Definition
A transponder operating mode that automatically reports the aircraft's pressure altitude to air traffic control radar in addition to the assigned four-digit identification code. Mode C requires the transponder to be connected to an altitude encoder and is required by regulation in most controlled airspace and above 10,000 feet MSL.
Plain English
A setting on the aircraft's transponder that sends both the aircraft's ID code and its altitude to controllers on the ground, so they can see how high the aircraft is on their radar screens.
Context Anchor
Pilots encounter Mode "C" when setting the transponder, talking with air traffic control, and reviewing equipment requirements for certain airspace.
Derivation
The 'Mode' part refers to the operating mode of the transponder. 'C' is simply the letter assigned to this particular mode in the international transponder system, where Mode A reports identity only, Mode C adds altitude, and Mode S adds selective addressing and data.
Why Pilots Care
ATC uses the altitude data for vertical separation; failure to squawk Mode C can result in loss of radar services or airspace violations.
Intuition Check
Mode "C" does not mean a clearance, a radio channel, or a type of airspace. It is a transponder setting that reports altitude.
Example Sentence 1
Before entering Class B airspace, the pilot confirmed the transponder was set to Mode C and squawking the assigned code.
Example Sentence 2
During the instrument proficiency check the examiner verified that the transponder was set to Mode C before departure.