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The Tenerife disaster took place at 17:06 local time on March 27, 1977, when two Boeing 747 airliners collided on the island of Tenerife, killing 583 people. The incident was the deadliest aircraft disaster in history until the September 11, 2001 attacks. It is noteworthy that one of the aircraft was on the ground when the collision occurred.

On March 27, 1977 Pan Am Flight 1736, had taken off from Los Angeles International Airport, bound for the Canary Islands, with an intermediate stop at New York's JFK International Airport. The aircraft was a B-747-121, registration N736PA and named Clipper Victor. This aircraft happened to be the first 747 to carry fare-paying passengers, on a flight from New York to London on 21 January 1970.

Upon approaching its final destination, Las Palmas, the Clipper Victor was told that the major airport was temporarily closed due to a terrorist bomb attack by Canary Island separatists. Clipper Victor was ordered to divert to Los Rodeos airport on the neighboring island of Tenerife, together with many other planes. It landed at Tenerife and was waiting to go.

The KLM Flight 4805, PH-BUF, a B-747-206B flying as a charter, full of vacationers, was getting ready to head back to Amsterdam via Las Palmas. The Pan Am jet could have taken off earlier, but the KLM plane needed to be fueled and blocked the Pan Am jet's access to the runway. Once refueling was done, the KLM plane was to take off first, followed by the Pan Am plane. Following the tower's instructions, the KLM jet taxied to the end of the main runway, made a 180 degree turn (difficult with a 747 on the narrow runway) and waited for takeoff clearance from air traffic control (ATC).

With KLM ready to go, Pan Am was instructed to taxi along the same main runway until it reached exit 3, then to take the exit, get off the main runway, and head to a parallel taxiway. Seemingly due to the heavy fog, they missed exit 3, which involved a sharp turn backwards and led straight back to the congested terminal area. They decided to go continue until exit 4, which was heading in the right direction.

Tenerife ATC gave the KLM plane clearance for post-takeoff travel route, but the KLM captain apparently mistook it to be permission for the takeoff itself. The co-pilot responded with a heavy Dutch accent with words that could either be "We are at take off" or "We are taking off" (see [1] (http://www.benjamins.com/jbp/series/LPLP/27-3/art/0002a.pdf)). The control tower was confused with the message and asked for the KLM plane to stand by. However, simultaneous communication from Pan Am made the response inaudible. Ironically, PanAm was reporting they have not finished taxiing. Both messages, if broadcast separately might have given KLM time to abort its takeoff.

Due to the dense fog, the KLM's pilots were unable to see the Pan Am 747 taxiing on the runway ahead of them. In addition, neither of the 747s could be seen from the control tower, and the airport was not equipped with runway radar.

KLM Captain Jacob Veldhuyzen van Zanten, impatient because the flight had been delayed for hours and thinking that they had permission to take off, applied full power. While the KLM had started its take-off run, the tower told Pan Am to "report when runway clear". Pan Am radioed back: "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, but it was overruled by the captain. The flight engineer apparently hesitated to further challenge van Zanten, possibly because van Zanten was not only senior in rank but also one of the most able and experienced pilots of the company.

Still taxiing along, the Pan Am spotted the KLM's landing lights. The Pan Am's pilots tried to apply full power and take a sharp left turn away from the runway, but the collision was only seconds away. The KLM plane attempted to avoid a collision by climbing away (scraping the tail of the plane along the runway for 65 feet), but was only slightly airborne at the time of impact, with the lower fuselage of the KLM plane hitting the upper fuselage of the Pan Am plane, ripping apart the center of the Pan Am jet (roughly what is directly above the wing.) The KLM plane slammed into the ground belly-up 150 m past the point of collision and slid down the runway. All 234 passengers and 14 crew members in the KLM plane were killed, and 335 of the 396 aboard the Pan Am flight (321 passengers and 14 crew members) perished, primarily due to the fire and explosions resulting from the fuel spilled in the impact (the KLM plane had been fully fueled). The Pan Am captain was among the survivors (54 passengers and 7 crew members).

Later investigation showed that there had been misinterpretations and false convictions. Analysis of the CVR transcript shows that the KLM pilot was convinced that he had been cleared for take-off, while the Tenerife control tower was certain that the KLM 747 was stationary at the end of the runway and awaiting takeoff clearance. While there is debate about their relative importance, the general conclusion is that the disaster was caused partly by squelched radio messages (calls from both planes to the tower and vice versa canceled each other because they happened to be at precisely the same instant), partly by non-standard phrases used by the KLM co-pilot ("We're at take off") and the Tenerife control tower ("O.K.")

Around 70 crash investigators from Spain, the Netherlands, the US, and the two airline companies were involved in the investigation. As a consequence of the accident, there were sweeping changes made to international airline regulations and to airplanes. It was made a worldwide rule that all control towers and pilot crews had to use English standard phrases. Airplane manufacturers began installing equipment that helped planes see through fog. Cockpit procedures were also changed. Hierarchical relations among crewmembers were played down. More emphasis was placed on decision-making by mutual agreement. This is known in the industry as crew resource management, and is now standard training in all major airlines.

Due to the frequent and dangerous fogs that cover the area around Los Rodeos airport in the North of the island, a second one was built in the South of the Island: the new Reina Sofía Airport, which serves the majority of Tenerife's domestic and international commercial flights; Los Rodeos, however, is still fully operational, mainly dedicated to cover regional flights within the Canary Islands.

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