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Radar transponders

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Transponder operation

VFR flights in Class E or G airspace squawk 1200 Mode C (ALT)

Standard transponder codes

Civil VFR flights in Class E or G airspace
Civil IFR flights in Class G airspace
Civil flights in A, C and D airspace, or IFR flights in Class E airspace
Military flights in Class G airspace
Unlawful interference
Communications failure

Important points in transponder operation

  • Select standby (STBY) before changing codes. Otherwise there is the real possibility of transmitting a non-authorised code during the process.
  • Do not press the IDENT feature unless requested by ATS. ‘Squawk’ does not mean press the IDENT. ‘Squawk IDENT’ is the request used for this purpose.
  • ‘Squawk STBY’ means switch to the STBY position.
  • ‘Squawk 5632’ for example, means select STBY, then select code 5632, then squawk ALT.
  • Transponders require a warm up before being selected ON or ALT. The STBY position is used to warm up the transponder.
  • In the TEST position the reply light should come on while the selector is held in this position.
  • The reply light comes on each time the transponder responds to an interrogation. This may be from ground-based secondary radar, or from a nearby TCAS-equipped aircraft.
  • In the ON position no altitude information is being transmitted.
  • On occasions transponders may require ‘recycling’ to restore correct encoding. To recycle, briefly select STBY then return to ALT.
  • A mode S transponder may have a GND selection on the switch. When you are taxiing on the ground, the transformer should be set to GND unless your installation includes a gear squat switch.

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New rules came into effect from 2 December 2021 that cover the general operating and flight rules. This website is now out of date and will be switched off in the new year. A PDF copy of the VFRG is available for download.
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