UK AIP
London Heathrow
EGLL AD 2.20 Section 2 a
viii Heathrow Airport is equipped with an advanced surface movement radar utilising Mode-S.
1 Aircraft operators intending to use London Heathrow Airport should ensure that Mode S transponders are able to operate when the aircraft is on the ground
2 Flight crew should:
Select XPNDR or the equivalent according to specific installation, AUTO if available, not OFF or STDBY, and the assigned Mode A code.
From the request for push back or taxi whichever is earlier;
After landing, continuously until the aircraft is fully parked on stand
After parking the Mode A code 2000 must be set before selecting OFF or STDBY.
3 Flight crew of aircraft equipped with Mode S having an aircraft identification feature should also set the aircraft identification. This setting is the aircraft identification specified in Item 7 of the ICAO ATC Flight Plan. The aircraft identification should be entered from the request for push back or taxi, whichever is earlier, through the FMS or the Transponder Control Panel.