In this mode, aircraft depart towards the west and approach from the east over London, thereby minimising the impact of noise on the most densely populated areas. Heathrow's two runways generally operate in segregated mode, whereby arriving aircraft are allocated to one runway and departing aircraft to the other. To further reduce noise nuisance to people beneath the approach and departure routes, the use of runways 27R and 27L is swapped at 15:00 each day if the wind is from the west. When landings are easterly there is no alternation; 09L remains the landing runway and 09R the departure runway due to the legacy of the now rescinded Cranford Agreement, pending taxiway works to allow the roles to be reversed. Occasionally, landings are allowed on the nominated departure runway, to help reduce airborne delays and to position landing aircraft closer to their terminal, reducing taxi times.
Night-time flights at Heathrow are subject to restrictions. Between 23:00 and 07:00, the noisiest aircraft (rated QC/8 and QC/16) cannot be scheduled for operation. In addition, during the night quota period (23:30–06:00) there are four limits: