Definition
The setting on a transponder selector switch that activates Mode C, causing the transponder to reply to ATC interrogations with both the aircraft's assigned squawk code and its pressure altitude. Selecting ALT enables altitude reporting; selecting ON enables the transponder without altitude reporting.
Plain English
The switch position on the transponder that tells it to send your altitude to air traffic control along with your code.
Context Anchor
Seen on the transponder or avionics control panel, especially during taxi, takeoff, radar service, and instrument flying.
Derivation
ALT is short for altitude. The label is placed on the switch position because that is the function it activates — altitude reporting via Mode C.
Why Pilots Care
Enables ATC radar to display your altitude, which is required for flight in most controlled airspace and improves traffic separation safety.
Intuition Check
ALT position does not mean the airplane’s position in the sky. It means the ALT setting on the transponder control, which enables altitude reporting.
Example Sentence 1
Before takeoff, the pilot turned the transponder selector to the ALT position so ATC would receive altitude information once airborne.
Example Sentence 2
With the transponder in ALT position, the controller could see the aircraft's altitude on the radar scope.