The following procedures and techniques are set out in the UK Aeronautical Information Publication (UK AIP) and form part of the noise policy framework set out by the Department for Transport (DfT).
Continuous descent approach
Subject to safety requirements, one of the main noise abatement measures identified is continuous descent approach (CDA).
A CDA is a noise abatement technique of flight during which a pilot descends at a rate with the intention of achieving a continuous descent to join the final approach at the correct height for the distance.
This avoids the need for extended periods of level flight. The intention is to keep aircraft higher for longer, using reduced thrust and thereby reducing arrival noise.
A CDA is not a precise art and relies on the accuracy of track miles to be provided to the flight crew by Air Traffic Control (ATC), pilot skill, weather and operational circumstances. Additionally, different aircraft types perform differently, requiring varying operating practices to be able to slow the aircraft down and meet speed restrictions. The requirement to fly a CDA is therefore not compulsory; rather, it is to fly a CDA whenever practicable.
There is often a requirement for a period of level flight prior to joining the Instrument Landing System (ILS) to meet the ICAO approach of joining the ILS from below and for ATC speed and sequencing requirements. There are no set height requirements for CDAs.
The CDA achievement levels are regularly reported back to our Noise and Track Keeping Working Group and the Heathrow Airport Consultative Committee, as well as to the Flight Operations Performance Committee (FLOPC), which includes airline and ATC representatives.
A voluntary code of practice is in place which was compiled by a group representing us, airlines, NATS, CAA and the DfT, and is primarily concerned with Heathrow, Gatwick and Stansted airports. The code encourages air traffic controllers and pilots to fly a CDA from 6000ft. The code was originally produced in 2002 and later revised in November 2006.
As well as reducing noise, CDAs reduce fuel burn, thereby cutting emissions and producing an overall environmental benefit.
Joining point rules
There are rules which dictate at which point aircraft must be established on the Instrument Landing System before being able to begin coming down further.
At night the requirement is 500ft higher than during the daytime. During the day and at night aircraft are not permitted to fly below the glideslope, ie they must maintain a certain height for their distance from touchdown.
The aim of these rules is to keep aircraft higher for longer and avoid prolonged periods of level flight at low levels.