, but a highly positive impact for the medium-sized
small firms. In percentage terms, this impact falls for the largest
small firms, but remains at a substantial level. While the FAA can make
no definitive inference on the impact of the proposed AD on firms
between the 21.8th and 64.3rd percentiles, the FAA notes the cost
varies from 9.1% up to 12.4% of annual business receipts for 96 firms
between the 81.9th and 64.3rd percentiles and from 5.5% to 9.1% for 92
firms between the 98.8th percentile and the 81.9th percentile. These
estimated percentage impacts are substantial and therefore, the FAA
concludes that this proposed AD will have a significant impact on a
substantial number of small entities.

Duplicative, Overlapping or Conflicting Federal Rules

    There are no Federal rules that duplicate, overlap, or conflict
with this proposed AD.

Significant Alternatives to the Proposed AD

    The FAA considered relying on repetitive inspections as the sole
safety method, but given that the past required repetitive inspections
have not fully addressed this critical safety issue, the FAA has
determined that a part life limit is also necessary. A life limit on
the wing front lower spar caps is the only available sufficient action
presently known to the FAA. Consequently, there are no significant
viable alternatives to the proposed AD.

Request for Comments

    The FAA has determined that this proposed rulemaking will have a
significant economic impact on a substantial number of small entities.
The FAA requests comments with supporting justification regarding this
determination.

International Trade Impact Analysis

    The Trade Agreement Act of 1979 prohibits Federal agencies from
establishing any standards or engaging in related activities that
create unnecessary obstacles to the foreign commerce of the United
States. The statute does not consider legitimate domestic objectives,
such as safety, as unnecessary. The statute also requires consideration
of international standards and, where appropriate, that they be the
basis for U.S. standards. The FAA is issuing this proposed AD because
of a known safety problem and, therefore, the proposed AD is not
considered an unnecessary obstacle to international trade.

Unfunded Mandates Reform Act Assessment

    Title II of the Unfunded Mandates Reform Act of 1995 (Pub. L. 104-
4) requires each Federal agency to prepare a written statement
assessing the effects of any Federal mandate in a proposed or final
agency rule that may result in an expenditure of $100 million or more
(adjusted annually for inflation with the base year 1995) in any one
year by State, local, and tribal governments in the aggregate, or by
the private sector. The Act deems such a mandate to be a ``significant
regulatory action.'' The FAA currently uses an inflation-adjusted value
of $136.1 million.
    This proposed AD does not contain such a mandate.

Regulatory Findings

    We have determined that this proposed AD would not have federalism
implications under Executive Order 13132. This proposed AD would not
have a substantial direct effect on the States, on the relationship
between the national Government and the States, or on the distribution
of power and responsibilities among the various levels of government.
    For the reasons discussed above, I certify that the proposed
regulation:
    1. Is not a ``significant regulatory action'' under Executive Order
12866;
    2. Is not a ``significant rule'' under the DOT Regulatory Policies
and Procedures (44 FR 11034, February 26, 1979); and
    ------3. Could have a significant economic impact on a substantial
number of small entities under the criteria of the Regulatory
Flexibility Act.

    We prepared a regulatory evaluation of the estimated costs to
comply with this proposed AD and placed it in the AD docket.

Examining the AD Docket

    You may examine the AD docket that contains the proposed AD, the
regulatory evaluation, any comments received, and other information on
the Internet at http://www.regulations.gov; or in person at the Docket
Management Facility between 9 a.m. and 5 p.m., Monday through Friday,
except Federal holidays. The Docket Office (telephone<PHONE_NUMBER>)
is located at the street address stated in the ADDRESSES section.
Comments will be available in the AD docket shortly after receipt.

List of Subjects in 14 CFR Part 39

    Air transportation, Aircraft, Aviation safety, Incorporation by
reference, Safety.

The Proposed Amendment

    Accordingly, under the authority delegated to me by the
Administrator, the FAA proposes to amend 14 CFR part 39 as follows:

PART 39--AIRWORTHINESS DIRECTIVES

    1. The authority citation for part 39 continues to read as follows:

    Authority:  49 U.S.C. 106(g), 40113, 44701.

Sec.  39.13  [Amended]

    2. The FAA amends Sec.  39.13 by removing Airworthiness Directive
(AD) 2006-07-15, Amendment 39-14542 (71 FR 16691, April 4, 2006), and
adding the following new AD:

Thrush Aircraft, Inc. (Type Certificate previously held by Quality
Aerospace, Inc. and Ayres Corporation): Docket No. FAA-2007-27862;
Directorate Identifier 2007-CE-036-AD.

Comments Due Date

    (a) We must receive comments on this airworthiness directive
(AD) action by July 6, 2009.

Affected ADs

    (b) The following lists a history of the ADs affected by this AD
action:
    (1) This AD supersedes AD 2006-07-15, Amendment 39-14542;
    (2) AD 2006-07-15 superseded AD 2003-07-01, Amendment 39-13097;
    (3) AD 2003-07-01 superseded AD 2000-11-16, Amendment 39-11764;
    (4) AD 2000-11-16 superseded AD 97-17-03, Amendment 39-10195;
and
    (5) AD 97-17-03 superseded AD 97-13-11, Amendment 39-10071.

Applicability

    (c) This AD affects the following airplane models and serial
numbers (S/Ns) in Table 1 that are certificated in any category when
wing front lower spar cap part numbers (P/N) 20207-1, 20207-2,
20207-11, 20207-12, 20207-13, 20207-14, 20207-15, or 20207-16 are
installed. This AD applies to the S/Ns in Table 1 with or without a
``DC'' suffix. This AD does not affect airplanes with wing front
lower spar cap P/N 22507 (any dash number). The table also
identifies the group that each airplane belongs in when determining
inspection compliance times and life limit times for the parts:

               Table 1--Applicability and Airplane Groups
------------------------------------------------------------------------
              Model                     Serial Nos. (S/N)        Group
------------------------------------------------------------------------
(1) S-2R.........................  5000R through 5100R,                1
                                    except 5010R, 5031R,
                                    5038R, 5047R, and 5085R.
(2) S2R-G1.......................  G1-101 through G1-106.....          1
(3) S2R-R1820....................  R1820-001 through R1820-            1
                                    035.
(4) S2R-T15......................  T15-001 through T15-033             1
                                    (also see paragraph (d)
                                    of this AD).
(5) S2R-T34......................  6000R through 6049R, T34-           1
                                    001 through T34-143, T34-
                                    145, T34-171,
                                    T34[dash]180, and T34-181
                                    (also see paragraph (e)
                                    of this AD).
(6) S2R-G10......................  G10-101 through G10-138,            2
                                    G10-140, and G10-141.
(7) S2R-G5.......................  G5-101 through G5-105.....          2
(8) S2R-G6.......................  G6-101 through G6-147.....          2
(9) S2RHG-T65....................  T65-002 through T65-018...          2
(10) S2R-R1820...................  R1820-036.................          2
(11) S2R-T34.....................  T34-144, T34-146 through            2
                                    T34-170, T34-172 through
                                    T34-179, and T34-189
                                    through T34-234 (also see
                                    paragraph (e) of this AD).
(12) S2R-T45.....................  T45-001 through T45-014...          2
(13) S2R-T65.....................  T65-001 through T65-018...          2
(14) 600 S2D.....................  All serial numbers                  3
                                    beginning with 600-1311D.
(15) S-2R........................  1380R, 1416R through                3
                                    2592R, 3000R, and 3002R.
(16) S2R-R1340...................  R1340-001 through R1340-            3
                                    035.
(17) S2R-R3S.....................  R3S-001 through R3S-011...          3
(18) S2R-T11.....................  T11-001 through T11-005...          3
(19) S2R-G1......................  G1-107 through G1-115.....          5
(20) S2R-G10.....................  G10-139, G10-142 through            5
                                    G10-165.
(21) S2R-G6......................  G6-148 through G6-155.....          5
(22) S2RHG-T34...................  T34HG-102.................          5
(23) S2R-T15.....................  T15-034 through T15-040             5
                                    (also see paragraph (d)
                                    of this AD).
(24) S2R-T34.....................  T34-236 through T34-270             5
                                    (also see paragraph (e)
                                    of this AD).
(25) S2R-T45.....................  T45-015...................          5
(26) S-2R........................  5010R, 5031R, 5038R,                6
                                    5047R, and 5085R.
------------------------------------------------------------------------

    (d) The S/Ns of Model S2R-T15 airplanes could incorporate T15-
xxx and T27-xxx (xxx is the variable for any of the S/Ns beginning
with T15- and T27-). This AD applies to both of these S/N
designations as they are both Model S2R-T15 airplanes.
    (e) The S/Ns of Model S2R-T34 airplanes could incorporate T34-
xxx, T36-xxx, T41-xxx, or T42-xxx (xxx is the variable for any of
the S/Ns beginning with T34-, T36-, T41-, and T42-). This AD applies
to all of these S/N designations as they are all Model S2R-T34
airplanes.
    (f) Any Group 3 airplane that has been modified with a hopper of
a capacity more than 410 gallons, a piston engine greater than 600
horsepower, or a gas turbine engine greater than 600 horsepower, is
a Group 1 airplane for the purposes of this AD. Inspect the airplane
at the Group 1 compliance time specified in this AD. Replace the
wing front lower spar caps in accordance with the formulas given in
paragraph (j) of this AD.
    (g) Group 6 airplanes were originally manufactured with higher
horsepower radial engines, but were converted to lower horsepower
radial engines. They are now configured identically to Group 3
airplanes.

Unsafe Condition

    (h) This AD is the result of the analysis of data from 117 wing
front lower spar cap fatigue cracks found on similar design Model
600 S2D and S2R (S-2R) series airplanes and the FAA's determination
that the replacement of high time wing front lower spar caps is
necessary to address the unsafe condition for certain airplanes.
Since we issued AD 2006-07-15, analysis reveals that inspections are
not detecting all existing cracks, and incidences of undetected
cracks are increasing. This AD retains the actions of AD 2006-07-15
and imposes a life limit on the wing front lower spar caps that
requires you to replace the wing front lower spar caps when the life
limit is reached. This AD also changes the requirements and
applicability of the groups discussed above and removes the
ultrasonic inspection method. We are issuing this AD to prevent wing
front lower spar cap failure caused by undetected fatigue cracks.
Such failure could result in loss of a wing.

Compliance

    (i) To address the problem, do the following, unless already
done:
    (1) If you have already done an inspection required by AD 2006-
07-15, within the next 30 days after the effective date of this AD,
identify the number of hours time-in-service (TIS) since your last
inspection required by AD 2006-07-15. You will need this to
establish the inspection interval for the next inspection required
by this AD.
    (2) Inspect the two outboard bolt hole areas (whether 1/4-inch
and 5/16-inch diameter bolt holes or both 5/16-inch diameter bolt
holes) on each wing front lower spar cap for fatigue cracking using
magnetic particle or eddy current procedures. If Kaplan splice
blocks, P/N 22515-1/-3 or 88-251, are installed following Quality
Aerospace, Inc. Custom Kit No. CK-AG-30, dated December 6, 2001,
inspect the three outboard bolt hole areas on each wing front lower
spar cap for fatigue cracking using magnetic particle or eddy
current procedures. Use the compliance times listed in paragraph
(i)(3) of this AD for the initial inspection and the compliance time
listed in paragraphs (i)(5), (i)(6), or (i)(7) of this AD for the
repetitive inspections. The cracks may emanate from the bolt hole on
the face of the wing front lower spar cap or they may occur in the
shaft of the hole. Inspect both of those areas.
    (i) If using the magnetic particle method, inspect using the
``Inspection'' portion of the ``Accomplishment Instructions'' and
``Lower Splice Fitting Removal and Installation Instructions'' in
Ayres Corporation Service Bulletin No. SB-AG-39, dated September 17,
1996. Do the inspection following American Society for Testing and
Materials E 1444-01, using wet particles meeting the requirements of
the Society for Automotive Engineers AMS 3046. CAUTION: Firmly
support the wings during the inspection to prevent movement of the
wing front lower spar caps when the splice blocks are removed. This
will allow easier realignment of the splice block holes and the
holes in the wing front lower spar cap for bolt insertion and
prevent damage to the bolt hole. Damage to the bolt hole inner
surface or edge of the bolt hole can cause cracks to begin
prematurely.
    (ii) The inspection must be done by or supervised by a Level 2
or Level 3 inspector certified following the guidelines established
by the American Society for Nondestructive Testing or MIL-STD-410.
    (iii) If using eddy current methods, a procedure must be sent to
the FAA, Atlanta Aircraft Certification Office (ACO), for approval
before doing the inspection. Send your proposed procedure to the
FAA, Atlanta ACO, ATTN: Cindy Lorenzen, One Crown Center, 1895
Phoenix Boulevard, Suite 450, Atlanta, Georgia 30349. You are not
required to remove the splice block for the eddy current
inspections, unless corrosion is visible. Eddy current inspection
procedures previously approved under AD 2006-07-15, AD 2003-07-01,
AD 2000-11-16, AD 97-13-11, and/or AD 97-17-03 remain valid for this
AD.
    (iv) If you change the inspection method used (magnetic particle
or eddy current), the TIS intervals for repetitive inspections are
based on the method used for the last inspection.
    (3) If airplanes have not yet reached the threshold for the
initial inspection required in AD 2006-07-15, initially inspect
following the wing front lower spar cap hours total TIS schedule
below or within the next 50 hours TIS after the effective date of
this AD, whichever occurs later:

                                        Table 2--Initial Inspection Times
----------------------------------------------------------------------------------------------------------------
                                              Initially inspect upon accumulating the following hours total TIS
               Airplane group                                  on the wing front lower spar cap
----------------------------------------------------------------------------------------------------------------
(i) Group 1................................  2,000 hours TIS.
(ii) Group 2...............................  1,400 hours TIS.
(iii) Group 3..............................  6,400 hours TIS.
(iv) Group 5...............................  1,000 hours TIS.
(v) Group 6................................  (A) S/N 5010R: 5,530 hours TIS.
                                             (B) S/N 5038R: 5,900 hours TIS.
                                             (C) S/N 5031R: 6,400 hours TIS.
                                             (D) S/N 5047R: 6,400 hours TIS.
                                             (E) S/N 5085R: 6,290 hours TIS.
(vi) Any airplane with the entire Custom     2,000 hours TIS.
 Kit CK-AG-41 installed.
----------------------------------------------------------------------------------------------------------------

    (4) Airplanes in all groups must meet the following conditions
before doing the repetitive inspections required in paragraphs
(i)(5), (i)(6), or (i)(7) of this AD:
    (i) No cracks have been found previously on wing front lower
spar cap; or
    (ii) Small cracks have been repaired through cold work (or done
as an option if never cracked) following Ayres Corporation Service
Bulletin No. SB-AG-39, dated September 17, 1996; or
    (iii) Small cracks have been repaired by reaming the 1/4-inch
bolt hole to 5/16 inches diameter (or done as an option if never
cracked) following Ayres Corporation Custom Kit No. CK-AG-29, Part
I, dated December 23, 1997; or
    (iv) Small cracks have been repaired through previous
alternative methods of compliance (AMOC); or
    (v) Small cracks have been repaired by installing Kaplan splice
blocks, P/N 22515-1/-3 or 88-251 (or done as an option if never
cracked) following Quality Aerospace, Inc. Custom Kit No. CK-AG-30,
dated December 6, 2001.
    (5) Repetitively inspect Groups 1, 2, 3, and 6 airplanes that do
not have ``big butterfly'' plates and lower splice plates, P/Ns
20211-09 and P/N 20211-11, installed following Ayres Corporation
Custom Kit No. CK-AG-29, Part II, dated December 23, 1997; or that
do not have ``big butterfly'' plates and lower splice plates, P/Ns
94418-5 and 94418-7 or P/Ns 94418-13 and 94418-15, installed
following Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A,
dated March 8, 2007; and meet the conditions in paragraph (i)(4) of
this AD. Follow the wing front lower spar cap hours TIS compliance
schedule below:

  Table 3--Repetitive Inspection Times for Airplane Groups 1, 2, 3, and 6 Without ``Big Butterfly'' Plates and
                                               Lower Splice Plates
----------------------------------------------------------------------------------------------------------------
 When airplanes accumulate the following
  hours TIS on the wing front lower spar   Inspect within the following   Inspect thereafter at intervals not to
cap since the last inspection required in       hours TIS after the                    exceed. . .
              AD 2006-07-15,                effective date of this AD,
----------------------------------------------------------------------------------------------------------------
(i) Magnetic Particle inspection:........  ............................  250 hours TIS.
    (A) 350 or more hours TIS............   (A) 50 hours TIS...........
    (B) 175 through 349 hours TIS........  (B) 75 hours TIS............
    (C) Less than 175 hours TIS..........  (C) upon accumulating 250
                                            hours TIS.
(ii) Eddy Current inspection:............  ............................  350 hours TIS.
    (A) 500 or more hours TIS............  (A) 50 hours TIS............
    (B) 275 through 499 hours TIS........  (B) 75 hours TIS............
    (C) Less than 275 hours TIS..........  (C) upon accumulating 350
                                            hours TIS.
----------------------------------------------------------------------------------------------------------------

    (6) Repetitively inspect Groups 1, 2, 3, 5, and 6 airplanes that
have ``big butterfly'' plates and lower splice plates, P/Ns 20211-09
and 20211-11, installed following Ayres Corporation Custom Kit No.
CK-AG-29, Part II, dated December 23, 1997; or that have ``big
butterfly'' plates and lower splice plates, P/Ns 94418-5 and 94418-
7, or 94418-13 and 94418-15, installed following Thrush Aircraft,
Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8, 2007; and
meet the conditions in paragraph (i)(4) of this AD. Follow the wing
front lower spar cap hours TIS compliance schedule below:

  Table 4--Repetitive Inspections Times for Airplane Groups 1, 2, 3, 5, and 6 With ``Big Butterfly'' Plates and
                                               Lower Splice Plates
----------------------------------------------------------------------------------------------------------------
 When airplanes accumulate the following
  hours TIS on the wing front lower spar   Inspect within the following   Inspect thereafter at intervals not to
cap since the last inspection required in       hours TIS after the                    exceed. . .
              AD 2006-07-15,                effective date of this AD,
----------------------------------------------------------------------------------------------------------------
(i) Magnetic particle inspection:........  ............................  450 hours TIS.
    (A) 650 or more hours TIS............  (A) 50 hours TIS............
    (B) 375 through 649 hours TIS........  (B) 75 hours TIS............
    (C) Less than 375 hours TIS..........  (C) upon accumulating 450
                                            hours TIS.
(ii) Eddy Current inspection:............  ............................  625 hours TIS.
    (A) 900 or more hours TIS............  (A) 50 hours TIS............
    (B) 550 through 899 hours TIS........  (B) 75 hours TIS............
    (C) Less than 550 hours TIS..........  (C) upon accumulating 625
                                            hours TIS.
----------------------------------------------------------------------------------------------------------------

    Note 1:  Group 5 airplanes had P/Ns 20211-09 and 20211-11
installed at the factory.

    (7) Repetitively inspect airplanes that incorporate Thrush
Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated March 8,
2007, in its entirety that meet the conditions in paragraph (i)(4)
of this AD. Follow the wing front lower spar cap hours TIS
compliance schedule below:

Table 5--Repetitive Inspection Times for Airplanes With Thrush Aircraft,
 Inc. Custom Kit No. CK-AG-41, Revision A, Incorporated in its Entirety
------------------------------------------------------------------------
     When using the following      Repetitively inspect at intervals not
       inspection methods,                     to exceed. . .
------------------------------------------------------------------------
(i) Magnetic particle inspection.  900 hours TIS
(ii) Eddy current inspection.....  1,250 hours TIS.
------------------------------------------------------------------------

     (8) Initially replace the wing front lower spar caps, P/Ns
20207-1, 20207-2, 20207-11, 20207-12, 20207-13, 20207-14, 20207-15,
or 20207-16, at the times specified in Table 6 of this AD.
Repetitively replace thereafter at the life limit times specified in
Table 7 of this AD.

     Table 6--Initial Compliance Time for Wing Front Lower Spar Cap
                               Replacement
------------------------------------------------------------------------
                                   Replace the wing front lower spar cap
Total hours TIS on the wing front  upon accumulating the following hours
          lower spar cap               TIS on the spar cap after the
                                         effective date of this AD.
------------------------------------------------------------------------
(i) Group 1 with a radial engine   500 hours.
 and more than 15,000 hours TIS.
(ii) Group 1 with a radial engine  1,000 hours.
 and 12,000 to 15,000 hours TIS.
(iii) Group 1 with a radial        1,500 hours.
 engine and 9,000 to 11,999 hours
 TIS.
(iv) Group 1 with a radial engine  2,000 hours.
 and 7,400 to 8,999 hours TIS.
(v) Group 1 with a radial engine   Use Table 7(xxii).
 and less than 7,400 hours TIS.
(vi) Group 1 with a turbine        500 hours.
 engine and more than 14,000
 hours TIS.

(vii) Group 1 with a turbine       1,000 hours.
 engine and 11,000 to 14,000
 hours TIS.
(viii) Group 1 with a turbine      1,500 hours.
 engine and 8,000 to 10,999 hours
 TIS.
(ix) Group 1 with a turbine        2,000 hours.
 engine and 4,200 to 7,999 hours
 TIS.
(x) Group 1 with a turbine engine  Use Table 7(xxiii).
 and less than 4,200 hours TIS.
(xi) Group 2 with more than 9,000  500 hours.
 hours TIS.
(xii) Group 2 with 6,000 to 9,000  1,000 hours.
 hours TIS.
(xiii) Group 2 with 3,900 hours    1,500 hours.
 to 5,999 hours TIS.
(xiv) Group 2 with less than       Use Table 7(xxiv).
 3,900 hours TIS.
(xv) Group 3 and 6 with more than  500 hours.
 28,800 hours TIS.
(xvi) Group 3 and 6 with 27,800    1,000 hours.
 to 28,799 hours TIS.
(xvii) Group 3 and 6 with less     Use Table 7(xxv).
 than 27,800 hours TIS.
(xviii) Group 5 with more than     500 hours.
 8,000 hours TIS.
(xix) Group 5 with 5,000 to 7,999  1,000 hours.
 hours TIS.
(xx) Group 5 with 2,400 to 4,999   1,500 hours.
 hours TIS.
(xxi) Group 5 with less than       Use Table 7(xxvi).
 2,400 hours TIS.
------------------------------------------------------------------------

             Table 7--Wing Front Lower Spar Cap Life Limits
------------------------------------------------------------------------
                                          Replace wing front lower spar
                                           cap upon the accumulation of
             Airplane Group               the following hours TIS on the
                                                    spar cap:
------------------------------------------------------------------------
(xxii) Group 1 with a radial engine....  9,400 hours TIS.
(xxiii) Group 1 with a turbine engine..  6,200 hours TIS.
(xxiv) Group 2.........................  5,400 hours TIS.
(xxv) Groups 3 and 6...................  28,800 hours TIS.
(xxvi) Group 5.........................  3,900 hours TIS with original
                                          wing front lower spar cap P/N
                                          20207-11 or 20207-12.
                                         5,400 hours TIS after original
                                          wing front lower spar cap has
                                          been replaced with any P/N
                                          20207-xx wing front lower spar
                                          cap.
------------------------------------------------------------------------

    Note 2: There is evidence of sharp, uneven edges on the spar cap
bolt holes that resulted from the manufacturing process in Group 5
airplanes. Once the original spar caps are replaced, the life limit
increases.

    (j) As previously stated in paragraph (f) of this AD, any Group
3 airplane that has been modified with a hopper of a capacity more
than 410 gallons, a piston engine greater than 600 horsepower, or a
gas turbine engine greater than 600 horsepower, is a Group 1
airplane for the purposes of this AD. Replace the spar caps using
the following formulas.
    (1) For airplanes that were originally Group 3 airplanes and
later modified by installing a piston engine of greater than 600
horsepower and/or a hopper capacity of greater than 410 gallons,
calculate the equivalent Group 1 hours TIS on each spar cap as
follows:
[GRAPHIC] [TIFF OMITTED] TP04MY09.003

    (2) For airplanes that were originally Group 3 airplanes and
later modified by installing a turbine engine of greater than 600
horsepower, with or without installing a hopper with greater than
410 gallon capacity, calculate the equivalent Group 1 hours TIS on
each spar cap as follows:
[GRAPHIC] [TIFF OMITTED] TP04MY09.004

    (ii) Equivalent Group 1 hours TIS = 6,200 x Usage Factor
    (3) When the equivalent Group 1 hours TIS on the wing front
lower spar cap equals the life limit of 9,400 hours TIS if a radial
piston engine is installed or reaches 6,200 hours TIS if a turbine
engine is installed, the wing front lower spar cap must be replaced.
Use Table 6 if over the life limit.
    (4) See the appendix to this AD for examples of how to calculate
the applicable life limit.
    (k) If any cracks are found during any inspection required by
this AD, you must

repair the cracks or replace the wing front lower spar cap before
further flight.
    (1) Use the cold work process to ream out small cracks as
defined in Ayres Corporation Service Bulletin No. SB-AG-39, dated
September 17, 1996, and deburr the bolt hole edges with the splice
blocks removed after cold work is performed; or
    (2) If the crack is found in a 1/4-inch bolt hole, ream the 1/4-
inch bolt hole to 5/16 inches diameter as defined in Part I of Ayres
Corporation Custom Kit No. CK-AG-29, dated December 23, 1997; or
    (3) Install Kaplan splice blocks, P/N 22515-1/-3 or 88-251,
following Quality Aerospace, Inc. Custom Kit No. CK-AG-30, dated
December 6, 2001; or
    (4) Replace the affected wing front lower spar cap following an
FAA-approved procedure (the applicable maintenance manual contains
these procedures) or replace both lower spar caps and the
surrounding structure following Thrush Aircraft, Inc. Custom Kit No.
CK-AG-41, Revision A, dated March 8, 2007. Although not mandatory,
the FAA recommends installing Custom Kit No. CK-AG-41, Revision A,
in its entirety. The additional structure provided in the custom kit
will provide a greater level of safety than the minimum acceptable
level of safety provided by replacing just the lower spar cap.
    (l) If a crack is found, the reaming associated with the cold
work process may remove a crack if it is small enough. Some aircraft
owners/operators were issued AMOCs with AD 97-17-03 to ream the \1/
4\-inch bolt hole to \5/16\ inches diameter to remove small cracks.
Ayres Corporation Custom Kit No. CK-AG-29, Part I, dated December
23, 1997, also provides procedures to ream the \1/4\-inch bolt hole
to \5/16\ inches diameter, which may remove a small crack. Resizing
the holes to the required size to install a Kaplan splice block may
also remove small cracks. If you use any of these methods to remove
cracks and the airplane is re-inspected before further flight and no
cracks are found, you may continue to follow the repetitive
inspection intervals for your airplane listed in paragraphs (i)(5),
(i)(6), or (i)(7) of this AD.
    (m) For all inspection methods (magnetic particle or eddy
current), hours TIS for initial and repetitive inspections intervals
and wing front lower spar cap life limit start over when the wing
front lower spar cap is replaced with a new P/N 20207-1, 20207-2,
20207-11, 20207-12, 20207-13, 20207-14, 20207-15, or 20207-16. These
wing front lower spar caps must be inspected as specified in
paragraphs (i)(3), (i)(5), (i)(6), and (i)(7) of this AD.
    (1) If the wings or wing front lower spar caps were replaced
with new or used wings or wing front lower spar caps during the life
of the airplane and the logbook records positively show the hours
TIS of the replacement wings or wing front lower spar caps, then
initially inspect at applicable times specified in paragraph (i)(3)
of this AD. Repetitively inspect thereafter at intervals specified
in paragraphs (i)(5), (i)(6), or (i)(7) of this AD. Replace the wing
front lower spar caps upon reaching the life limit specified in
Table 7 of this AD.
    (2) If the wings or wing front lower spar caps were replaced
with new or used wings or wing front lower spar caps during the life
of the airplane and logbook records do not positively show the hours
TIS of the replacement wings or wing front lower spar caps, then
inspect within 50 hours TIS after the effective date of this AD,
unless already done. Repetitively inspect thereafter at intervals
specified in paragraphs (i)(5), (i)(6), or (i)(7) of this AD.
Replace the wing front lower spar caps within 500 hours TIS after
the effective date of this AD.
    (3) If both wing front lower spar caps are replaced by
installing the entire Thrush Aircraft, Inc. Custom Kit No. CK-AG-41,
Revision A, dated March 8, 2007, then initially inspect at 2,000
hours TIS as shown in paragraph (i)(3) of this AD. Repetitively
inspect thereafter at intervals specified in paragraph (i)(7) of
this AD. Replace the wing front lower spar caps at times specified
in paragraph (i)(8) of this AD.
    (n) Any wing front lower spar cap that is removed and is at or
beyond the replacement time specified in this AD must be disposed of
following the procedures in 14 CFR Part 43.10.
    (o) Replacement times start over when the wing front lower spar
cap is replaced with a new P/N 20207-1, 20207-2, 20207-11, 20207-12,
20207-13, 20207-14, 20207-15, or 20207-16. These wing front lower
spar caps are now life-limited parts and must be replaced upon the
accumulation of the hours TIS specified in Table 7 of this AD.
    (p) Report any cracks you find within 10 days after the cracks
are found or within 10 days after the effective date of this AD,
whichever occurs later. Send your report to Cindy Lorenzen,
Aerospace Engineer, ACE-115A, Atlanta ACO, One Crown Center, 1895
Phoenix Blvd., Suite 450, Atlanta, GA 30349; telephone: (770) 703-
6078; facsimile:<PHONE_NUMBER>; e-mail<EMAIL_ADDRESS>The
Office of Management and Budget (OMB) approved the information
collection requirements contained in this regulation under the
provisions of the Paperwork Reduction Act and assigned OMB Control
Number 2120-0056. Include in your report the following information:
    (1) Aircraft model and serial number;
    (2) Engine model;
    (3) Aircraft hours TIS;
    (4) Left and right wing front lower spar cap hours TIS;
    (5) Hours TIS on the spar cap since last inspection;
    (6) Crack location and size;
    (7) Procedure (magnetic particle, ultrasonic, or eddy current)
used for the last inspection;
    (8) Description of any previous modifications and hours TIS when
the modification was done, such as engine model change, installation
of winglets, hopper capacity increase, cold working procedure done
on bolt holes, or installation of butterfly plates; and
    (9) Information on corrective action taken or installation of
Thrush Aircraft, Inc. Custom Kit No. CK-AG-41, Revision A, dated
March 8, 2007, and when this corrective action was taken.

Special Flight Permits

    (q) Under 14 CFR part 39.23, we are limiting the special flight
permits for this AD by the following conditions:
    (1) The hopper is empty;
    (2) Vne is reduced to 126 miles per hour (109 knots) indicated
airspeed (IAS); and
    (3) Flight into known turbulence is prohibited.

Alternative Methods of Compliance (AMOCs)

    (r) The Manager, Atlanta Aircraft Certification Office, FAA,
ATTN: Cindy Lorenzen, Aerospace Engineer, ACE-115A, Atlanta Aircraft
Certification Office, One Crown Center, 1895 Phoenix Blvd., Suite
450, Atlanta, GA 30349; telephone:<PHONE_NUMBER>; facsimile: (770)
703-6097; e-mail<EMAIL_ADDRESS>or Keith Noles, Aerospace
Engineer, ACE-117A, Atlanta Aircraft Certification Office, One Crown
Center, 1895 Phoenix Blvd., Suite 450, Atlanta, Georgia 30349;
telephone:<PHONE_NUMBER>; facsimile:<PHONE_NUMBER>; e-mail:
<EMAIL_ADDRESS>has the authority to approve AMOCs for this
AD, if requested using the procedures found in 14 CFR 39.19. Before
using any approved AMOC on any airplane to which the AMOC applies,
notify your appropriate principal inspector (PI) in the FAA Flight
Standards District Office (FSDO), or lacking a PI, your local FSDO.
    (s) AMOCs approved for AD 2006-07-15, AD 2003-07-01, AD 2000-11-
16, AD 97-13-11, and/or AD 97-17-03 are approved as AMOCs for this
AD except for those pertaining to ultrasonic inspection methods.

Related Information

    (t) To get copies of the service information referenced in this
AD, contact Thrush Aircraft, Inc. at 300 Old Pretoria Road, P.O. Box
3149, Albany, Georgia 31706-3149 or go to http://
www.thrushaircraft.comm. To view the AD docket, go to the U.S.
Department of Transportation, Docket Operations, M-30 West Building
Ground Floor, Room W12-140, New Jersey Avenue, SE., Washington, DC,
or on the Internet at http://www.regulations.gov. The docket number
is Docket No. FAA-2007-27862; Directorate Identifier 2007-CE-036-AD.

Appendix to Docket No. FAA-2007-27862

    The following are examples of calculating Equivalent Group 1
hours.
    Example 1: S/N xxx was originally a Group 3 airplane; later it
was modified with a Wright R-1820-71, 1200 horsepower, radial engine
when the wing front lower spar caps had 15,700 hours TIS on them.
The wing front lower spar caps have accumulated an additional 8,200
hours since the engine conversion for a total of 23,900 hours TIS on
the wing front lower spar caps.

Usage Factor = 15,700 hours/28,800 + 8,200 hours/9,400= 1.417
Equivalent Group 1 hours = 9,400 x 1.417 = 13,320 hours.

    The spar caps will need to be replaced within the next 1,000
hours TIS after the effective date of this AD as determined by Table
6 for a Group 1 airplane with a radial engine with between 12,000
and 15,000 hours TIS.

    ------Example 2: S/N yyy was originally a Group 3 airplane;
later it was modified with a PT6A-34, 750 horsepower, turbine engine
when the wing front lower spar caps had 5,300 hours TIS on them. The
wing front lower spar caps now have 7,700 hours TIS.

Usage Factor = 5,300 hours/28,800 + (7,700 - 5,300)/6,200 = 0.571
Equivalent Group 1 hours = 6,200 x 0.571 = 3,540 hours.

    The spar caps will need to be replaced at 6,200 Equivalent Group
1 total hours TIS, which is within the next 2,660 hours TIS (6,200 -
3,540=2,660).

    Issued in Kansas City, Missouri, on April 27, 2009.
Kim Smith,
Manager, Small Airplane Directorate, Aircraft Certification Service.
[FR Doc. E9-10162 Filed 5-1-09; 8:45 am]
BILLING CODE 4910-13-P