Radar transponders

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

Standard transponder codes

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.