Definition
The section of Title 14 of the Code of Federal Regulations (14 CFR Part 91) that sets the rules for ATC transponder and altitude reporting equipment, including when a transponder is required, where it must be installed and operating, and the equipment standards it must meet. It mandates an operable coded radar beacon transponder with Mode 3/A 4096-code capability and Mode C automatic pressure altitude reporting in defined airspace, such as Class A, Class B, Class C, and at and above 10,000 feet MSL (with limited exceptions).
Plain English
It is the federal rule that says when you must have a working transponder on board, what kind of transponder it has to be, and where in the sky it must be turned on and reporting your altitude.
Context Anchor
Seen in operating-rules discussions for Class A airspace and other airspace where transponder and altitude-reporting equipment are required.
Derivation
Section comes from a Latin word meaning “to cut.” In regulations, a section is one cut-out part of a larger rulebook. The number 91.215 points to a specific rule inside Part 91, which covers general operating and flight rules.
Why Pilots Care
Failure to meet this equipment requirement makes flight in Class A airspace illegal and can result in enforcement action.
Analogy
Think of section 91.215 like a street address in the rulebook. Part 91 is the neighborhood, and 91.215 is the exact house where the transponder-equipment rule lives.
Intuition Check
Do not read “section 91.215” as a handbook page or a training chapter. It is a specific federal aviation regulation that a pilot may be required to follow.
Example Sentence 1
Before entering Class B airspace, the pilot confirmed the aircraft met the transponder requirements of section 91.215.
Example Sentence 2
An inoperative transponder would have placed the flight in violation of section 91.215.