Page 407 - PPL BEG-engelsk

Communication

4.10.3.2 SSR radar

Hence, the SSR Radar system (Secondary Surveillance Radar) was invented,

where the aircraft is equipped with a transponder that sends a response back

to the radar when it picks up the primary radar signal.

4.10.3.3 The transponder

The response from the transponder in the aircraft consists of 4 digits between

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

which digits the pilot has set the transponder to.

You can see a picture of a

transponder here.

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

We set the selector to SBY (Stand By) when we start the electrical systems, as

per the checklist. In this position, the transponder warms up, so it is ready when

we set it to ON on the runway, just before takeoff.

In the ON position, the transponder sends the 4 digits displayed on the

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

If the selector is set to ALT, the transponder sends the aircraft's altitude with

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

encoder that provides the transponder with information about the aircraft's

pressure altitude (altitude relative to 1013 hPa).

When the transponder sends altitude information, we say it is in mode C

(CHARLI).

If the aircraft is equipped with a transponder, it must be on during flight. If the

transponder is in mode C, it must be set to ALT.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© marts 2026 407