3.4_ Transponder Codes

A Transponder code, also known as a Squawk code, is a unique four digit octal code (8's and 9's are not allowed) which the pilot's radar transponder should be set to, in order for him to show up correctly on the radar.

In controlled airspace ATC will assign you a Squawk code on clearance.
If in uncontrolled airspace, or when ATC has not assigned you a Squawk code the standard code for IFR flight is 2000 (this may vary according to the country).
If flying VFR in north america the Squawk code 1200 should be used, in most of Europe 7000 is used in place of 1200.

3.4.1_ Transponder modes

The aircraft's transponder has several modes:

OFF: self explanatory

Mode Standby: The first notch... displays code only, used on the ground. In this mode the transponder is not transmitting your Squawk code.

Mode Alpha: The second notch... Transmits aircraft IDonly, but no altitude information. when Squawking mode A a little light will turn on and off from time to time on the transponder. (When flying online squawking mode A is identical to squawking mode C)

Mode Charlie: The third notch... Displays a data box beside your blip on ATC radar (typically including your current altitude and ground-speed). Set transponder to mode C only when holding short of the active runway, ready for departure, or as instructed by ATC. when Squawking mode C a little light will turn on and off from time to time on the transponder.

Mode Delta: The fourth notch... Used for testing only, in mode D the transponder constantly transmits an ident signal.

Squawk Ident: ATC may instruct you to "squawk ident". in this case press the Ident button located on the transponder, your radar blip will flash on ATC radar, and the transponder light should turn on (steady) for a few seconds.

3.4.2_ Reserved Squawk codes:

Some squawk codes have been reserved for special use, using these codes set off an alarm and inform ATC you have a problem.
code 7500 is used in the case of a hijack situation - NOT to be used when flying online.
code 7600 is used in the case of a communication failure.
code 7700 is used in the case of a serious emergency - if you lose one engine it's not an emergency. If you lose both engines it's a different story...
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