Definition
A radio device installed in the aircraft that automatically replies to interrogations from ground-based secondary surveillance radar. When triggered, it transmits a coded signal back to air traffic control, identifying the aircraft and (in Mode C or Mode S) reporting its pressure altitude.
Plain English
A small box in the cockpit that talks to ATC's radar. When the radar pings the aircraft, the transponder pings back with an identifying code and altitude, so controllers can see exactly which dot on their screen is which aircraft.
Context Anchor
Seen on avionics displays, transponder controls, and cockpit checklists when setting or checking the aircraft’s assigned transponder code.
Derivation
A common shorthand built from the X (often used for 'trans-') plus PDR for 'ponder', giving a compact four-letter label that fits on small panel placards and avionics screens.
Why Pilots Care
Keeps the aircraft visible and correctly identified to air traffic control, a legal requirement in most controlled airspace and essential for safe separation.
Intuition Check
XPDR is not a navigation display or a radio for talking. It is the transponder control or status label—the system that electronically answers tracking signals.
Example Sentence 1
Before takeoff, the pilot set the XPDR to the assigned squawk code and switched it to ALT so altitude reporting was active.
Example Sentence 2
ATC instructed the aircraft to squawk 4321 on the XPDR while entering Class B airspace.