Transponder
- Transponder A: sends back a transponder code to ATC.
- Transponder C: sends a transponder code plus altitude information to ATC.
- Transponder S: sends a transponder code plus altitude information to ATC, and receives and reports information to other transponders of the same type.
Code 7600. Code 7600 is the code for a loss of radio. If a pilot's radio goes out during a fight, they can enter 7600 into the transponder. Then they should proceed to their intended airport following a standard landing plan. Pilots should be extra careful and on alert for other aircraft around them in the sky.
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7000. ICAO. VFR standard squawk code when no other code has been assigned. EASA countries. Code that pilot shall set when not receiving air traffic services, unless otherwise prescribed by the competent authority.
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For example, 1200 in the USA means that the flight is flying under Visual Flight Rules (VFR) and is not typically in direct contact with ATC. 1200 is a shared code so you may see many aircraft transmitting it at the same time in a given area.
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Aircraft in an emergency, who are not receiving an air traffic service should set the transponder to EMERGENCY (Mode 3A Code 7700) as part of their initial actions. A pilot may select Mode 3A Code 7700 whenever he has specific reason to believe that this would be the best course of action [ ICAO Doc 8168 Vol 1.]
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