Bovingdon stack

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The Bovingdon stack is a section of airspace to the north west of London where inbound planes to London Heathrow Airport, which is 20 miles (30 km) to the south, are held. It is a busy example of a hold. It extends above the village of Bovingdon and the town of Chesham, and requires the VOR navigational beacon BNN which is situated on the former RAF airfield at Bovingdon. At busy times on a clear day a dozen planes may be seen circling overhead. Other holding patterns serving Heathrow are at Biggin Hill, Kent (BIG - SE Arrivals), Lambourne, Essex (LAM - NE Arrivals) and Ockham, Surrey (OCK - SW Arrivals), where inbound aircraft will normally use the pattern closest to their arrival route. They can be visualised as invisible helter skelters in the sky.

The stack descends in 1000ft (300m) intervals from 13,000ft (4km) down to 7000ft (2100m).

On December 1, 2003, at 6 am, a major disaster in the stack was narrowly avoided. An air traffic controller was blamed by a later enquiry for misdirecting traffic when he ordered a United Airlines Boeing 777 into a level of the Bovingdon Hold (or stack) already occupied by a similar British Airways plane. The two planes, carrying 500 passengers, flew within 600 vertical feet of each other.