Definition
A specific paragraph within Title 14 of the Code of Federal Regulations, Part 91 (General Operating and Flight Rules), Section 91.215, paragraph (a). This paragraph specifies the technical performance standards (TSO classes) that an ATC transponder must meet when installed in an aircraft operated under Part 91. It establishes which transponder equipment is acceptable for use in U.S. airspace.
Plain English
It is the part of the federal aviation rules that lists which types of transponders are approved for use in an aircraft. It tells you what the transponder itself must be capable of, by referencing approved equipment standards.
Context Anchor
Seen in aircraft inspection and equipment discussions, especially when checking whether a transponder installation meets FAA requirements.
Derivation
CFR stands for Code of Federal Regulations, the official collection of U.S. federal rules. Title 14 covers Aeronautics and Space. Part 91 contains the general flight rules most pilots operate under. The numbering pattern 91.215(a) means Part 91, Section 215, paragraph (a) -- a standard way of pointing to one specific rule within a much larger body of regulations.
Why Pilots Care
Compliance is required to operate legally in controlled airspace and to avoid violations during ramp checks or radar contact.
Intuition Check
Do not read this as just a page reference. It is a legal citation to a specific FAA rule, and the letters and numbers tell you exactly where that rule is found.
Example Sentence 1
The avionics shop confirmed the new transponder met the equipment standards in 14 CFR 91.215(a) before signing off the installation.
Example Sentence 2
During the preflight review the pilot checked that the aircraft satisfied 14 CFR part 91 section 91.215(a) for the planned flight into Class B airspace.