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