Transponder code

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Transponder codes are four digit numbers transmitted by the transponder in an aircraft in response to a secondary surveillance radar interrogation signal to assist air traffic controllers in traffic separation. A transponder code (often called a squawk code) is assigned by air traffic controllers for operation under IFR or a VFR flight following. Most squawk codes are simply numbers assigned by ATC but there are some that have their own meaning and are used when radio communication is impractical or impossible.

Routine codes

1200: VFR flight, this is the standard squawk code used in North American airspace when no other has been assigned.
7000: VFR standard squawk code for most European airspace.
7004: Aerobatic and display code.
0021: VFR squawk code for German airspace (5000 feet and below)
0022: VFR squawk code for German airspace (above 5000 feet)
0001: Military code for highspeed uncontrolled (non-ATC directed) flight (US)
7001: Sudden military climbout from low-level operations (UK)
2000: The code to be squawked when entering an SSR area from a non-SSR area (used as a VFR squawk code in some European countries)
0000: military escort (in the US), suspected transponder failure (in the UK).

Emergency codes

If a plane squawks any of these codes, it officially declares an emergency. Warning alarms will go off at nearby ATC centers and the aircraft will be given immediate priority over all other non-emergency traffic.

7500: Unlawful interference (hijacking) code. A plane squawking this code will be given any assistance requested.
7600: Lost Communications. Used when the primary VHF communications radio(s) are not functioning, preventing verbal contact with ATC. ATC may then attempt to communicate with the aircraft via aviation light signals.
7700: Onboard Emergency ("Distress" or "Urgency" conditions requiring the plane to get on the ground as soon as possible; i.e. those that would merit a Mayday call, an engine failure, medical emergency, etc.)

Other aspects

Squawk codes are 4-digit octal numbers; the dials on a transponder read from 0 to 7 inclusive. Thus the lowest possible squawk is 0000 and the highest is 7777. Because these squawks are sensitive, care must be taken not to squawk 7500, 7600 or 7700 during a squawk code change. For example, when changing from 1200 to 6501, one might turn the second wheel to a 5, and then rotate the first wheel backwards in the sequence 1-0-7-6 to get to 6. This would momentarily have the transponder squawking 7500, which might lead to more attention than one desires, including a possible interception by a fighter jet. Modern digital transponders are operated by buttons and avoid this problem.

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