Definition
The section of the Federal Aviation Regulations that sets out transponder and altitude reporting equipment requirements for aircraft operating in U.S. controlled airspace. It specifies which airspace requires an operable transponder with Mode C (altitude reporting), the conditions under which a transponder must be turned on and operating, and the limited circumstances in which ATC may authorize deviations.
Plain English
This is the rule that tells pilots when they must have a working transponder with altitude reporting on board, and when it has to be switched on. It mainly applies in busier airspace where ATC needs to see your aircraft on radar.
Context Anchor
Seen when studying Class A airspace and the equipment a pilot must have before operating there.
Derivation
The reference is a regulatory address. '14 CFR' means Title 14 of the Code of Federal Regulations, which covers aviation. 'Part 91' is the section of those rules that governs General Operating and Flight Rules. 'Section 91.215' is the specific rule within Part 91 dealing with transponders. Knowing the structure helps you find any FAR quickly: Title, Part, Section.
Why Pilots Care
Compliance ensures your aircraft is visible to air traffic control, preventing conflicts and maintaining safe separation in high-altitude flight.
Intuition Check
Do not read this as just a handbook reference or page number. It is a legal rule citation: it points to an enforceable FAA requirement.
Example Sentence 1
Before entering Class A airspace, the pilot confirmed the aircraft met the transponder requirements of 14 CFR part 91, section 91.215.
Example Sentence 2
The regulation in 14 CFR part 91, section 91.215 applies to most IFR operations above 10,000 feet MSL.