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Transponder codes are four digit numbers broadcast 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.

• 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 of European airspace.
• 0021: VFR squawk code for German airspace (5000 feet and below)
• 0022: VFR squawk code for German airspace (above 5000 feet)

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 NORAD and the aircraft will be given immediate priority over all other non-emergency traffic.

• 7700: Basic in-flight emergency code. This code will cause alarms to go off at all stations that pick it up and grant immediate attention from air traffic controllers monitoring the area.
• 7600: No radio. This code lets controllers know that a radio failure has occurred on the plane. Planes with a radio failure are given priority over other, non-emergency traffic, and ATC will communicate with them via aviation light signals.
• 7500: Unlawful interference (hijacking) code. A plane squawking this code will be given any assistance requested. If the plane alternates between 7500 and 7700 rapidly, it means a request for immediate, armed intervention in the hijacking.

• 0000: military escort.

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 the 7000 series squawks are so sensitive in North American airspace, care must be taken not to squawk 7500, 7600 or 7700 during a squawk code change. For example, when changing from 1200 (normal VFR squawk) to 6501 (an assigned ATC squawk), 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.

• Aviation