Tenerife airport disaster

On March 27, 1977, two Boeing 747 passenger jets, operating KLM Flight 4805 and Pan Am Flight 1736, collided on the runway at Los Rodeos Airport on the Spanish island of Tenerife. Resulting in 583 fatalities, this accident is the deadliest in aviation history.

A terrorist incident at Gran Canaria Airport had caused many flights to be diverted to Los Rodeos, including the two aircraft involved in the accident. The airport quickly became congested with parked airplanes blocking the only taxiway and forcing departing aircraft to taxi on the runway instead. Patches of thick fog were drifting across the airfield, hence visibility was greatly reduced for pilots and the control tower.

Wikipedia