On March 27, 1977, two Boeing 747s — KLM Flight 4805 and Pan Am Flight 1736 — collided on the runway at Los Rodeos Airport in Tenerife, in the Canary Islands. The airport was crowded that day because a bomb explosion at Gran Canaria Airport had diverted dozens of flights, including both 747s, to the smaller Tenerife airfield, where they waited in a mass of parked aircraft.
Fog had settled over the airfield, and neither crew could see the other aircraft. The KLM jet, with 248 people aboard, began its takeoff roll while the Pan Am jet, carrying 396 people, was still taxiing on the same runway. The KLM captain had not been cleared to take off — air traffic control had cleared him only to line up and prepare for departure — and a radio transmission confirming that Pan Am was not yet clear of the runway was masked by a simultaneous transmission. The two aircraft met at speed in the fog.
All 248 people aboard the KLM aircraft died. On the Pan Am jet, 335 people were killed and 61 survived. The total of 583 dead makes Tenerife the deadliest accident in aviation history.
The Spanish investigation (report A-102/1977, published in 1978) found that the fundamental cause was the KLM captain taking off without clearance. Contributing factors included the fog, the blocked radio transmission, non-standard phraseology, and the congestion that had placed both aircraft on a single runway. Dutch investigators appended their own comments to the report, placing more emphasis on the misunderstandings between the tower and the KLM crew, but the central facts were not in dispute.
The accident drove worldwide changes in how pilots and controllers communicate: the word "takeoff" is now reserved for the clearance itself, clearances must be read back, and crew resource management training — teaching crews to question and confirm decisions — was reshaped around the lessons of Tenerife.