Definition
A transponder operating mode in which the aircraft's pressure altitude (referenced to 29.92 inches of mercury) is automatically encoded and transmitted along with the assigned beacon code in reply to interrogations from air traffic control's secondary surveillance radar. This allows the controller's radar display to show both the aircraft's identity and its current altitude alongside its position.
Plain English
When your transponder is set to Mode C, it tells ATC's radar not just where you are and who you are, but also how high you're flying. The altitude is read automatically from a small device tied to the static system and sent every time the radar pings you.
Context Anchor
You will see MODE C in transponder use, airspace equipment requirements, and ATC instructions involving altitude reporting.
Derivation
The 'C' is simply the third designation in the transponder mode lettering scheme (Mode A reports identity only; Mode C adds altitude; Mode S adds selective addressing and data). The letter itself carries no meaning beyond its place in the sequence.
Why Pilots Care
Mode C data lets controllers maintain safe vertical separation, issue accurate traffic advisories, and assign altitudes without relying solely on pilot reports.
Intuition Check
Do not read MODE C as just another cockpit setting with no special meaning. In aviation use, MODE C specifically means the transponder reply includes altitude information.
Example Sentence 1
Before entering Class C airspace, the pilot confirmed the transponder was set to ALT so Mode C was active and reporting altitude.
Example Sentence 2
With Mode C active, the radar scope displayed the aircraft at 6500 feet.