43 Years ago today: On 27 March 1977 a KLM Boeing 747 and a PanAm Boeing 747 collided on the Tenerife, Spain runway in fog; killing all 248 KLM occupants and 335 out of 396 PanAm occupants [world’s worst air accident]

Date: Sunday 27 March 1977
Time: 17:06
Boeing 747-121
Operator: Pan American World Airways (Pan Am)
Registration: N736PA
C/n / msn: 19643/11
First flight: 1969-12-24 (7 years 3 months)
Total airframe hrs: 25725
Cycles: 7195
Engines: Pratt & Whitney JT9D-7A
Crew: Fatalities: 9 / Occupants: 16
Passengers: Fatalities: 326 / Occupants: 380
Total: Fatalities: 335 / Occupants: 396
Collision casualties: Fatalities: 248
Aircraft damage: Destroyed
Aircraft fate: Written off (damaged beyond repair)
Location: Tenerife-Los Rodeos International Airport (TCI) (   Spain)
Crash site elevation: 632 m (2073 feet) amsl
Phase: Taxi (TXI)
Nature: Int’l Non Scheduled Passenger
Departure airport: Tenerife-Los Rodeos International Airport (TCI/GCXO), Spain
Destination airport: Las Palmas-Airport de Gran Canaria (LPA/GCLP), Spain
Flightnumber: PA1736

At 12:30 a bomb explodes in the Las Palmas passenger terminal. Because of warnings of a possible second bomb, the airport was closed. A large number of flights were diverted to Tenerife, a.o. KLM Flight 4805 from Amsterdam and PanAm Flight 1736 (coming from Los Angeles and New York).
Las Palmas Airport opened to traffic again at 15:00.
Because the PanAm passengers remained on aboard it was possible to leave Tenerife at once. The taxiways were congested by other aircraft however. This meant the PanAm crew had to backtrack on runway 12 for takeoff on runway 30. The entrance to runway 12 however, was blocked by the KLM Boeing. The PanAm flight had to wait for almost 2 hours before all KLM passengers (except 1) had reboarded and refueling had taken place.
The KLM flight was then cleared to backtrack runway 12 and make a 180deg. turn at the end. Three minutes later (at 17:02) Pan Am 1736 was cleared to follow the KLM aircraft and backtrack runway 12. The PanAm crew were told to leave the runway at the third taxiway and report leaving the runway. At 17:05:44
KLM 4805 reported ready for takeoff and was given instructions for a Papa beacon departure. The KLM crew repeated the instructions and added “We are now at takeoff”. The brakes were released and KLM 4805 started the takeoff roll.
Tenerife tower, knowing that Pan Am 1736 was still taxiing down the runway replied “OK …… Stand by for takeoff, I will call you.” This message coincided with the PanAm crew’s transmission “No … uh we’re still taxiing down the runway, the Clipper 1736”. These communications caused a shrill noise in the KLM cockpit, lasting approx. 3.74 seconds.
Tenerife tower replied: “Papa Alpha 1736 report runway clear.”, whereupon the PanAm crew replied: “OK, will report when we’re clear”. This caused some concerns with the KLM flight engineer asking the captain: “Is he not clear then?” After repeating his question the captain answers emphatically: “Oh, yes”.
A number of second before impact the KLM crew saw the PanAm Boeing still taxiing down the runway. The crew tried to climb away and became airborne after a 65 feet tail drag in an excessive rotation.
The PanAm crew immediately turned the aircraft to the left and applied full power. The KLM aircraft was airborne, but the fuselage skidded over the PanAm’s aft fuselage, destroying it and shearing off the tail. The KLM aircraft flew on and crashed out of control 150 m further on, sliding another 300 m bursting into flames.

Probable Cause:

PROBABLE CAUSE: “The KLM aircraft had taken off without take-off clearance, in the absolute conviction that this clearance had been obtained, which was the result of a misunderstanding between the tower and the KLM aircraft.
This misunderstanding had arisen from the mutual use of usual terminology which, however, gave rise to misinterpretation. In combination with a number of other coinciding circumstances, the premature take-off of the KLM aircraft resulted in a collision with the Pan Am aircraft, because the latter was still on the runway since it had missed the correct intersection.”