Definition
The section of Federal Aviation Regulations that establishes the minimum altitudes for IFR operations. It requires pilots flying under instrument flight rules to operate at or above the minimum altitude prescribed for the route or, if none is prescribed, at least 1,000 feet above the highest obstacle within 4 nautical miles of course (2,000 feet in designated mountainous terrain), unless otherwise authorized by ATC or necessary for takeoff and landing.
Plain English
This is the rule that tells you how low you are allowed to fly when on an IFR flight plan. In flat country you must stay at least 1,000 feet above anything within 4 miles of your course; in mountain country, 2,000 feet. If a chart gives a higher minimum, you use that.
Context Anchor
Seen in instrument flying discussions about IFR cruising altitude, obstacle clearance, and when a pilot must climb to meet a higher minimum altitude.
Derivation
CFR stands for Code of Federal Regulations. Part 91 is the section covering general operating and flight rules for civil aircraft. The number 91.177 is simply the catalog address of this specific rule within Part 91.
Why Pilots Care
Violating this regulation risks controlled flight into terrain and regulatory enforcement action during IFR operations.
Grounding Statement
If you are in cloud and cannot see the ground, this rule helps make sure there is a required vertical buffer between the aircraft and the highest nearby obstacle.
Intuition Check
Do not read this as a general cruising-altitude preference. Section 91.177 is about the minimum altitude you may use under IFR; another rule or an ATC clearance may still affect the altitude you actually fly.
Example Sentence 1
Before accepting a direct routing across the ridge, the pilot checked 14 CFR 91.177 and confirmed an altitude that gave at least 2,000 feet of clearance over the highest terrain within 4 miles of the new course.
Example Sentence 2
During the IFR flight review, the examiner confirmed compliance with 14 CFR Part 91.177 for the planned route.