Enroute Altitudes & Terrain Clearance
Key IFR enroute altitude definitions and who owns terrain clearance in each situation.
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Questions Covered in This Set
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MEA — what does it guarantee?
Obstacle clearance (1,000 ft non-mountainous / 2,000 ft mountainous) AND acceptable nav signal coverage for the entire segment; comms usually assured unless 'MEA GAP' is annotated.
MOCA — what's the catch?
Shown with an asterisk (e.g. *4300). Same obstacle clearance as MEA, but nav signal only guaranteed within 22 NM of the VOR — per 91.177 you may only fly it inside that 22 NM.
What is an MCA and why does it exist?
Minimum Crossing Altitude — the altitude you must already have before crossing a fix where the MEA rises ahead, forcing you to begin the climb early.
MRA
Minimum Reception Altitude — lowest altitude at which an intersection can be identified using off-course navaids; irrelevant if identifying the fix by GPS.
MAA
Maximum Authorized Altitude — the ceiling for that segment, usually due to frequency interference.
OROCA — what does it guarantee and not guarantee?
Gray lat/long quadrangle number giving 1,000 ft (non-mountainous) or 2,000 ft (mountainous) above the highest obstacle. Guarantees NO nav or comm coverage, is not ATC-assignable — it's your personal terrain sanity check. Jeppesen equivalent: Grid MORA.
MVA / MIA — whose numbers are they?
The controller's: polygons on the radar scope you can't see. Often LOWER than MEAs because radar coverage substitutes for nav coverage and obstacle buffers are drawn tightly.
Who owns terrain clearance on a radar vector with an ATC-assigned altitude?
The controller — they must assign at or above the MVA/MIA.
What does 'maintain own terrain and obstruction clearance until reaching X' mean?
ATC is explicitly handing you the responsibility. Cross-check OROCA and nearby high terrain before accepting.
ATC offers 'Direct PUEBLO, maintain eight thousand' but the quadrangle OROCA is 14,300. What do you do?
Do not silently turn. Say 'unable' and request a higher altitude or to remain on the airway — never accept a clearance below terrain-safe altitude off-airway.
