Arrivals into Heathrow - The Holding Stack

London's Heathrow airport is the fifth busiest airport in the world in terms of total passenger traffic (67 million annual passengers, approximately), and handles nearly 700 landings each day. It is located about 12 nautical miles west of central London, and has two parallel east-west oriented runways (there were six shorter runways earlier, arranged in three pairs at different angles, but this arrangement has now been done away with). Most of the time, airplanes landing at Heathrow have to fly over the city, due to the nature of prevailing winds, which generally require an approach from east to west (a landing on the west-facing runway). On days on which the winds favour the use of the east-facing runway, the airplanes climb out over the city, before proceeding on course. This, in recent years, has raised concerns about air pollution, safety, noise pollution, and the impact of Heathrow on its surroundings.

Heathrow is about 12 nautical miles to the west of central London.

Normally, airplanes arriving into Heathrow are directed by controllers to one of four holding "stacks", each located over a navigation beacon. These navigation beacons, and hence the holding stacks, are located at Bovingdon, Lambourne, Ockham, and Biggin, all on the outskirts of London. Sometimes, depending on traffic load, airplanes may first be directed to an "outer stack" (eg. Daventry), before proceeding to one of the above "inner stacks".

Location of "inner stacks" at Bovingdon, Lambourne, Ockham, and Biggin.

Arrivals are first directed to the top of the stack until controllers are able to clear traffic in lower layers of the stack. As lower levels of the stack clear up, airplanes holding at higher levels are moved down in sequence. The height and number of levels in each stack, and the number of stacks in use all depend on how busy the terminal environment is and the pattern of arrivals into the terminal area. The lowest level will be at least 7000 feet above the ground, and adjacent levels are separated by a minimum of 1000 vertical feet. Traffic and other factors permitting, controllers will vector the aircraft onto the final approach course when the aircraft reaches an appropriate altitude/level in the stack.

Leaving the stack for final approach course and landing.

As mentioned earlier, there have been environmental concerns regarding the current system of handling arrivals into Heathrow. While there have been some organizations that have suggested measures like banning night flights, altering flight paths, extending runways, reducing the number of short-haul flights, and imposing passenger duty on transfer passengers, there have been others that have proposed changes to airspace management and handling procedures.

HEART1A concept proposed by Environmentally Responsible Air Transport (ERAT) that makes use of just two holding stacks, instead of the existing four.