Inbound aircraft to London Heathrow Airport typically follow one of a number of Standard Terminal Arrival Routes (STARs).
The STARs each terminate at one of four different RNAV waypoints (co-located with VOR navigational aids), and these also define four "stacks"[1] 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.
[3] At 6 am on 1 December 2003, a major disaster in the stack was narrowly avoided when two planes, carrying a total of 500 passengers, flew within 600 feet (180 m) of vertical clearance.
An air traffic controller was blamed by a later inquiry for misdirecting traffic when he ordered a United Airlines Boeing 777 into a level of the Bovingdon Hold already occupied by a similar British Airways aircraft.