The following procedures and techniques are set out in full in the UK Aeronautical Information Publication (UK AIP) and form part of the noise policy framework set by the Department for Transport (DfT).
Noise preferential routing
Aircraft departing from Heathrow are required to follow specific paths called noise preferential routes (NPRs) up to an altitude of 4,000ft, unless directed otherwise by air traffic control (ATC).
NPRs were designed to avoid overflight of built-up areas where possible. They lead from the take-off runway to the main UK air traffic routes.
Each NPR has a swathe extending 1.5 km each side of the nominal NPR centre line, within which aircraft are considered to be flying on track. This takes account of various factors that affect track-keeping, including tolerances in navigational equipment, type and weight of aircraft, and weather conditions – particularly winds that may cause drifting when aircraft are turning.
Aircraft reaching 4,000ft at any point along an NPR may be turned off the route by ATC onto more direct headings to their destinations – a practice known as ‘vectoring’. ATC may also vector aircraft from NPRs below 4,000ft for safety reasons, including in certain weather conditions – for example to avoid storms.
The NPRs have not been altered since the 1960s. Stability is important, so that people know where aircraft noise will be experienced. Significant changes to the NPRs would be subject to public consultation by the Government.
The frequency with which any particular NPR is used will vary, and is an operational decision for ATC, taking account of the final destination of individual flights together with other considerations such as overall air traffic and weather conditions, both locally and along intended routes.
After take-off the aircraft shall be operated in such a way that it is at a height of not less than 1,000ft aal (above aerodrome level) at 6.5 km from the start of roll, as measured along the departure track of that aircraft.
This encourages aircraft operators to gain height as quickly as possible and then reduce engine power and noise at the earliest opportunity.
This point was also chosen as few residential areas lay closer to major airports than that, and this would result in a noise benefit for residents who live farther from the airport.