Each is defined by a VOR radio-navi

Each is defined by a VOR radio-navigational beacon. When the airport is busy, aircraft orbit in the associated hold patterns. These holding areas lie to the northwest, northeast, southeast and southwest of the London conurbation. Aircraft hold between 7000 feet and 15000 feet at 1000 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.