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 only guarantees the navigation signal within 22 nautical miles of the navigation aid.
The general clearance values behind these, per ICAO Doc 8168 and the FAA, are 1000 feet in non-mountainous areas and 2000 feet in designated mountainous areas.
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: 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 navigation aid 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.
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.
- MEA and MOCA differ on signal, not clearance. Both clear the terrain; only the MEA promises navigation coverage for the full segment.
- Mountainous means more margin. The clearance built into these figures 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.