Definition
A function of the aircraft's transponder (Mode C or Mode S) that automatically transmits the aircraft's pressure altitude to ground-based air traffic control radar in 100-foot increments, without any action required by the pilot beyond turning the equipment on.
Plain English
When the transponder is turned on in the right mode, it automatically tells the controller's radar what altitude the aircraft is flying at. The pilot doesn't have to read the altitude over the radio.
Context Anchor
You encounter this when setting or checking the transponder before flight, when receiving radar services, or when ATC asks you to verify altitude reporting.
Why Pilots Care
It supplies controllers with continuous, accurate altitude data needed for safe separation between aircraft.
Intuition Check
Automatic does not mean the pilot can ignore it. It means the equipment sends altitude without a radio call, but the pilot still must make sure the transponder is set correctly and respond if ATC questions the altitude.
Example Sentence 1
After takeoff, the pilot confirmed the transponder was in ALT mode so automatic altitude reporting would be active for departure control.
Example Sentence 2
When automatic altitude reporting is inoperative, the pilot must state altitude on every radio call.