Definition
A transponder operating mode that transmits a four-digit identification code (0000–7777 in octal) when interrogated by ATC radar. Mode A returns identity only and does not include altitude information. The civilian designation is Mode A; the military designation for the same function is Mode 3, and the two are interoperable, which is why they are written together as Mode A/3.
Plain English
A transponder setting that sends out a four-digit code so air traffic control can identify which target on their radar screen is your aircraft. It does not send altitude — that is a separate mode.
Context Anchor
Seen on transponder controls and in discussions of radar identification, assigned codes, and transponder operation.
Derivation
"Mode" comes from the Latin modus, meaning "manner" or "method" — here, a method of replying to a radar interrogation. The letter A is the civilian label and the number 3 is the military label for the same identity-reporting function, dating back to early shared civil/military Secondary Surveillance Radar standards.
Why Pilots Care
Enables positive aircraft identification by ATC without providing altitude data.
Grounding Statement
When MODE A/3 is working, the radar system can match your aircraft to the code you were assigned.
Intuition Check
MODE A/3 does not mean “automatic” and it does not mean “three modes at once.” Here, mode means the type of transponder reply, and A/3 means the civil and military names for the identification-code reply.
Example Sentence 1
After contacting departure, the pilot set the assigned code in Mode A/3 and confirmed the transponder was transmitting.
Example Sentence 2
With the transponder in MODE A/3 the controller could identify the aircraft but had to ask for altitude separately.