Definition
A transponder operating mode in which the aircraft automatically transmits its pressure altitude to air traffic control radar along with its assigned beacon code. The altitude is referenced to a standard pressure setting of 29.92 inches of mercury, regardless of the local altimeter setting, and is corrected at the controller's display so the altitude shown matches the pilot's altimeter when set correctly.
Plain English
Mode C is the setting on your transponder that lets controllers see your altitude on their radar screen, not just a blip showing where you are. It sends your altitude up automatically every time the radar interrogates the aircraft.
Context Anchor
Seen in transponder use, instrument flying, ATC radar services, and airspace where altitude-reporting equipment is required.
Derivation
The 'Mode' designations (A, C, S) come from the original military Identification Friend or Foe (IFF) system. Mode A reports identity (the squawk code), Mode C adds altitude, and Mode S adds selective addressing and data. The letter 'C' is simply the system's label, not an abbreviation.
Why Pilots Care
It gives controllers real-time altitude data needed for safe vertical separation between aircraft.
Intuition Check
Mode C is not a radio channel or a cockpit setting for climbing. It is the transponder’s altitude-reporting function.
Example Sentence 1
Before takeoff, the pilot set the transponder to Mode C so ATC would receive altitude reports throughout the flight.
Example Sentence 2
With Mode C enabled, the controller could issue a descent clearance based on the aircraft's reported altitude.