Definition
The lowest altitudes at which an aircraft may legally operate under Instrument Flight Rules, established by 14 CFR 91.177. Except when necessary for takeoff or landing, no person may operate under IFR below the applicable minimum altitudes prescribed in parts 95 and 97, or, if no minimum is prescribed, below 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown in designated mountainous terrain, or 1,000 feet above the highest obstacle within that same distance in non-mountainous terrain.
Plain English
The lowest height you are allowed to fly when operating on instruments. The rule guarantees a safe buffer above terrain and obstacles: 2,000 feet over mountains, 1,000 feet over flatter areas, measured within 4 nautical miles either side of your route.
Context Anchor
Seen on IFR charts, departure procedures, route descriptions, and clearances when a pilot must know how low the aircraft may legally and safely fly.
Why Pilots Care
Keeps the aircraft safely above terrain and obstacles while maintaining reliable navigation signals during IFR flight.
Intuition Check
Do not read “minimum” as a suggestion or a comfortable low point. In this context, it is the lowest authorized altitude for that IFR segment unless a different valid clearance or rule applies.
Example Sentence 1
Before accepting the lower altitude from ATC, the pilot checked the chart to confirm it was at or above the minimum IFR altitude for that segment.
Example Sentence 2
Departure procedures list minimum IFR altitudes to follow until reaching the enroute structure.