Regulation Text
(a) Operation of aircraft at minimum altitudes. 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. 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]
Research Notes
Section 91.177 sets minimum altitudes for IFR operations. The reg establishes the floor below which IFR flight is not permitted absent specific authorization.
Paragraph (a) — General floor: Except when necessary for takeoff or landing, no person may operate an aircraft under IFR below:
- (1) The published MEA (Minimum En route Altitude) on the airway/route being flown; OR
- (2) If no published MEA exists, an altitude of 2,000 feet above the highest obstacle within a horizontal distance of 4 NM from the course to be flown (in DESIGNATED MOUNTAINOUS AREAS), or 1,000 feet above the highest obstacle within a horizontal distance of 4 NM from the course (in OTHER areas).
The 'designated mountainous area' overlay: Mountainous areas are designated in 14 CFR Part 95 — covering the Rockies, Sierra Nevada, Cascades, Appalachians, certain Alaska areas, and others. In these areas, the 2,000-foot obstacle clearance applies instead of 1,000 feet.
Off-airway/random IFR: When flying IFR off published routes, the pilot must select altitudes that satisfy § 91.177(a)(2) — 1,000 or 2,000 above the highest obstacle within 4 NM of the course. This is a significant flight-planning step for direct IFR routings.
MOCA — Minimum Obstacle Clearance Altitude: An MOCA is an altitude that provides obstacle clearance for navigation purposes but does not necessarily provide adequate VOR reception. MOCAs are typically lower than MEAs. § 91.177 does not directly govern MOCA use — pilots may descend to the MOCA within 22 NM of a VOR when ATC authorizes (or when off-route).
MAA — Maximum Authorized Altitude: Some routes have an upper altitude limit (MAA) to ensure separation from other route structures. The pilot must respect both the MEA (floor) and the MAA (ceiling).
Reference: FAA-H-8083-16 Chapter 3; AIM 5-3 on enroute procedures.
Amendment History
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