Page 70 - PPL-engelsk 2025

Air Law

1.5.3.3 Transponder

The response from the aircraft's transponder consists of four digits between 0

and 7. There are 4096 different responses from the aircraft, depending on the

digits set by the pilot.

Here is a picture showing a

transponder.

If the switch is set to OFF, the transponder is turned off.

The switch is set to STB (Standby) when we reach that point in the checklist

during the aircraft's electrical equipment startup.

In this position, the transponder is warmed up and ready when we are on the

runway before takeoff, and the transponder is set to ON. In the ON position,

the transponder sends the four digits that were set in the four windows. The

transponder sends a secondary reply signal in mode A (ALPHA).

If the switch is set to ALT, the transponder sends the aircraft’s altitude

information with the reply signal. However, this only happens if the aircraft is

equipped with an encoder that provides the transponder with the aircraft’s

pressure altitude (altitude relative to 1013 hPa). When the transponder sends

altitude information, it is in mode C (CHARLIE).

The IDENT function makes the transponder signal flash on the air traffic

controller's screen for identification purposes. This function must only be used

if instructed to do so.

If an aircraft is equipped with a transponder, it must be on during flight. If the
transponder is in mode C, it must be in the ALT position and activated during

flight.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 70