All Those Ms

  • E-Mail this Article
  • View Printable Article
  • Text size:

    • A
    • A
    • A

The FAA's TERPS wizards chart MEAs, MOCAs, MCAs and MRAs to make sure we can fly through the clag without encountering embedded rocks and trees. Sometimes we have to fly at or above these minimum altitudes, but not always. Here's the skinny.

During a typical IFR flight, you probably make lots of decisions without much forethought. One of them is the altitude to file and fly. Unless there's ice around or known rocks in the clouds or an intense head or tailwind, what difference does it make?

All you have to do is look at the published MEAs and be sure you're above them, right? And besides, you'd never get a clearance that could be below those MEAs, right? Yeah, well as someone famous once said "Trust, but verify." Sometimes what appears to be a simple truth is not what it seems, especially when it comes to altitudes.

Let's dig around MEAs and their cousins on the en route chart and review some things that you may not have thought about in a while.

Building an Airway

MEAs and MOCAs are the meat and potatoes of the en route chart as far as obstacle clearance is concerned; knowing the underlying structure can serve you well. The airways are themselves regulations, FAR 95 and the 8260.3 (TERPS) manual govern the establishment and publication of the routes.

The required obstruction clearance is basically the same as described in FAR 91.177, requiring 1000 feet of obstacle clearance in non-mountainous terrain and 2000 feet in designated mountainous areas. The reason for the extra clearance over mountains is not to give you a nice cushion over rough (read remote) areas in case of an emergency, but mainly because of the Bernoulli effect as air flows over the mountains. The huge venturi created as the upper winds are forced over the terrain can cause large variations in the pressure gradient at given true altitudes, resulting in large altimeter errors.

In certain mountainous areas, that 2000-foot minimum may be reduced. A 1500-foot clearance is allowed in the mountainous areas of the Eastern U.S., Puerto Rico and Hawaii. In the Western U.S. and Alaska, the clearance may be reduced to 1700 feet. When designating these areas, the specialists must take a careful look at the geography. TERPS requires them to consider areas of precipitous terrain, weather phenomena peculiar to the area and any conditions conducive to marked pressure differentials, such as canyons, which can act like giant venturis.

Other considerations are the type of navaids used and the distance between them and the availability of weather and altimeter information in the area. The width of the airway for both non-radar separation and obstruction-clearance purposes is 4 nautical miles either side of the centerline out to 50 nautical miles from the VOR defining the route. If the airway segment extends more than 50 miles, the protected airspace fans out 4.5 degrees from the centerline.

If you could see the protected area of an airway and could slice out a section of it, you'd be left with the trapezoidal shaped section shown. The airway has a "primary" obstacle clearance area that extends 4 miles either side of the centerline. A secondary, or buffer zone extends an additional 2 miles either side of the airway and angles up to intersect the airway's extreme outer edge, as shown schematically in the drawing. So, you can see that even if you're a mile or two off the centerline, there's plenty of obstacle clearance.

The width of the airway and/or secondary protected airspace is expanded at points where the airway turns over a navaid or intersection. The amount of extra protection depends on how sharp the turn is, the MSL altitude of the area to be protected (due to the higher true speeds), and the distance from the appropriate navaids.The TERPS nerds have formulas to calculate all of this; you don't need to worry about it.

On to the MEA

So once all this is done, we have an MEA right? No, not really, we just have a MOCA or minimum obstacle clearance altitude. That altitude is set by the highest or "controlling" obstacle (terrain or man-made) that comes within 1000 feet of the bottom of the airway's primary clearance area. Fly at the MOCA and you're guaranteed obstacle clearance but reception of navaids is only promised within 22 nautical miles of the stations.

The MEA, on the other hand, guarantees obstruction clearance and navaid reception along its entire length, unless there's an MEA gap, which will be indicated on the chart. That's a rather taller order than just 1000 feet of clearance so most MEAs really provide a lot more than the minimum required clearance. They're higher primarily to assure navigation reception of the VORs that anchor both ends of the route.

The flight inspection folks will tell you that radio communication is also checked and is supposed to be available along the entire route but you probably know from experience that this isn't always true. Centers and tracons have transceiver sites scattered all over the place and some work better than others.

The Flight Check guys fly the airway and make sure that what looks good on paper actually "plays" along the entire proposed route. If it doesn't work, they'll jack up the MEA until it does. In certain mountainous areas, airways are established beyond usable navaid limits and a gap is shown on the chart where dead-reckoning or some other form of navigation is needed.

The gap can't be larger than a specified size, depending on the altitude of the MEAs leading to the gap. If it's larger, up goes the MEA until it shrinks or the airway is moved somewhere else.

Even when no gap exists, there are problems. The VOR system being what it is_that is, crumbling a bit at the edges_flight inspectors sometimes have to be resourceful about making an airway work without resorting to a stratospheric MEA. The notorious Harrisburg VOR took years of work to support enough airways to earn its keep. It's still a highly restricted navaid.

You probably know that VORs have standard service volumes of 40 miles for L-class and H-class below 14,500 feet and 25 miles for T-class. Very often, though, a T-class will be pressed into service to make an airway play beyond the 25-mile limit. A good example of a VOR with "expanded service volume" is Bradley (BDL) Connecticut.

It's All Charted

MEAs and MOCAs are posted on charts, of course. On Jeppesen products, the MEA is given under the airway designator blocks, while the MOCA (if one is shown) is given as a MSL value, followed by a "T." If either is directional, that is, applicable only when flying the airway in a certain direction, Jepp charts have a little arrow to point the way. NOS depicts the MEA above the MOCA, usually (but not always) near the airway designator block. The MOCA also has an asterisk. Often (again, not always) when an MEA changes due to higher terrain beyond, this will be depicted on the chart as an MCA or minimum crossing altitude. The MCA is marked on NOS en route charts by a flag with an X inside. (Jeppesen uses a note under the fix name.) MCAs aren't as common in the East as out West, but you'll still find them just about everywhere except in the plains states. With an MCA, you must cross the fix where the new MEA applies at or above the specified altitude. The altitude might not be the higher MEA, but it will get you to either the MOCA or a point where the standard rate of climb will allow you to get to the MEA safely.

What is the standard rate of climb? Actually, it's a climb gradient that varies with altitude. Below 5000 feet, it's 150 feet per nautical mile, it's 120 feet per mile between 5000 and 10,000 feet and above that, it's 100 feet per mile. That last figure is a very low rate; about 150 feet per minute at 100 knots.

Yet another symbol is about to appear on NOS en route charts. Jeppesen users may be familiar with the MORA, minimum off-route altitudes, published in some areas. NOS will be adding what it will term an OROCA or off-route obstruction clearance altitude to the en routes. Basically it should be just about the same as a MORA, one-degree square blocks with a minimum IFR altitude depicted.

When MEAs Apply

Now that you know the Zen of MEAs, when do the limitations they impose apply? That's easy. Always, at least when you're on an IFR flight plan. Except...when you're unable to maintain the MEA for whatever reason or when you're in radar contact and being vectored or when you're navigating off airway and being monitored on radar.

All this is explained in FAR 91.177 which says that if there's both an MEA and a MOCA on a published route segment, you have to maintain at least the MEA unless you're within 22 nautical miles of the navaid defining the route, in which case you can fly at the MOCA. If you can't hold the MOCA, say because of icing or performance limitations, you can still fly lower but you'll have to do it under your emergency authority and, of course, the guarantees for obstacle clearance no longer apply.

Wait A Minute...

But wait, you say, there are plenty of places that have a MOCA well beyond the 22-mile limitation. What good is that? Emergency use? Sure, that's a possibility. But remember what the real difference between an MEA and a MOCA is: navaid limitations. The ATC Handbook (7110.65) is helpful here. It says "...navaid use limitations do not apply when routing is pilot requested or controller initiated and... radar monitoring and course correction as necessary is applied." In other words, radar is a legal substitute for a navaid.

As far as the regs are concerned, "normal" en route ATC is non-radar. The reality is just the reverse. As an example, see the chart insert showing V292 west of Barnes VOR near Sky Park. The MEA for the entire segment is 10,000 feet but the 5200-foot MOCA, 40 miles out, is no problem if you're in radar contact. The airway is useable all the way down to 5200 feet. There's just no guarantee you'll receive Barnes VOR.

The bad news is that the only way the controller is going to let you down to 6000 feet out there is if he can continue radar coverage. If you drift off the airway, it won't be your fault. The controller must take the responsibility to monitor your progress and issue vectors to correct it as needed.

What if you lose communications and can't get vectors? Easy. First the controller is supposed to issue alternate instructions. If that doesn't happen, the lost comm provisions in FAR 91.185 take effect: Fly the higher of the altitude assigned, told to expect, or the MEA for the route segment. So just climb to 10,000 feet and navigation reception is assured.

This cool radar stuff also eliminates the MAA, maximum authorized altitude, which you sometimes see on jet routes. Occasionally these are established for protection of a military special use airspace but more commonly they exist because of frequency overlap. The FAA's frequency management people try to allocate frequencies for navigation and communication so they don't interfere with one another and limiting the altitude at which an airway can be used will do the trick. But, once again, radar eliminates the limitation.

Radar wipes out yet another limitation: airway intersections affected by navaid coverage. If an off-airway navaid that provides the crossing radial for an intersection is unusable at the MEA, an MRA, minimum reception altitude is published. You say you're talking to Center and in radar contact? Forget the MRA.

I know this question will have occurred to you: "Hey, what about with GPS, there shouldn't be any navaid limitations with that?" You're right. Other than the pretty predicable RAIM holes (which supposedly will be eliminated when Wide-Area Augmentation System is established) there are virtually no limitations. So those of you who save these articles can throw this one out once everybody is equipped with GPS and we move from ground-based to space-based nav. If your nav gear can walk the walk now, there's no regulatory reason why you can't operate down at the MOCA all the time. At least the regs don't say you can't.

Real Life

As far as your average controller goes, we've already hit some pretty arcane stuff. V292 is heavily used between 6000 feet and 10,000 feet but you'll probably find most of the controllers assign these altitudes due to habit; they don't really think about the MEA versus the MOCA. That's another reason you won't hear controllers proposing "what ifs" for lost comm. It's just not something controllers consider. (That doesn't mean you shouldn't be thinking about it, though. You're in the airplane, the controller's in a nice, safe ATC facility.)

All this is fine until you want to do something non-standard in airspace where the controller isn't used to such twists. That's when the sweet talk and diplomacy comes in. If the controller allows you to fly an airway below the MEA, navigational responsibility now rests on his or her shoulders. One reason airways exist in a radar world is so the controller knows with reasonable accuracy where you're going, and what other airways/routes you might conflict with.

If you drift off into other traffic, other airspace or worse, terrain, it's not your fault, it's ATC's. Taking the navigational responsibility increases workload and ATC may not always want to play. Sometimes the opposite will occur, where you may want a lower altitude at or above the MEA and the controller will mumble something incomprehensible about a minimum altitude. In this case, the problem is usually airspace configuration, meaning he or she would have to coordinate with someone else to use that altitude or perhaps allowing you lower will drop you out of radar contact.

Of course, nothing says you have to be in radar contact and all controllers are trained in non-radar procedures (snicker, snicker). But unless you're in an area where ATC is proficient at this (not often the case), it tends to make a scope dope nervous to have a non-radar flib floating around in a radar sector. Many controllers will handle non-radar by just asking for a bazillion position reports and manually moving the track along till you fade back in.

Controller's Job

What exactly is the controller's responsibility regarding altitude clearances and all this minimum maximum stuff? The 7110.65 states very plainly that an assigned altitude must be at or above the appropriate minimum IFR altitude. However, a controller is not responsible for leading the pilot by the hand to that altitude. How you get there is up to you. This is especially critical when departing an uncontrolled airport.

Look again at the chart insert. Near Columbia County airport is ATHOS intersection with an MCA of 4000 feet, heading west along V270. The controller must assign at least 4000 feet to anyone cleared out of Columbia along that airway.

There's no published departure procedure for the airport so it's entirely up to the pilot to get to 4000 feet before ATHOS. That means knowing enough about the local obstacle environment to devise your own departure procedure.

If you can make it work by going direct ATHOS, fine. If you can't make it, or the controller can't open 4000 feet, a holding pattern over the NDB (PFH) would be a good idea. If ATC doesn't have 4000 feet open, you'd get a "paper-stop" clearance to PFH. Why not hold at ATHOS? Remember you must cross ATHOS at 4000 feet westbound. Your first crossing in a hold would be below the MCA, and you might not have enough terrain clearance.

In the case of a clearance below the MEA, the controller has to be sure of radar monitoring, otherwise you'll get a climb to the MEA. If the airway you're on changes MEA (no MOCA) and you haven't been assigned a higher altitude yet, pipe up! It won't be the first time a controller has forgotten about an MEA. The controller might get grumpy, but that's tough. He forgot the MEA, not you.

Just about all Centers and tracons are equipped with some version of the E-MSAW (En route Minimum Safe Altitude Warning) as a backup device, but knowing what's really beneath you should keep you from ever having to hear that alert.