Definition
An encoder is a device that converts the aircraft's pressure altitude into a digital signal and sends it to the transponder, which then transmits that altitude information to air traffic control radar.
Plain English
It is the small box that reads the airplane's altitude and feeds that number to the transponder, so controllers on the ground can see how high the aircraft is on their radar screens.
Context Anchor
Seen in aircraft equipment checks, maintenance records, and preflight discussions of the transponder and altitude-reporting system.
Derivation
From 'encode,' meaning to convert information into a coded form. The device encodes altitude as a digital signal the transponder can transmit.
Why Pilots Care
Without a working encoder the transponder cannot report altitude to ATC, which is required for flight in most controlled airspace.
Analogy
It works like a translator: the altitude system knows the airplane’s height, and the encoder turns that information into a language the transponder can send.
Intuition Check
An encoder is not the same thing as the transponder. The encoder prepares the altitude information; the transponder sends it.
Example Sentence 1
During the pitot-static and transponder check, the technician verified that the encoder was reporting the correct altitude to ATC.
Example Sentence 2
The controller asked the pilot to check the encoder after receiving an incorrect Mode C readout.