Instrument charts are full of minimum altitudes, and each one exists for a different reason. Mixing them up is a classic exam trap and, in real flying, a terrain-clearance risk. Here are the four you will meet most often: MSA, MORA, MOCA and MEA.
MSA: Minimum Sector Altitude
Published on approach charts, the MSA gives the lowest safe altitude within 25 nautical miles of a navigation aid or waypoint, divided into sectors. It provides at least 1,000 ft of obstacle clearance. MSA is an emergency and situational-awareness altitude: if you are unsure of your position near the airport, climbing to the MSA keeps you clear of terrain. It does not guarantee radio or navigation coverage.
MORA: Minimum Off-Route Altitude
MORA is a term used on Jeppesen charts. A route MORA gives terrain clearance within 10 nautical miles of the route centreline, and a grid MORA gives it for each latitude-longitude square on the chart. Clearance is 1,000 ft where the highest terrain is at or below 5,000 ft, and 2,000 ft where it is higher. MORA is useful when an aircraft must leave its planned route, for example to avoid weather.
Watch: the four altitudes
MOCA: Minimum Obstacle Clearance Altitude
The MOCA for an airway segment guarantees obstacle clearance along that segment. It does not guarantee that the navigation signal can be received along the whole segment, so it may be lower than the MEA.
MEA: Minimum En-route Altitude
The MEA is the lowest altitude on an airway segment that guarantees both obstacle clearance and adequate navigation signal coverage. It is the altitude normally used as the lowest planned cruising level on that segment.
Side by side
| Altitude | Where | Guarantees |
|---|---|---|
| MSA | Within 25 NM of a fix (approach charts) | 1,000 ft obstacle clearance |
| MORA | Off route (route or grid) | 1,000 or 2,000 ft terrain clearance |
| MOCA | Airway segment | Obstacle clearance only |
| MEA | Airway segment | Obstacle clearance + navigation coverage |
How pilots use them
- During flight planning, check that planned levels are at or above the MEA for every segment.
- If a diversion off the airway is needed, use MORA to stay clear of terrain.
- On arrival, brief the MSA so the whole crew knows the safe altitude around the airport.
- In cold weather, apply temperature corrections: a cold atmosphere makes the aircraft lower than the altimeter shows.
Exam summary
- MSA: 25 NM, 1,000 ft clearance, no nav-signal guarantee.
- MORA: off-route, 1,000/2,000 ft depending on terrain height.
- MOCA ≤ MEA: MOCA is obstacles only, MEA adds navigation coverage.
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