section_path: Operations/Arrival stacks
section_name: Arrival stacks
paragraph_text: Arrival stacks
  
  Inbound aircraft to London Heathrow Airport typically follow one of several Standard Arrival Routes (STARs). The STARs each terminate at one of four different VOR installations, 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 a helix in the sky. Each stack descends in 1,000 feet (305 m) intervals from 16,000 feet (4,877 m) down to 8,000 feet (2,438 m). Aircraft hold between 7,000 and 15,000 feet (2,134 and 4,572 m) 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 following four stacks are currently in place:
  
  In high-traffic situations, air traffic controllers can opt to use a number of RNAV STARs either to send traffic to a non-standard stack or to move traffic from one stack to another. These are not allowed to be used for flight planning and will be assigned by ATC tactically.