AIM ¶ 3-3-3 — IFR Altitude Requirements
AIM 3-3-3 explains IFR altitude rules in Class G airspace and the 1,000/2,000-foot obstacle clearance requirement under 14 CFR 91.177.
AIM 3-3-3 reminds pilots that 14 CFR sets the pilot and equipment requirements for IFR flight, and points specifically to 14 CFR 91.177 for minimum IFR altitudes. Beyond simply flying at a charted altitude or flight level, you must stay clear of obstacles by a defined margin.
Obstacle clearance under 91.177:
- At least 1,000 feet above the highest obstacle within a 4 NM horizontal distance of your course.
- At least 2,000 feet above obstacles in designated mountainous terrain.
IFR cruising altitudes in Class G airspace below 18,000 feet MSL (based on magnetic course/ground track):
- 0°–179°: odd thousands MSL (3,000; 5,000; 7,000…)
- 180°–359°: even thousands MSL (2,000; 4,000; 6,000…)
Why it matters: ATC assigns altitudes in controlled airspace, but in Class G you are responsible for picking a legal IFR altitude. Always cross-check the MEA, MOCA, or OROCA to ensure obstacle clearance is met, especially in mountainous areas.