What happened
On 27 March 1977, several flights were diverted to Los Rodeos Airport in Tenerife due to a bomb explosion and subsequent terminal closure at Las Palmas. Among the diverted aircraft were KLM flight 4805, a Boeing 747 with registration PH-BUF, and Pan Am flight 1736, a Boeing 747 with registration N736PA.
As the aircraft moved through the airport, the crew of the Pan Am flight found their path blocked by the KLM aircraft on a taxiway. While waiting for the KLM plane to move, the Pan Am crew received instructions from the tower to taxi down the active runway. Simultaneously, the KLM crew requested permission to backtrack on the same runway for departure.
During the taxiing process, a series of communications occurred between the tower and both aircraft. The controller instructed the KLM flight to continue to the end of the runway to backtrack, but the KLM crew expressed uncertainty regarding which taxiway to use. Despite these exchanges, the KLM aircraft began its takeoff roll. Approximately 20 seconds after releasing the brakes, the Boeing 747 (PH-BUF) collided with the Pan Am aircraft, which was still occupying the runway. The collision resulted in 248 fatalities on the KLM flight and 335 fatalities on the Pan Am flight, with 61 survivors from the latter aircraft.
Findings
Investigations concluded that the primary cause of the disaster was a critical misunderstanding between the tower controller and the KLM crew regarding takeoff clearance. The KLM pilot proceeded with the takeoff under the mistaken belief that authorization had been granted. This error was compounded by the fact that the Pan Am aircraft remained on the runway after failing to exit at the correct intersection.