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Last Updated: Friday, 18 January 2008, 17:15 GMT
Landing at Heathrow
Heathrow approach flight paths

Up to 700 aircraft arrive every day at London's Heathrow Airport, the world's busiest international airport.

In order to land safely, air traffic controllers and pilots follow a set procedure.

The landing sequence generally starts with incoming flights being directed to one of four holding "stacks" on the outskirts of London.

Within the stacks, the aircraft are kept 1,000 vertical feet (305m) apart and at least 7,000ft (2,133m) above the ground.

The number of stacks in use and the number of aircraft in each stack depends on how busy the airport is at the time and where the flights are coming in from.

The four stacks used by Heathrow are located over navigation beacons at Bovingdon, Lambourne, Ockham and Biggin.

When air traffic control is ready for an aircraft to land, the pilot is instructed to move out of the holding pattern and begin the descent to the airport.

Graphic showing Heathrow final approach

There is no set route between the stack and the start of the final approach because there are many variables for air traffic control to consider, such as weather and the position of other aircraft.

However, by the time the aircraft is within 7.5 nautical miles (13.9km) of touching down, it must have lined up with a guide known as the Instrument Landing System.

This is a ground-based system which, among other things, sends radio signals to the plane to guide it onto the centre of the runway at a shallow descent of 3 above horizontal.

This equates to the aircraft being approximately 2,500ft (762m) above the ground at 7.5 nautical miles away from landing.

At night, the distance for joining ILS is increased and pilots must be "established" on the trajectory by 10 nautical miles (18.5km) out.

Pilots generally turn off the plane's auto-pilot system (which is separate from ILS) when they are about a minute away from landing and make the final touchdown by hand.

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