This subject was covered in
GASIL 4 of 2002
Transponder use in the circuit
For many years we have been advising pilots to use their transponders at all times, “squawking” both mode A and mode C, except in the visual circuit pattern. With the increasing use of airborne collision avoidance systems (ACAS) in commercial aircraft, it has been decided to update the published procedures. The UKAIP is about to be amended to reflect these.
Whenever possible, pilots with a serviceable transponder should select and transmit code 7000 with altitude reporting (mode C) at all times, unless one of the following conditions apply:
• An air traffic service unit has allocated a different code
• A Special purpose code (such as the distress code of 7700) is more appropriate
• The aircraft is operating within a visual circuit and local procedures require the transponder to be set to standby
The change means that while previously pilots were asked to switch the transponder to STANDBY when in an aerodrome traffic pattern, unless instructed otherwise, we are now asking you to keep it on ALT (ON with mode C) unless instructed otherwise.
RC