Definition
A transponder operating mode that automatically reports the aircraft's pressure altitude to air traffic control, in addition to the four-digit identification code transmitted in Mode A. The altitude data is referenced to 29.92 inches of mercury (the standard altimeter setting) and is transmitted in 100-foot increments when the transponder is interrogated by ATC radar.
Plain English
Mode C is the setting on your transponder that sends your altitude to ATC automatically, so the controller's screen shows not just who you are but how high you are.
Context Anchor
Seen in instrument flying, transponder use, radar services, and airspace rules that require altitude reporting.
Derivation
The 'Mode' designations (A, C, S) come from the secondary surveillance radar system developed after WWII. 'C' simply identifies this mode in the lettered sequence -- it does not stand for a word. Mode A sends identification, Mode C adds altitude, Mode S adds selective addressing and data.
Why Pilots Care
ATC uses the altitude data for vertical separation, terrain clearance advisories, and conflict alerts, directly affecting safety in controlled airspace.
Intuition Check
Mode C does not mean a special voice radio channel. It means the transponder is automatically reporting altitude.
Example Sentence 1
Before entering the Class B airspace, the pilot confirmed the transponder was set to Mode C so ATC could see the aircraft's altitude.
Example Sentence 2
ATC instructed the aircraft to remain in Mode C so the controller could monitor its altitude throughout the climb.