Page 406 - PPL-engelsk 2025

Communication

In this position, the transponder warms up, so it is ready when we set it to ON

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.

It has been agreed that:

- aircraft flying VFR set the code 7000.

- aircraft is in an emergency; the code 7700 is set.

- aircraft has a radio failure; the code 7600 is set.
- aircraft is subject to unlawful interference (hijacking), the code 7500 is set.

Today, new transponders send the aircraft's call sign, for example, OY-CTZ. This

is known as mode S (SIERRA).

The newest system provides flight controllers with this information directly on

their computer screens, so they are immediately aware of the aircraft's

identity, flight altitude, and whether it is climbing or descending.

Additionally, computer systems generate vectors showing the aircraft's future

position based on current flight direction and speed. The flight controller can

then see if conflicting traffic is or will be present.

Flight Theory PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 406