Heathrow is the busiest two-runway airport in the world, welcoming around 1,300 combined take-offs and landings each day. With safety, weather, scheduling, aircraft technology, airspace and noise reduction factors all to take into account, landing an aircraft at the UK’s busiest airport isn’t as simple as it seems.
Efforts to limit noise mean that fewer people are affected by noise today than at any time since the 1970s, however, noise remains an issue for people living near or under the various flight paths used for take-offs and landings at the airport. On average there are around 650 arrivals into Heathrow each day with planes arriving into Heathrow from 04:30.
Because Heathrow is so busy, most planes coming into land at Heathrow wait in ‘holding stacks’. The stack acts as a waiting room, allowing NATS air traffic controllers to efficiently organise planes for landing.
There are four stacks around Heathrow called Bovingdon, Lambourne, Ockham and Biggin. The UK Government sets their locations and they have been in the same places since the 1960s.
Once the planes leave the holding stack they are directed by NATS air traffic controllers to the final approach into Heathrow. The controllers sequence the planes from all four stacks into a single stream of traffic and guide them safely onto one of Heathrow’s two runways.
There are no set routes for planes moving from the holding stacks to the final approach for landing and so the position of aircraft in the skies will vary from day to day.
Factors such as how busy the stack is, weather conditions, or the position of other aircraft on route into Heathrow will impact how aircraft are sequenced by air traffic controllers to leave the stack and make their way to the final approach. However, the overall patterns have remained similar for many years.
Continuous Descent Approaches
An arrivals procedure known as ‘Continuous Descent Approaches’ has been in operation at Heathrow for many years. This procedure involves aircraft maintaining a steady angle of approach when landing at the airport, as opposed to approaches which involve prolonged periods of level flight. The intention of a CDA is to keep aircraft higher for longer, thereby reducing arrival noise.
We report daily on our Operational Data website the percentage of arrivals that performed Continuous Descent Approaches. On average around 85% of aircraft coming into Heathrow use CDA and this is consistent with previous years.
The final approaches into Heathrow’s runways are fixed flight paths that extend about 29 miles from the end of each of the airports two runways. Aircraft follow a radio beam known as the Instrument Landing System (ILS).
There is one beam aligned with the centrelines of each of the runways (northern and southern) to ensure aircraft approach in a straight line as they come into land, consequently these fixed approach paths haven’t moved.
We’ve recently begun a process of replacing the ILS antennae at the end of each runway with new enhanced systems known as eILS.
Heights of arriving aircraft
Once aircraft reach the final approach they cannot lose too much altitude as they need to be at a certain height when they join the final approach into Heathrow. The angle of landing for the final approach is set at 3° and as a result aircraft will be at a set height for distance from the runway.
There are rules laid down in the Airport’s AIP (Aeronautical Information Publication) which states that the minimum height at which aircraft can join the ILS during the day (between 6am and 11pm) is 2,500ft which approximately 7.5 nautical miles from Heathrow. At night (11pm to 6am) they must be no lower than 3,000ft at 10 nautical miles from Heathrow.
As the ILS beam extends about 29 miles out, aircraft can join the final approach at any point after the distances mentioned above. However, this will vary depending on how aircraft are sequenced each day by the controllers.
To test whether the implementation of steeper approaches of up to 3.5 degrees at the airport is possible, Heathrow ran a steeper approach angle trial between September 2015 and March 2016. The trial tested an approach angle of 3.2 degrees and the results are now being analysed with a final report due later on this year.
Heathrow is heavily restricted by the Government in terms of the numbers and types of aircraft that are allowed to operate between 23:30 and 06:00.
Between 11:30pm and 6am Heathrow is restricted to 5,800 night-time take-offs and landings a year. There is also a night quota limit, which caps the amount of noise the airport can make at night.
Around 80% of the night flights at Heathrow are between 4.30-6am with on average around 16 aircraft arriving each day between these hours. Heathrow also has a voluntary ban in place that prevents flights scheduled between 4:30am-6am from landing before 4:30am.
We also do not schedule any departures between 11pm and 6am.
Late running arrivals and weather affects
Unfortunately sometimes extreme weather conditions have an impact on our operations which can mean that the spacing between arriving aircraft has to be increased for safety reasons.
The knock-on effect of this means that the normal schedule is significantly impacted, therefore in order for us to recover from this disruption, some arrivals are allowed to land later than usual. In exceptional circumstances like this, we have to be granted dispensation by the Department for Transport (DfT) in order to operate flights during these hours.
On occasions, planes can also be delayed coming into land at Heathrow due to weather conditions or issues at the airport they are flying from.