Inbound aircraft to London Heathrow Airport typically follow one of four Standard Arrival Routes. The STARs each terminate at a different radio beacon or RNAV waypoint, and these also define four "stacks" where aircraft can be held, if necessary, until they are cleared to begin their approach to land. Stacks are sections of airspace where inbound aircraft will normally use the pattern closest to their arrival route. They can be visualised as invisible helter skelters in the sky. Each stack descends in 1000 ft intervals from 16,000 ft down to 8000 ft. Aircraft hold between 7,000 feet and 15,000 feet at 1,000 foot intervals. If these holds become full, aircraft are held at more distant points before being cleared onward to one of the four main holds.
The Bovingdon stack is for arrivals from the north west. It extends above the village of Bovingdon and the town of Chesham, and requires the VORnavigational beacon BNN which is situated on the former RAF Bovingdon airfield. At busy times on a clear day a dozen planes may be seen circling overhead. In more recent times, the co-located RNAV waypoint BOVVA has been designated an official replacement when the Bovingdon VOR is unserviceable.
The Biggin Hill stack on the south east edge of Greater London is for arrivals from the south east. In more recent times, the co-located RNAV waypoint WEALD has been designated an official replacement when the Biggin Hill VOR is unserviceable.
The Lambourne stack in Essex is for arrivals from the north east. In more recent times, the co-located RNAV waypoint TAWNY has been designated an official replacement when the Lambourne VOR is unserviceable.
The Ockham stack in Surrey is for arrivals from the south west.
Incidents
On 1 December 2003 at 6am, a major disaster in the stack was narrowly avoided. An air traffic controller was blamed by a later inquiry for misdirecting traffic when he ordered a United AirlinesBoeing 777 into a level of the Bovingdon Hold already occupied by a similar British Airways plane. The two planes, carrying 500 passengers, flew within 600 vertical feet of each other.
