FAR 91.177 — IFR Minimum Altitudes
FAR 91.177 sets minimum IFR altitudes: MEA, MOCA, and obstacle clearance over mountainous and non-mountainous terrain. Study guide for pilot students.
In Plain English
FAR 91.177 sets the floor for how low you can legally fly under IFR. Except for takeoff, landing, or specific FAA authorization, you must stay at or above the published minimum altitudes in Parts 95 and 97 (charts, approach plates, airway altitudes).
Key points:
- MEA vs. MOCA: If a route segment has both a Minimum En Route Altitude (MEA) and a Minimum Obstruction Clearance Altitude (MOCA), you may descend below the MEA down to (but not below) the MOCA — but only if navigation signals are usable. For VOR navigation, the MOCA only guarantees signal reception within 22 NM of the VOR.
- No published minimum? Then use obstacle clearance:
- Mountainous areas: 2,000 ft above the highest obstacle within 4 NM of course.
- Non-mountainous areas: 1,000 ft above the highest obstacle within 4 NM of course.
- Climbs: Begin climbing to a higher minimum altitude immediately after passing the point where it applies. If obstacles intervene, cross the Minimum Crossing Altitude (MCA) before that point.
Why it matters: these minimums keep you clear of terrain and ensure reliable navigation when you can't see outside.
Regulation Text
14 CFR § 91.177§ 91.177 Minimum altitudes for IFR operations.
(a)Except when necessary for takeoff or landing, or unless otherwise authorized by the FAA, no person may operate an aircraft under IFR below—
(1) The applicable minimum altitudes prescribed in parts 95 and 97 of this chapter. However, if both a MEA and a MOCA are prescribed for a particular route or route segment, a person may operate an aircraft below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. For aircraft using VOR for navigation, this applies only when the aircraft is within 22 nautical miles of that VOR (based on the reasonable estimate by the pilot operating the aircraft of that distance); or
(2) If no applicable minimum altitude is prescribed in parts 95 and 97 of this chapter, then—
(i) In the case of operations over an area designated as a mountainous area in part 95 of this chapter, an altitude of 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown; or
(ii) In any other case, an altitude of 1,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown.
(b)Climb to a higher minimum IFR altitude shall begin immediately after passing the point beyond which that minimum altitude applies, except that when ground obstructions intervene, the point beyond which that higher minimum altitude applies shall be crossed at or above the applicable MCA.
[Docket 18334, 54 FR 34294, Aug. 18, 1989, as amended by Amdt. 91-296, 72 FR 31678, June 7, 2007; Amdt. 91-315, 75 FR 30690, June 2, 2010]
Oral Exam Questions a DPE Might Ask
Q1What's the difference between MEA and MOCA, and when can you fly below the MEA?
Per FAR 91.177, the MEA guarantees both obstacle clearance and navigation signal reception, while the MOCA guarantees obstacle clearance but only assures VOR signal reception within 22 NM of the station. You may descend below the MEA down to the MOCA only if the navigation signals are available.
Q2If no minimum IFR altitude is published for your route, how do you determine a legal altitude?
FAR 91.177 requires 2,000 feet above the highest obstacle within 4 NM of course in designated mountainous areas, and 1,000 feet above the highest obstacle within 4 NM of course everywhere else.
Q3When ATC assigns you a higher minimum IFR altitude on your route, when must you begin the climb?
Under FAR 91.177(b), you begin the climb immediately after passing the point where the higher minimum altitude applies, unless an MCA is published due to intervening obstacles — in that case you must cross the fix at or above the MCA.
Studying for a checkride?
Related Sections in Part 91