Air traffic control
Upon arrival in the Northolt area both aircraft were placed in a stacking pattern, which in addition to the two involved aircraft included two other aircraft at higher altitudes. Each stack had an intermediate distance of 500 feet. The stacking was regulated by the air traffic control of Metropolitan Zone. Any aircraft entering the stack had to follow orders from air traffic control, which indicated their altitudes and route, and issued gates allowing the aircraft to enter and leave. Air traffic control issued atmospheric pressure measurements (QFE), allowing the aircraft to synchronise their altimeters.