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In the ON position, the transponder sends the 4 digits that have been turned into

the 4 windows. The transponder sends a secondary response signal in mode A

(ALPHA).

If the selector is set to ALT, information about the aircraft's altitude is sent with

the response 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

information about altitude, we say that it is in mode C (CHARLI).

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

If the transponder has mode C, it must be in the ALT position.

9.10.4.4 Squawk codes

It has been agreed that aircraft that:

- are flying VFR, set the code 7000

- are in an emergency, set 7700
- have radio failure, set 7600

- are subject to an illegal act (hijacking), set 7500.

9.10.4.5 Transponder modes

The transponder has different modes

Mode A (ALPHA): In the ON position, the transponder sends the 4-digit

identification transponder code (Squawk code) that has been entered.

Mode C (CHARLI): If the selector is set to the ALT position, information about the

aircraft's pressure altitude is sent with the transponder code.

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 in relation to 1013 HPa).

Flyveteori PPL(A)(UL)/LAPL Henning Andersen, Midtjysk Flyveskole© 2025 902