Definition
An aircraft fitted with a working transponder — an onboard radio device that receives interrogation signals from ground or airborne radar and replies with a coded signal containing identity (squawk code) and, if Mode C or Mode S, pressure altitude. This reply allows air traffic control and other suitably equipped aircraft to detect, identify, and track the aircraft on radar displays.
Plain English
An aircraft carrying a small radio box that automatically answers radar with an ID code and altitude, so controllers and other aircraft can see exactly who and where it is.
Context Anchor
Seen in traffic avoidance discussions, especially when explaining which aircraft can be detected by traffic-alert systems and air traffic control radar.
Derivation
Transponder is a blend of transmitter and responder — it transmits a response. So a transponder-equipped aircraft is one carrying a device that automatically responds to radar interrogations.
Why Pilots Care
Determines whether the aircraft will appear on radar and collision-avoidance systems, directly affecting separation standards and traffic advisories.
Grounding Statement
A transponder-equipped aircraft is easier for radar and compatible traffic-alert systems to notice because it actively replies instead of only being seen by reflection.
Intuition Check
Do not read equipped as merely having the box in the panel. In this context, the important point is that the transponder is installed, turned on, and able to reply.
Example Sentence 1
Traffic advisory systems can only display nearby transponder-equipped aircraft, so an airplane flying without one will not appear on the screen.
Example Sentence 2
Controllers routed the transponder-equipped aircraft around slower traffic that was not replying to radar.