Definition
The section of Title 14, Code of Federal Regulations, Part 91 that requires the pilot-in-command of an aircraft operated in controlled airspace under IFR to report immediately to ATC any malfunction of navigational, approach, or communication equipment occurring in flight.
Plain English
This is the rule that says: if you're flying IFR in controlled airspace and something breaks on your nav, approach, or radio gear, you have to tell ATC right away. The rule also lists exactly what information you must include in that report.
Context Anchor
Seen in instrument flying, especially in communication-failure discussions and when reviewing pilot responsibilities after equipment stops working in flight.
Derivation
CFR stands for Code of Federal Regulations — the published set of U.S. federal rules. Title 14 covers aeronautics and space. Part 91 contains the General Operating and Flight Rules. The number 91.187 simply identifies the specific section within Part 91 that addresses equipment malfunction reports.
Why Pilots Care
Reporting keeps ATC aware so they can provide assistance, adjust routing, and maintain separation when your equipment is unreliable.
Intuition Check
Do not assume this rule only applies after a total radio failure. It applies to reportable failures of navigation, approach, or communication equipment during IFR flight in controlled airspace.
Example Sentence 1
When his glideslope receiver failed on the descent, the pilot complied with 14 CFR 91.187 and reported the malfunction to approach control before continuing.
Example Sentence 2
The examiner asked how 14 CFR Part 91.187 applies when both comm radios become inoperative inside Class C airspace.