Definition
A specific subsection of the Federal Aviation Regulations that establishes the technical standards a transponder and its altitude reporting equipment must meet to be approved for use in U.S. airspace. It identifies the FAA Technical Standard Orders (TSOs) that the equipment must comply with, and it ties into the broader rule requiring transponder inspections every 24 calendar months under 14 CFR 91.413.
Plain English
It is the rule that lists which technical standards your transponder must meet to be legal for flight. If your transponder does not meet one of the approved standards listed in this section, it cannot be used in U.S. airspace.
Context Anchor
Seen when checking whether an aircraft’s transponder equipment is legal for use, especially in discussions of transponder requirements and inspections.
Derivation
‘CFR’ stands for Code of Federal Regulations -- the official collection of U.S. government rules. ‘Part 91’ is the section covering general operating and flight rules. ‘Section 91.215’ is the specific rule about transponders, and ‘(a)’ points to the first paragraph within that rule, which lists the approved equipment standards.
Why Pilots Care
Pilots must ensure their aircraft meets this equipment standard to legally enter controlled airspace and receive ATC services.
Analogy
A regulation citation works like a street address. “14 CFR part 91, section 91.215(a)” tells you exactly where to find one specific rule inside the federal aviation rules.
Intuition Check
Do not read this as the inspection itself. It is a legal reference pointing to the rule that says what standard the installed transponder equipment must meet.
Example Sentence 1
Before signing off the installation, the avionics shop confirmed the new transponder met the equipment standards required by 14 CFR part 91, section 91.215(a).
Example Sentence 2
During the annual inspection, verify compliance with 14 CFR part 91, section 91.215(a) for the transponder.