Terrain does not move, and the altitudes that keep you above it are some of the most important numbers on a chart. The trouble is that there are several of them, with similar-looking abbreviations, and they do not all promise the same thing. Knowing exactly what each one guarantees, and which one is not a safe altitude at all, is basic terrain awareness.
This is general educational information, not operational, legal, or regulatory advice. Rules differ by authority and change over time. Always verify against current official sources and follow your operator's approved procedures.
The altitudes that guarantee clearance
These four are designed to keep you safely above terrain and obstacles, with the clearance already built in. The FAA Instrument Procedures Handbook, the FAA AIM and the Jeppesen Airway Manual define them:
- MSA, minimum sector altitude: shown on approach charts, it gives at least 1000 feet of obstacle clearance within 25 nautical miles of a reference point, usually split into sectors. It is an emergency terrain-awareness figure, not a routine approach altitude.
- MORA, minimum off-route altitude (a Jeppesen figure): clears terrain and obstacles within a defined band either side of a route, or in a grid square, by 1000 feet where the highest point is up to 5000 feet, and by 2000 feet where it is higher.
- MEA, minimum en-route altitude: along an airway, it assures both obstacle clearance and an acceptable navigation signal for the whole segment.
- MOCA, minimum obstruction clearance altitude: assures the same obstacle clearance along the airway, but comes with a signal caveat that is specifically American. See below, because this is the one people most often state as though it were universal.
The 22 nautical miles is a US figure
MOCA is routinely taught as "obstacle clearance, but the navigation signal is only guaranteed within 22 nautical miles of the aid", and that sentence needs a jurisdiction attached to it. The Jeppesen glossary brackets it itself: the MOCA "in the USA" assures acceptable navigational signal coverage only within 22 NM of a VOR. Two things are being said there and both matter. The 22 NM promise is a United States convention, and the aid it is measured from is specifically a VOR, not any navaid you please.
ICAO's MOCA carries no signal element at all. It is an obstacle-clearance altitude, full stop. So on an ICAO-convention chart, do not read a MOCA as an implicit promise of 22 NM of anything: it promises terrain clearance and says nothing about what you will be able to receive. Where a chart follows the US convention, the 22 NM from a VOR is the figure that applies.
The general clearance values behind these are less universal than they look, too. The 1000 feet in non-mountainous areas and 2000 feet in designated mountainous areas pairing is the FAA and Jeppesen presentation, and it is the one you will meet on a US chart. ICAO Doc 8168 does not use that binary split: at 1.4.1 it sets a graded scale keyed to how much the terrain varies, starting at 300 m (1000 ft) and rising to 450 m (1476 ft) and then 600 m (1969 ft) as the terrain becomes more broken. The ICAO numbers land near the FAA's at both ends, which is why the shorthand survives, but they are not the same rule, and "1000 or 2000" is not what PANS-OPS says.
The one that is not a safe altitude
The MEF, Maximum Elevation Figure, is different in kind, and the difference catches people out. Printed in each latitude and longitude quadrangle on a VFR chart, the MEF is the elevation of the highest terrain or obstacle in that box, rounded up. It has no clearance margin built in. It tells you the highest thing in the square and nothing more; the safe cruising altitude is whatever you choose to add above it.
A worked example
Planning a VFR leg across a chart, you read an MEF of 5400 feet in the quadrangle your track crosses. That is the top of the highest obstacle in that box with no margin. To set a cruising altitude you add your own clearance, say 1500 to 2000 feet for comfort over rising ground, giving roughly 7000 to 7400 feet, before you then round to a VFR cruising level appropriate to your direction of flight.
Contrast the IFR picture on the same route, flown on a US chart: an airway segment shows an MEA of 7000 feet and a MOCA of 5500 feet. You may, in the right circumstances, descend to the MOCA and still be clear of obstacles, but beyond 22 nautical miles from the VOR you are no longer assured of the signal, so the MEA is the altitude that keeps both your terrain clearance and your navigation honest along the whole segment. On an ICAO-convention chart the same MOCA would promise you the obstacle clearance and nothing at all about the signal, which is a different trade to weigh.
Common pitfalls
- MEF is the obstacle, not your altitude. Never fly at the MEF; it is the height of the highest obstacle, with nothing added.
- Quoting MOCA's 22 NM as universal. It is a US convention, measured from a VOR; ICAO's MOCA has no navigation-signal element at all.
- MEA and MOCA differ on signal, not clearance. Both clear the terrain; only the MEA promises navigation coverage for the full segment.
- Treating 1000 and 2000 feet as the international rule. That binary is the FAA and Jeppesen presentation. ICAO Doc 8168 uses a graded scale, from 300 m (1000 ft) up to 450 m (1476 ft) and 600 m (1969 ft), keyed to how much the terrain varies.
- Mountainous means more margin. Under the US convention the clearance rises to 2000 feet in designated mountainous areas, so the same airway feels very different over high ground.
In Pilot EFB
Pilot EFB is a study and planning companion for terrain-aware route planning, sitting alongside your weather, NOTAMs and flight time in one offline-first place. It does not replace your charts or set your safe altitude for you, so read the published figures from your official source of record and add your own margin. Pilot EFB is not a certified Electronic Flight Bag, so treat it as a study and planning aid and brief from your official source of record.