Minimum altitudes for IFR operations14 CFR 91.177 Minimum altitudes for IFR operations
Under IFR you may not fly below the minimum altitudes prescribed in Parts 95 and 97, except when necessary for takeoff or landing or when the FAA authorizes otherwise. Where none are prescribed, the minimum is 2,000 feet above the highest obstacle within 4 nautical miles of the course in a designated mountainous area, and 1,000 feet elsewhere.
By the ATIS Alerts team · Updated October 15, 2026 · Regulation text current as of October 14, 2026. A study aid based on the official regulation text, not legal advice or an FAA interpretation.
At a glance
- Fly at or above the minimum altitudes prescribed in Parts 95 and 97, except when necessary for takeoff or landing or unless otherwise authorized by the FAA (a)(1).
- When both an MEA and a MOCA are prescribed, you may fly below the MEA down to, but not below, the MOCA if the applicable navigation signals are available (a)(1).
- When navigating by VOR, that MOCA allowance applies only within 22 nautical miles of the VOR, based on the pilot's reasonable estimate (a)(1).
- No prescribed altitude, designated mountainous area: 2,000 feet above the highest obstacle within 4 nautical miles of the course (a)(2)(i).
- No prescribed altitude, anywhere else: 1,000 feet above the highest obstacle within 4 nautical miles of the course (a)(2)(ii).
- Climb to a higher minimum IFR altitude immediately after passing the point where it applies, and cross that point at or above the MCA when ground obstructions intervene (b).
In plain English
Section 91.177 sets the floor for IFR flight. Paragraph (a)(1) points to the minimum altitudes prescribed in Parts 95 and 97 of 14 CFR, including the MEA and MOCA it names for routes and route segments. The rule allows flying lower only when necessary for takeoff or landing, or when the FAA authorizes it.
Some route segments have both a minimum en route altitude (MEA) and a minimum obstruction clearance altitude (MOCA). The rule lets you fly below the MEA down to, but not below, the MOCA, provided the applicable navigation signals are available. If you are using VOR for navigation, that applies only when you are within 22 nautical miles of the VOR, based on your reasonable estimate of the distance.
When no minimum altitude is prescribed in Parts 95 and 97, paragraph (a)(2) supplies one. Over an area designated as mountainous in Part 95, fly at least 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course. Everywhere else the margin is 1,000 feet above the highest obstacle within 4 nautical miles of the course.
Paragraph (b) covers climbs. When a higher minimum IFR altitude applies ahead, begin the climb immediately after passing the point beyond which it applies. When ground obstructions intervene, that point itself must be crossed at or above the applicable minimum crossing altitude (MCA), so the climb has to be complete before you reach it.
Common mistakes
- Using the MOCA on a VOR route beyond 22 nautical miles from the station. When navigating by VOR, the MOCA allowance applies only within that distance (a)(1).
- Starting a climb to a higher minimum altitude late. The climb begins immediately after passing the point where the higher altitude applies, and an MCA must be met at the point itself (b).
- Using the 1,000-foot margin in a designated mountainous area. Where Part 95 designates the area as mountainous, the margin is 2,000 feet (a)(2)(i).
The official text
14 CFR 91.177 as published on the eCFR, current as of October 14, 2026.
(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.
Test yourself
Practice questions from our free FAA written test prep that cite 14 CFR 91.177.
Instrument Rating
Unless otherwise authorized, what is the minimum altitude for IFR operations over mountainous terrain outside designated routes?
- A. 2,000 feet above the highest obstacle within 4 nautical miles of the course to be flown.
- B. 1,000 feet above the highest obstacle within 4 nautical miles of the course to be flown.
- C. 2,000 feet above the highest obstacle within 5 statute miles of the course to be flown.
- D. 1,500 feet above the highest obstacle within 5 nautical miles of the course to be flown.
Show answer
A. 2,000 feet above the highest obstacle within 4 nautical miles of the course to be flown. Mountainous terrain gets 2,000 feet of clearance; non-mountainous gets 1,000 feet. Both use a horizontal radius of 4 NM from the course. It governs off-route altitude selection where no MEA or other minimum altitude is prescribed; a charted OROCA uses similar buffers as a planning aid.
More Instrument Rating practice →CFII Airplane
MOCA guarantees obstacle clearance for the whole route segment but assures navigation signal coverage only within:
- A. 22 nautical miles of the VOR.
- B. 25 nautical miles.
- C. 12 nautical miles.
- D. 40 nautical miles.
Show answer
A. 22 nautical miles of the VOR. Twenty-two nautical miles. MEA guarantees both obstacle clearance and signal coverage for the entire segment, which is why MEA is the altitude normally flown and MOCA is a request-and-coordinate exception.
More CFII Airplane practice →Instrument Ground Instructor
A minimum en route altitude (MEA) guarantees:
- A. acceptable navigation signal coverage and obstacle clearance for the entire route segment.
- B. obstacle clearance only.
- C. radar coverage along the airway.
- D. two-way communications with ATC for the entire segment.
Show answer
A. acceptable navigation signal coverage and obstacle clearance for the entire route segment. The MEA is the both-and altitude: navigation signal AND obstruction clearance across the whole segment. Neither radar nor communications coverage is promised. The MOCA keeps the full-segment obstacle clearance but guarantees the signal only near the VOR.
More Instrument Ground Instructor practice →Instrument Ground Instructor
A minimum obstruction clearance altitude (MOCA) differs from the MEA in that the MOCA assures:
- A. obstacle clearance over the whole segment, but acceptable VOR signal coverage only within 22 NM of the VOR.
- B. navigation signal coverage everywhere, but no obstacle clearance.
- C. both signal and obstacle protection at a lower altitude than the MEA.
- D. obstacle clearance only in designated mountainous areas.
Show answer
A. obstacle clearance over the whole segment, but acceptable VOR signal coverage only within 22 NM of the VOR. The 22 NM signal limit is the tested number; obstacle protection applies to the entire segment either way. Under 91.177 a pilot navigating by VOR may descend from the MEA to the MOCA only within that 22 NM of the VOR.
More Instrument Ground Instructor practice →
Questions pilots ask
Can I fly below the MEA on an IFR flight?
Down to, but not below, the MOCA, if both are prescribed for the route segment and the applicable navigation signals are available. When using VOR for navigation, only within 22 nautical miles of that VOR (91.177(a)(1)).
What is the minimum IFR altitude when no altitude is published?
2,000 feet above the highest obstacle within 4 nautical miles of the course over an area designated as mountainous in Part 95, or 1,000 feet above the highest obstacle within 4 nautical miles of the course elsewhere (91.177(a)(2)).
When do I climb to a higher MEA?
Immediately after passing the point beyond which the higher minimum altitude applies. If ground obstructions intervene, cross that point at or above the applicable MCA (91.177(b)).