Updated: Aug 29, 2019
It is 1977 and there is a lot of tension in the Canary Islands. In 1964 a Canarian lawyer, Antonio Cubillo, started the violent, “Canary Islands Independence Movement” (CIIM), in an attempt to force the Spanish government to create an independent state in the Canary Islands. Its armed wing was called the Fuerzas Armadas Guanches (FAG) which wanted to avenge the deaths of two of the CIIM activists, at the hands of the Spanish security forces.
They started a series of bomb attacks. The first was in 1976 in Las Palmas, and then, after officially declaring armed struggle to the Spanish government, they blew up the Galerias Preciados in Madrid. After this series of attacks they decided to aim at a more special target, to receive much more exposure: the international airport of Gran Canaria, also known as “Las Palmas”.
As a result, CIIM terrorists bombed a florist shop at the airport on 27 March 1977, injuring eight people, and then threatened to explode a second bomb in the airport, forcing police to shut down air traffic while they searched for it. As a safety precaution, the competent authorities decided to close the airport, and all the flights scheduled were diverted to the Los Rodeos airport in Tenerife, 120 km away.
Los Rodeos was a regional airport with only one runway and one parallel taxiway, with four short connections linking the two. It could not easily accommodate all of the traffic diverted from Gran Canaria, which included five large airliners. Among the flights diverted, were 2 Boeing 747s, one operated by KLM from Amsterdam and one by Pan Am from Los Angeles.
The KLM Flight 4805 was a charter flight, and its pilot was one of the carrier's greatest at the time: Jacob Veldhuyzen Van Zanten. His photograph was used for publicity materials such as magazine advertisements, including the in-flight magazine on board. At the time of the accident, Van Zanten was KLM's chief flight instructor, with 11,700 flight hours, of which 1,545 hours were on the 747.
The aircraft was named Rijn (Rhine), and was carrying 14 crew members and 235 passengers, including 52 children.
Pan Am Flight 1736 was operated by an aircraft named Clipper Victor. This particular aircraft had operated the inaugural 747 commercial flight on January 22, 1970. On August 2, 1970, in its first year of service, it also became the first 747 to be hijacked. It was carrying 380 passengers, of which 14 had boarded in New York, where the crew was also changed. The new captain was Victor Grubbs, aged 56, with 21,043 hours of flight time, of which 564 hours were on the 747.
In the meantime, the authorities reopened Gran Canaria airport once the bomb threat had been contained. So, the Pan Am plane was ready to depart from Tenerife, but although it was standing by, the access to the runway was being obstructed by the KLM plane and a refueling vehicle. In fact, the diverted airplanes took up so much space that they were parked on the long taxiway, forcing the departing planes to taxi along the main runway to position themselves for takeoff, a procedure known as a “backtaxi”. The KLM captain, apparently to save time for the next flight, had decided to fully refuel at Los Rodeos instead of Las Palmas, causing about 35 minutes of delay. Unfortunately, during this time, a thick fog covered the airport, limiting visibility to only 500m which later decreased to less than 100m.
After refueling, the KLM flight, started taxiing on the main runway and began to position itself, turning 180 degrees, lining up for the takeoff roll. Shortly afterward, the Pan Am crew was instructed by ATC to follow the KLM down the same runway, but they were told to leave the runway at the third exit using the parallel taxiway. In 1977, at Los Rodeos airport, there were no markings or signs to identify the runway exits and complemented by poor visibility, the Pan Am crew became unsure of its position on the runway. Anyway, the third exit was practically impossible to take for a big plane such as a 747 because of the difficult turns that should have accomplished. So, The Pan Am crew, unaware of its position, imagined that the exit instructed by ATC was the fourth one which was much easier to taxi through. In turn, the Pan Am aircraft was on the runway for longer than expected.
Meanwhile, just after completing the line up on the runway, the KLM captain advanced the throttles and the aircraft started moving - the Pan Am plane in front. The first officer of the KLM flight advised Van Zanten that ATC clearance had not yet been given, but he soon replied that he had it already. The first officer, to verify the statement of his captain, read the flight clearance back to the ATC controller, saying: "we are now at takeoff”. Shortly after, the captain interrupted the co-pilot's read-back with the comment, "we're going". The controller, who could not see the runway due to the fog, initially responded with "OK" (non-standard terminology), reinforcing the KLM captain's misinterpretation that they had takeoff clearance. The controller then immediately added "stand by for takeoff, I will call you", indicating that he had not given them the takeoff clearance.
Meanwhile, a simultaneous radio call from the Pan Am crew caused a mutual interference on the radio frequency. This caused the KLM crew to miss the crucial portion of the controller's response. The Pan Am crew's transmission was "We're still taxiing down the runway, the Clipper 1736!". This message was also blocked by the interference and inaudible to the KLM crew. Due to the fog, neither crew was able to see the other plane on the runway ahead of them. In addition, neither of the aircraft could be seen from the control tower, and to make matters worse, the airport was not equipped with ground radar.
The tower instructed the Pan Am crew to report when the runway was cleared. The Pan Am crew replied: "OK, will report when we're clear". On hearing this, the KLM flight engineer expressed his concern about the Pan Am still being on the runway by asking the pilots in his own cockpit if the Pan Am left the runway or not, but the KLM captain, confidently replied "Oh, yes" and continued with the takeoff roll.
The KLM reached V1 speed, and by the time the KLM pilots saw the Pan Am aircraft still on the runway, they were already traveling too fast to stop. In desperation, the pilots prematurely rotated the aircraft and attempted to lift off, causing a severe tail-strike for 22m. After that, it took off. The KLM was moving at approximately 140 knots when it left the ground. Its nose landing gear didn’t touch the Pan Am, but its left-side engines, lower fuselage, and main landing gear struck the upper right side of the Pan Am's fuselage, ripping apart the center of the jet.
The KLM plane remained airborne for a very brief amount of time, but immediately went into a stall, rolled sharply, and hit the ground, approximately 150 m past the collision and slid down the runway for a further 300m. The full load of fuel on the KLM ignited immediately, into a huge fireball.
Both airplanes were destroyed in the collision. All 248 passengers and crew aboard the KLM plane died. In the Pan Am aircraft, 335 people were killed, primarily due to the explosions caused by the fuel spilled which ignited upon impact. Only 61 people aboard the Pan Am aircraft survived, including the captain, first officer and flight engineer.
The following day, the Canary Islands Independence Movement, responsible for the bombing at Gran Canaria that started the chain of events that led to the disaster, denied any responsibility for the accident.
The investigation started soon and led to the conclusion that the fundamental cause of the accident was that KLM captain Van Zanten started the takeoff roll without adequate ATC clearance. Other major factors contributing to the accident were the sudden fog, that limited visibility, the inability of the control tower and the crews of both planes to see one another and the interference from simultaneous radio transmissions. Furthermore the use of ambiguous non-standard phrases by the KLM crew and ATC and the fact that the Pan Am aircraft had not left the runway at the third intersection, contributed to the chain of events.
This was one of the first accident investigations during which the contribution of "human factors" was studied. Captain Van Zanten, was a KLM training captain and instructor for over 10 years, but had not flown on regular routes during the 12 weeks prior to the accident. In addition, the flight engineer and the first officer hesitated to challenge Van Zanten, because the captain was not only senior in rank, but also one of the most respected pilots working for the airline.
Lastly, the extra fuel taken on by the KLM plane delayed takeoff by an extra 35 minutes, which gave time for the fog to settle in. Moreover, it added over 40 tons of weight to the plane, which increased the takeoff distance and made it more difficult to clear the Pan Am plane when taking off. Finally it increased the size of the fire after the crash, that unfortunately killed everyone on board the KLM plane.
As a consequence of the accident, aviation authorities around the world introduced standard phrases and a greater emphasis on English as a common working language. Air traffic instruction should not be acknowledged just with a colloquial phrase such as "OK" but with a read-back of the key parts of the instruction, to show mutual understanding. The phrase "take off" is now spoken only when the actual takeoff clearance is given. Up until that point, aircrew and controllers should use the phrase "departure". Cockpit procedures were also changed. Hierarchical relations among crew members were played down. More emphasis was placed on team decision-making by mutual agreement.
In the aftermath, the Dutch authorities were initially reluctant to blame captain Van Zanten and his crew, but later on, the airline accepted responsibility for the accident, paying compensations to the victims and their families. A total of 583 people perished, making it one of, if not the deadliest disaster in aviation history.