Flight history

For both planes, Tenerife was an unscheduled stop. Their destination was Gran Canaria International Airport (also known as Las Palmas Airport), serving Las Palmas on the nearby island of Gran Canaria. Both are in the Canary Islands, an autonomous community of Spain located in the Atlantic Ocean off the southwest coast of Morocco.

Pan Am Flight 1736 had originated at Los Angeles International Airport, with an intermediate stop at New York's John F. Kennedy International Airport. The aircraft was a Boeing 747-121, registration N736PA, named Clipper Victor. Of the 380 passengers (mostly in retirement age, but also including two children), 14 had boarded in New York, where the crew was also changed. The new crew consisted of Captain Victor Grubbs, First Officer Robert Bragg, and Flight Engineer George Warns; there were 13 other crew members. The same aircraft had operated the inaugural 747 commercial flight on January 22, 1970.

KLM Flight 4805, a charter flight for Holland International Travel Group, had arrived from Amsterdam Airport Schiphol, the Netherlands. Its captain was Jacob Veldhuyzen van Zanten, KLM's chief flying instructor. The First Officer was Klaas Meurs and the Flight Engineer was Willem Schreuder. The aircraft was a Boeing 747-206B, registration PH-BUF, named Rijn (Rhine). The KLM jet had 14 crew members and 235 passengers, including 52 children. Most of the KLM passengers were Dutch; four Germans, two Austrians and two Americans were also on the plane. After the aircraft landed at Tenerife, the passengers were transported to the airport terminal. One of the inbound passengers, who lived on the island, chose not to re-board the 747, leaving 234 passengers on board.

Disaster

Diversion of aircraft to Los Rodeos

Both flights had been routine until they approached the islands. At 1:15 pm, a bomb (planted by the separatist anti-Francoist Fuerzas Armadas Guanches) exploded in the terminal of Gran Canaria International Airport, injuring one person. There had been a phone call warning of the bomb, and soon after another call claimed that a second bomb was at the airport. The civil aviation authorities had therefore closed the airport temporarily after the bomb detonated and diverted all of its incoming flights to Los Rodeos, including the two Boeing 747 aircraft involved in the disaster. The Pan Am crew indicated that they would prefer to circle in a holding pattern until landing clearance was given, but were ordered to divert to Los Rodeos.

In all, five large aircraft were diverted to Los Rodeos, a regional airport that could not easily accommodate them. The airport had only one runway and one major taxiway parallel to it, as well as several small taxiways connecting the main taxiway and the runway. While waiting for Gran Canaria airport to reopen, the diverted aircraft took up so much space that they were parked on the long taxiway, meaning that it could not be used for taxiing. Instead, departing aircraft had to taxi along the runway to position themselves for takeoff, a procedure known as a runway backtaxi or backtrack.

After the threat at Gran Canaria had been contained, authorities reopened that airport. The Pan Am aircraft was ready to depart from Tenerife, but the KLM plane and a refueling vehicle obstructed its access to the runway. The Pan Am aircraft was unable to maneuver around the fueling KLM, reach the runway and depart due to a lack of just 12 ft (3.7 m) of clearance. Captain Veldhuyzen van Zanten had decided to fully refuel at Los Rodeos instead of Las Palmas, apparently to save time. The refueling took about 35 minutes. After that, the passengers were brought back to the plane. The search for a missing family of four delayed the flight even further.

Taxiing and takeoff preparations

Following the tower's instructions, the KLM was cleared to taxi the full length of the runway and make a 180° turn to get into takeoff position. While the KLM was backtaxiing on the runway, the controller asked the flight crew to report when it was ready to copy the ATC clearance. Because the flight crew was performing the checklist, copying this clearance was postponed until the aircraft was in takeoff position on Runway 30.

Shortly afterward, the Pan Am was instructed to follow the KLM down the same runway, exit it by taking the third exit on their left and then use the parallel taxiway. Initially, the crew was unclear as to whether the controller had told them to take the first or third exit. The crew asked for clarification and the controller responded emphatically by replying: "The third one, sir; one, two, three; third, third one". The crew began the taxi and proceeded to identify the unmarked taxiways using an airport diagram as they reached them.

The crew successfully identified the first two taxiways (C-1 and C-2), but their discussion in the cockpit never indicated that they had sighted the third taxiway (C-3), which they had been instructed to use. There were no markings or signs to identify the runway exits and they were in conditions of poor visibility. The Pan Am crew appeared to remain unsure of their position on the runway until the collision, which occurred near the intersection with the fourth taxiway (C-4).

The angle of the third taxiway would have required the plane to perform a turn of approximately 148°, which would lead back toward the still-crowded main apron. At the end of C-3, the Pan Am would have to make another 148° turn in order to continue taxiing towards the start of the runway. Taxiway C-4 would have required two 35° turns. A study carried out by the Air Line Pilots Association after the accident concluded that making the second 148° turn at the end of taxiway C-3 would have been "a practical impossibility". Subsequent performance calculations and taxi tests with a Boeing 747 turning off on an intersection comparable to the C-3 at Tenerife, as part of the Dutch investigation, indicate that in all probability the turns could have been made. The official report from the Spanish authorities explains that the controller instructed the Pan Am aircraft to use the third taxiway because this was the earliest exit that they could take to reach the unobstructed section of the parallel taxiway.

Weather conditions at Los Rodeos

Tenerife / Los Rodeos airport is at 633 metres (2,077 feet) above sea level, which accounts for cloud behaviour that differs from that at most other airports. Clouds at 600 m (2,000 ft) above ground level at the nearby coast, are at ground level at Los Rodeos / Tenerife North. Drifting clouds from different densities cause wildly varying visibilities, from unhindered at one moment to below the minimums the next. The collision took place in a high density cloud.

The Pan Am crew found themselves in poor and rapidly deteriorating visibility almost as soon as they entered the runway. According to the ALPA report, as the Pan Am aircraft taxied to the runway, the visibility was about 500 m (1,600 ft). Shortly after they turned onto the runway it decreased to less than 100 m (330 ft).

Meanwhile, the KLM was still in good visibility, but with clouds blowing down the runway towards them. "The KLM aircraft completed its 180 degree turn in relatively clear weather and lined up on Runway 30. The next cloud was some 900 m (3,000 ft) down the runway and moving towards the aircraft at about 12 knots (6 meters per second)".

Communication misunderstandings

Immediately after lining up, the KLM pilot advanced the throttles and the aircraft started to move forward. The co-pilot advised the captain that ATC clearance had not yet been given, and Captain Veldhuyzen van Zanten responded, "I know that. Go ahead, ask." Meurs then radioed the tower that they were "ready for takeoff" and "waiting for our ATC clearance". The KLM crew then received instructions which specified the route that the aircraft was to follow after takeoff. The instructions used the word "takeoff," but did not include an explicit statement that they were cleared for takeoff.

Meurs read the flight clearance back to the controller, completing the readback with the statement: "We are now at takeoff." Captain Veldhuyzen van Zanten 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" (terminology which is nonstandard), which reinforced the KLM captain's misinterpretation that they had takeoff clearance. The controller's response of "OK" to the co-pilot's nonstandard statement that they were "now at takeoff" was likely due to his misinterpretation that they were in takeoff position and ready to begin the roll when takeoff clearance was received, but not in the process of taking off. The controller then immediately added "stand by for takeoff, I will call you," indicating that he had not intended the clearance to be interpreted as a takeoff clearance.

A simultaneous radio call from the Pan Am crew caused mutual interference on the radio frequency, which was audible in the KLM cockpit as a three second long whistling sound (or heterodyne). This caused the KLM crew to miss the crucial latter portion of the tower'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. Either message, if heard in the KLM cockpit, would have alerted the crew to the situation and given them time to abort the takeoff attempt.

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 the airport was not equipped with ground radar.

After the KLM plane had started its takeoff roll, the tower instructed the Pan Am crew to "report when runway clear." The Pan Am crew replied: "OK, we'll report when we're clear." On hearing this, the KLM flight engineer expressed his concern about the Pan Am not being clear of the runway by asking the pilots in his own cockpit, "Is he not clear, that Pan American?" Veldhuyzen van Zanten emphatically replied "Oh, yes" and continued with the takeoff.

Collision

According to the CVR, the Pan Am pilot said, "There he is!" when he spotted the KLM's landing lights through the fog just as his plane approached exit C-4. When it became clear that the KLM was approaching at takeoff speed, Grubbs exclaimed, "Goddamn, that son-of-a-bitch is coming straight at us!" while the co-pilot Robert Bragg yelled, "Get off! Get off! Get off!". The Pan Am crew applied full power to the throttles and took a sharp left turn towards the grass in an attempt to avoid a collision. By the time the KLM pilots saw the Pan Am, they were already traveling too fast to stop. In desperation the pilots prematurely rotated the aircraft and attempted to clear the Pan Am by climbing away, causing a severe tailstrike for 20 m (66 ft).

The KLM was within 100 m (330 ft) of the Pan Am when it left the ground. Its nose gear cleared the Pan Am, but the engines, lower fuselage and main landing gear struck the upper right side of the Pan Am's fuselage at approximately 140 knots (260 km/h; 160 mph), ripping apart the center of the Pan Am jet almost directly above the wing. The right side engines crashed through the Pan Am's upper deck immediately behind the cockpit.

The KLM plane remained briefly airborne following the collision, but the impact with the Pan Am had sheared off the outer left engine, caused significant amounts of shredded materials to be ingested by the inner left engine, and damaged the wings. The KLM immediately went into a stall, rolled sharply, and hit the ground at a point approximately 150 m (500 ft) past the collision, sliding a further 300 m (1,000 ft) down the runway. The full load of fuel, which had caused the earlier delay, ignited immediately in a large fireball.

A survivor of the Pan Am flight, John Coombs of Haleiwa, Hawaii, said that sitting in the nose of the plane probably saved his life: "We all settled back, and the next thing an explosion took place and the whole port side, left side of the plane, was just torn wide open."

Both airplanes were destroyed. All 234 passengers and 14 crew members in the KLM plane died, as did 326 passengers and 9 crew members aboard the Pan Am, primarily due to the fire and explosions resulting from the fuel spilled and ignited in the impact. The other 56 passengers and 5 crew members aboard the Pan Am aircraft survived, including the pilots and flight engineer. Most of the survivors on the Pan Am walked out onto the left wing, the side away from the collision, through holes in the fuselage structure. The Pan Am's engines were still running at takeoff power for a few minutes after the accident despite First Officer Bragg's intention to turn them off. The top part of the cockpit, where the engine switches were located, had been destroyed in the collision, and all control lines were severed, leaving no method for the flight crew to control the aircraft's systems. After a short time of running at full power, the engines began to disintegrate, throwing engine parts at high speed that killed a flight attendant who had escaped the burning plane. Survivors waited for rescue, but it did not come promptly, as the firefighters were initially unaware that there were two aircraft involved and were concentrating on the KLM wreck some distance away in the thick fog. Eventually, most of the survivors on the wings dropped to the ground below.

Captain Veldhuyzen van Zanten was KLM's chief of flight training. His photograph was used for publicity materials such as magazine advertisements, including the inflight magazine on board PH-BUF. As such, KLM suggested that he should be part of the investigation team, before learning that he was the captain involved. He had given the co-pilot on the flight his Boeing 747 qualification check about two months before the accident.

Investigation

About 70 crash investigators from Spain, the Netherlands, the United States, and the two airline companies were involved in the investigation. Facts showed that there had been misinterpretations and false assumptions. Analysis of the CVR transcript showed that the KLM pilot was convinced that he had been cleared for takeoff, while the Tenerife control tower was certain that the KLM 747 was stationary at the end of the runway and awaiting takeoff clearance. It appears KLM's co-pilot was not as certain about take-off clearance as the captain.

Subsequent to the crash, first officer Robert Bragg, who was responsible for handling the Pan Am's radio communications, made a public statement which conflicts with statements made by the Pan Am crew in the official transcript of the CVR. In the documentary Crash of the Century (produced by the makers of Mayday), he stated he was convinced the tower controller had intended they take the fourth exit C-4 because the controller delivered the message to take "the third one, sir; one, two, three; third, third one" after the Pan Am had already passed C-1 (making C-4 the third exit counting from there).

Probable cause

The investigation concluded that the fundamental cause of the accident was that Captain Veldhuyzen van Zanten took off without takeoff clearance. The investigators suggested the reason for his mistake might have been a desire to leave as soon as possible in order to comply with KLM's duty-time regulations, and before the weather deteriorated further.

Other major factors contributing to the accident were:

• The sudden fog greatly limited visibility. The control tower and the crews of both planes were unable to see one another.
• Interference from simultaneous radio transmissions, with the result that neither message could be heard.

The following factors were considered contributing but not critical:

• Use of ambiguous non-standard phrases by the KLM co-pilot ("We're at take off") and the Tenerife control tower ("OK").
• The Pan Am aircraft had not exited the runway at C-3.
• The airport was (due to rerouting from the bomb threat) forced to accommodate a great number of large aircraft, resulting in disruption of the normal use of taxiways.

Dutch response

The Dutch authorities were reluctant to accept the Spanish report blaming the KLM captain for the accident. The Netherlands Department of Civil Aviation published a response that, whilst accepting that the KLM aircraft had taken off "prematurely", argued that he alone should not be blamed for the "mutual misunderstanding" that occurred between the controller and the KLM crew, and that limitations of using radio as a means of communication should have been given greater consideration.

In particular, the Dutch response pointed out that

• The crowded airport had placed additional pressure on all parties, KLM, Pan Am, and the controller;
• Sounds on the CVR suggested that during the incident the Spanish control tower crew had been listening to a football match on the radio and may have been distracted.
• The transmission from the tower in which the controller passed KLM their ATC clearance was ambiguous and could have been interpreted as also giving take-off clearance. In support of this part of their response, the Dutch investigators pointed out that Pan Am's messages "No! Eh?" and "We are still taxiing down the runway, the Clipper 1736!" indicated that Captain Grubbs and First Officer Bragg had recognised the ambiguity (this message was not audible to the control tower or KLM crew due to simultaneous cross-communication);
• The Pan Am had taxied beyond the third exit. Had the plane turned at the third exit as instructed, the collision would not have occurred.

Although the Dutch authorities were initially reluctant to blame Captain Veldhuyzen van Zanten and his crew, the airline ultimately accepted responsibility for the accident. KLM paid the victims or their families compensation ranging between $58,000 and $600,000. The sum of settlements for property and damages was $110 million (an average of $189,000 per victim, due to limitations imposed by European Compensation Conventions in effect at the time).

Speculations

This was one of the first accident investigations during which the contribution of "human factors" was studied. The human factors included:

• The flight was one of Captain Veldhuyzen van Zanten's first after spending six months training new pilots on a flight simulator, where he had been in charge of everything (including simulated ATC), hence having been away from the real world of flying for an extended period.
• The flight engineer's and the first officer's apparent hesitation to challenge Veldhuyzen van Zanten further. The official investigation suggested that this might have been because the captain was not only senior in rank, but also one of the most respected pilots working for the airline. This view is questioned by Jan Bartelski, a former KLM Captain and the president of the International Federation of Air Line Pilots' Associations (IFALPA), who knew both men and believes that this explanation is inconsistent with his experience of their personalities.
• The reason only the flight engineer on the KLM plane reacted to the radio transmission "OK, we'll report when we're clear" might lie in the fact that by then he had completed his pre-flight checks, whereas his colleagues were experiencing an increased workload, just at the same time as the visibility worsened.
• The ALPA study group concluded that the KLM crew did not realise that the transmission "Papa Alpha one seven three six, report when runway clear" was directed at the Pan Am because this was the first and only time the Pan Am was referred to by that name. Before that, the Pan Am was called "Clipper one seven three six", with the proper callsign.

The extra fuel the KLM plane took on added several factors:

• it delayed takeoff an extra 35 minutes, which gave time for the fog to settle in;
• it added over forty tons of weight to the plane, which increased the takeoff distance and made it more difficult to clear the Pan Am when taking off;
• it increased the size of the fire from the crash that ultimately killed everyone on board.

Safety response

As a consequence of the accident, sweeping changes were made to international airline regulations and to aircraft. Aviation authorities around the world introduced requirements for standard phrases and a greater emphasis on English as a common working language. For example, ICAO calls for the phrase "line up and wait" as an instruction to an aircraft moving into position but not cleared for takeoff. Also several national air safety boards began penalizing pilots for disobeying air traffic controllers' orders. Air traffic instruction should not be acknowledged solely with a colloquial phrase such as "OK" or even "Roger" (which simply means the last transmission was received), but with a readback of the key parts of the instruction, to show mutual understanding. Additionally, the phrase "takeoff" is spoken only when the actual takeoff clearance is given. Up until that point, both aircrew and controllers should use the phrase "departure" in its place (e.g. "ready for 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, part of what has become known in the industry as Crew Resource Management.

In 1978 a second airport was inaugurated on the island: the new Tenerife South Airport (TFS). This airport now serves the majority of international tourist flights. Los Rodeos, renamed to Tenerife North Airport (TFN), was then used only for domestic and inter-island flights, but in 2002 a new terminal was opened and it carries international traffic once again, including budget airlines. The Spanish authorities installed a ground radar at Tenerife North following the accident.

Memorials

A Dutch national memorial and final resting place for the victims of the KLM plane is located in Amsterdam, at Westgaarde cemetery. There is also a memorial at the Westminster Memorial Park and Mortuary in Westminster, California.

The 30th anniversary marked the first time that Dutch and American next of kin, and aid helpers from Tenerife, joined in international commemoration service held at the Auditorio de Tenerife in Santa Cruz; the International Tenerife Memorial March 27, 1977 was inaugurated at the Mesa Mota March 27, 2007. The monument was designed by Dutch sculptor Rudi van de Wint.

Notable victims

• Eve Meyer, a pin-up model, film actress and producer and former wife of Russ Meyer.
• A. P. Hamann, the former city manager of San Jose, California, and his wife Frances Hamann.

Documentaries

The disaster has been featured in many TV shows and documentaries. These include the Mayday special episode "Crash of the Century", the Survival in the Sky episode "Blaming the Pilot", the Seconds From Disaster episode "Collision on the Runway", PBS's NOVA episode "The Deadliest Plane Crash" in 2006, the PBS special "Surviving Disaster: How the Brain Works Under Extreme Duress" (based on Amanda Ripley's book "The Unthinkable") in 2011, and the Discovery Channel TV series Most Deadly and Destroyed in Seconds.