All Those Ms

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 decisionswithout much forethought. One of them is the altitude to fileand fly. Unless there’s ice around or known rocks in the cloudsor 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’reabove them, right? And besides, you’d never get a clearance thatcould be below those MEAs, right? Yeah, well as someone famousonce said "Trust, but verify." Sometimes what appearsto be a simple truth is not what it seems, especially when itcomes to altitudes.

Let’s dig around MEAs and their cousins on the en route chartand review some things that you may not have thought about ina while.

Building an Airway

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

The required obstruction clearance is basically the same as describedin FAR 91.177, requiring 1000 feet of obstacle clearance in non-mountainousterrain and 2000 feet in designated mountainous areas. The reasonfor the extra clearance over mountains is not to give you a nicecushion over rough (read remote) areas in case of an emergency,but mainly because of the Bernoulli effect as air flows over themountains. The huge venturi created as the upper winds are forcedover the terrain can cause large variations in the pressure gradientat 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 theEastern U.S., Puerto Rico and Hawaii. In the Western U.S. andAlaska, the clearance may be reduced to 1700 feet. When designatingthese 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 conduciveto marked pressure differentials, such as canyons, which can actlike giant venturis.

Other considerations are thetype of navaids used and the distance between them and the availabilityof weather and altimeter information in the area. The width ofthe airway for both non-radar separation and obstruction-clearancepurposes is 4 nautical miles either side of the centerline outto 50 nautical miles from the VOR defining the route. If the airwaysegment extends more than 50 miles, the protected airspace fansout 4.5 degrees from the centerline.

If you could see the protectedarea of an airway and could slice out a section of it, you’d beleft with the trapezoidal shaped section shown. The airway hasa "primary" obstacle clearance area that extends 4 mileseither side of the centerline. A secondary, or buffer zone extendsan additional 2 miles either side of the airway and angles upto intersect the airway’s extreme outer edge, as shown schematicallyin the drawing. So, you can see that even if you’re a mile ortwo off the centerline, there’s plenty of obstacle clearance.

The width of the airway and/or secondary protected airspace isexpanded 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 highertrue speeds), and the distance from the appropriate navaids.TheTERPS nerds have formulas to calculate all of this; you don’tneed 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. Thataltitude is set by the highest or "controlling" obstacle(terrain or man-made) that comes within 1000 feet of the bottomof the airway’s primary clearance area. Fly at the MOCA and you’reguaranteed obstacle clearance but reception of navaids is onlypromised within 22 nautical miles of the stations.

The MEA, on the other hand, guarantees obstruction clearance andnavaid reception along its entire length, unless there’s an MEAgap, which will be indicated on the chart. That’s a rather tallerorder than just 1000 feet of clearance so most MEAs really providea lot more than the minimum required clearance. They’re higherprimarily to assure navigation reception of the VORs that anchorboth ends of the route.

The flight inspection folks will tell you that radio communicationis also checked and is supposed to be available along the entireroute but you probably know from experience that this isn’t alwaystrue. Centers and tracons have transceiver sites scattered allover the place and some work better than others.

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

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

Even when no gap exists, there are problems. The VOR system beingwhat it is_that is, crumbling a bit at the edges_flight inspectorssometimes have to be resourceful about making an airway work withoutresorting to a stratospheric MEA. The notorious Harrisburg VORtook 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 40miles for L-class and H-class below 14,500 feet and 25 miles forT-class. Very often, though, a T-class will be pressed into serviceto make an airway play beyond the 25-mile limit. A good exampleof 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 theMOCA (if one is shown) is given as a MSL value, followed by a"T." If either is directional, that is, applicable onlywhen flying the airway in a certain direction, Jepp charts havea little arrow to point the way. NOS depicts the MEA above theMOCA, usually (but not always) near the airway designator block.The MOCA also has an asterisk. Often (again, not always) whenan MEA changes due to higher terrain beyond, this will be depictedon the chart as an MCA or minimum crossing altitude. The MCA ismarked on NOS en route charts by a flag with an X inside. (Jeppesenuses a note under the fix name.) MCAs aren’t as common in theEast as out West, but you’ll still find them just about everywhereexcept in the plains states. With an MCA, you must cross the fixwhere the new MEA applies at or above the specified altitude.The altitude might not be the higher MEA, but it will get youto either the MOCA or a point where the standard rate of climbwill allow you to get to the MEA safely.

What is the standard rate of climb? Actually, it’s a climb gradientthat varies with altitude. Below 5000 feet, it’s 150 feet pernautical mile, it’s 120 feet per mile between 5000 and 10,000feet and above that, it’s 100 feet per mile. That last figureis 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-routealtitudes, published in some areas. NOS will be adding what itwill term an OROCA or off-route obstruction clearance altitudeto the en routes. Basically it should be just about the sameas a MORA, one-degree square blocks with a minimum IFR altitudedepicted.

When MEAs Apply

Now that you know the Zen of MEAs, when do the limitations theyimpose apply? That’s easy. Always, at least when you’re on anIFR flight plan. Except…when you’re unable to maintain the MEAfor whatever reason or when you’re in radar contact and beingvectored or when you’re navigating off airway and being monitoredon radar.

All this is explained in FAR 91.177 which says that if there’sboth an MEA and a MOCA on a published route segment, you haveto maintain at least the MEA unless you’re within 22 nauticalmiles of the navaid defining the route, in which case you canfly at the MOCA. If you can’t hold the MOCA, say because of icingor performance limitations, you can still fly lower but you’llhave to do it under your emergency authority and, of course, theguarantees for obstacle clearance no longer apply.

Wait A Minute…

But wait, you say, there are plenty of places that have a MOCAwell beyond the 22-mile limitation. What good is that? Emergencyuse? Sure, that’s a possibility. But remember what the real differencebetween an MEA and a MOCA is: navaid limitations. The ATC Handbook(7110.65) is helpful here. It says "…navaid use limitationsdo not apply when routing is pilot requested or controller initiatedand… radar monitoring and course correction as necessary isapplied." In other words, radar is a legal substitute fora navaid.

As far as the regs are concerned,"normal" en route ATC is non-radar. The reality is justthe reverse. As an example, see the chart insert showing V292west of Barnes VOR near Sky Park. The MEA for the entire segmentis 10,000 feet but the 5200-foot MOCA, 40 miles out, is no problemif you’re in radar contact. The airway is useable all the waydown to 5200 feet. There’s just no guarantee you’ll receive BarnesVOR.

The bad news is that the only way the controller is going to letyou 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 controllermust take the responsibility to monitor your progress and issuevectors to correct it as needed.

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

This cool radar stuff also eliminates the MAA, maximum authorizedaltitude, which you sometimes see on jet routes. Occasionallythese are established for protection of a military special useairspace but more commonly they exist because of frequency overlap.The FAA’s frequency management people try to allocate frequenciesfor navigation and communication so they don’t interfere withone another and limiting the altitude at which an airway can beused will do the trick. But, once again, radar eliminates thelimitation.

Radar wipes out yet another limitation: airway intersections affectedby navaid coverage. If an off-airway navaid that provides thecrossing radial for an intersection is unusable at the MEA, anMRA, minimum reception altitude is published. You say you’re talkingto Center and in radar contact? Forget the MRA.

I know this question will have occurred to you: "Hey, whatabout with GPS, there shouldn’t be any navaid limitations withthat?" You’re right. Other than the pretty predicable RAIMholes (which supposedly will be eliminated when Wide-Area AugmentationSystem is established) there are virtually no limitations. Sothose of you who save these articles can throw this one out onceeverybody is equipped with GPS and we move from ground-based tospace-based nav. If your nav gear can walk the walk now, there’sno regulatory reason why you can’t operate down at the MOCA allthe time. At least the regs don’t say you can’t.

Real Life

As far as your average controller goes, we’ve already hit somepretty arcane stuff. V292 is heavily used between 6000 feet and10,000 feet but you’ll probably find most of the controllers assignthese altitudes due to habit; they don’t really think about theMEA versus the MOCA. That’s another reason you won’t hear controllersproposing "what ifs" for lost comm. It’s just not somethingcontrollers consider. (That doesn’t mean you shouldn’t be thinkingabout it, though. You’re in the airplane, the controller’s ina nice, safe ATC facility.)

All this is fine until you want to do something non-standard inairspace where the controller isn’t used to such twists. That’swhen the sweet talk and diplomacy comes in. If the controllerallows you to fly an airway below the MEA, navigational responsibilitynow rests on his or her shoulders. One reason airways exist ina radar world is so the controller knows with reasonable accuracywhere you’re going, and what other airways/routes you might conflictwith.

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

Of course, nothing says you have to be in radar contact and allcontrollers are trained in non-radar procedures (snicker, snicker).But unless you’re in an area where ATC is proficient at this (notoften the case), it tends to make a scope dope nervous to havea non-radar flib floating around in a radar sector. Many controllerswill handle non-radar by just asking for a bazillion positionreports and manually moving the track along till you fade backin.

Controller’s Job

What exactly is the controller’s responsibility regarding altitudeclearances and all this minimum maximum stuff? The 7110.65 statesvery plainly that an assigned altitude must be at or above theappropriate minimum IFR altitude. However, a controller is notresponsible for leading the pilot by the hand to that altitude.How you get there is up to you. This is especially critical whendeparting an uncontrolled airport.

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

There’s no published departure procedure for the airport so it’sentirely up to the pilot to get to 4000 feet before ATHOS. Thatmeans knowing enough about the local obstacle environment to deviseyour own departure procedure.

If you can make it work by going direct ATHOS, fine. If you can’tmake it, or the controller can’t open 4000 feet, a holding patternover the NDB (PFH) would be a good idea. If ATC doesn’t have 4000feet open, you’d get a "paper-stop" clearance to PFH.Why not hold at ATHOS? Remember you must cross ATHOS at 4000 feetwestbound. 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 tobe sure of radar monitoring, otherwise you’ll get a climb to theMEA. If the airway you’re on changes MEA (no MOCA) and you haven’tbeen assigned a higher altitude yet, pipe up! It won’t be thefirst time a controller has forgotten about an MEA. The controllermight get grumpy, but that’s tough. He forgot the MEA, not you.

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