Definition
An air traffic control radar beacon system (ATCRBS) transponder mode that replies to a ground interrogation with a four-digit octal identification code (0000–7777) assigned by ATC, allowing controllers to identify and track the aircraft on radar. Mode A returns identity only; it does not report altitude.
Plain English
A transponder setting that sends back a four-digit code so the controller's radar screen can show which blip is your aircraft. It tells ATC who you are, but not how high you are flying.
Context Anchor
Seen when setting or discussing a transponder code with air traffic control, especially when comparing code-only replies with altitude-reporting replies.
Derivation
The 'mode' designations (A, C, S) are simply sequential labels for different transponder interrogation/reply formats developed by the military and civil aviation. Mode A was the original identity-only mode; Mode C added altitude reporting; Mode S added selective addressing and data exchange.
Why Pilots Care
Allows the aircraft to be identified on radar in situations where full Mode C altitude reporting is unavailable or not required.
Intuition Check
Do not read Mode A as a general aircraft setting or performance mode. Here it specifically means a transponder reply that sends the assigned code, not altitude.
Example Sentence 1
After takeoff, the pilot confirmed the transponder was on and the assigned Mode A code of 4302 was set correctly.
Example Sentence 2
During the practice flight in the local training area the instructor had the student leave the transponder in Mode A only.