Definition
ALT is the standard abbreviation for altitude — the vertical distance of an aircraft, measured from a stated reference such as mean sea level or the ground. In the context of Mode C, ALT refers specifically to the pressure altitude data that the transponder automatically reports to air traffic control radar.
Plain English
ALT is just shorthand for 'altitude' — how high the aircraft is. On a transponder, selecting ALT tells the box to send the aircraft's height to ATC along with its identifying code.
Context Anchor
Seen on transponder controls and in discussions of Mode C altitude reporting.
Derivation
From the Latin altus, meaning 'high.' The same root gives us 'altitude.' The three-letter form ALT is used because cockpit labels and ATC messages need to be short and unambiguous.
Why Pilots Care
Allows ATC to maintain safe vertical separation between aircraft without relying solely on pilot reports.
Intuition Check
ALT does not mean alternator in this context. Here it means the transponder mode that sends altitude information.
Example Sentence 1
Before takeoff, the pilot selected ALT on the transponder so that ATC would receive altitude information once airborne.
Example Sentence 2
ATC asked the pilot to verify ALT was selected before entering the approach corridor.