Definition
An aircraft transponder system that automatically transmits the aircraft's pressure altitude to ATC radar in addition to the assigned transponder code. Commonly known as Mode C, it requires an encoding altimeter or altitude encoder connected to the transponder, and is required equipment for flight in Class B airspace, within 30 nautical miles of a Class B primary airport, in and above Class C airspace, and at or above 10,000 feet MSL (with limited exceptions).
Plain English
A transponder feature that sends your altitude down to air traffic control automatically, so the controller sees your height on their radar screen without having to ask.
Context Anchor
You will see this term in equipment requirements for operating in Class B airspace and other controlled airspace where air traffic control needs altitude information from your aircraft.
Derivation
Altitude comes from the Latin word altus, meaning “high.” In this term, the equipment is reporting the aircraft’s height automatically, without the pilot having to call out each altitude change by radio.
Why Pilots Care
Enables ATC to see the aircraft's altitude on radar and maintain safe separation.
Intuition Check
Do not read “automatic” as meaning the airplane controls its altitude. Here it means the equipment sends altitude information by itself once it is operating.
Example Sentence 1
Before entering the Class B airspace, the pilot confirmed the transponder was set to ALT so the automatic altitude reporting equipment was active.
Example Sentence 2
Controllers could not provide radar vectors until the airplane's automatic altitude reporting equipment became operational.