On March 27, 1977 Pan Am Flight 1736 had taken off from New York's JFK International Airport, bound for the Canary Islands. Upon approaching its final destination Las Palmas it was told the major airport was temporarily closed due to a bomb attack by Canary Island separatists, and was ordered to divert to the airport at the neighboring island of Tenerife, together with many other planes.
The KLM Flight 4805, a 747 flying as a charter full with vacationers, was getting ready to head back to Amsterdam. KLM had instructions to depart first; the PanAm 747 would follow. Following the towers instructions, the KLM jet taxied to the end of the main runway and waited for takeoff clearance.
What happened next would turn out to be a fatal chain of events:
With KLM ready to go, PanAm was instructed to taxi along the same main runway until they reached exit 3, then to head further to the take off point via a parallel taxiway. 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 on till exit 4, which was heading in the right direction.
Air traffic control gave the KLM plane clearance for the route it was to take after takeoff, but the KLM crew mistakenly thought that was permission for the take off itself. Since there was dense fog, the KLM's pilots were unable to see the PanAm 747 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.
Later investigation showed that the KLM pilots misinterpreted some of Tenerife's instructions. This was partly caused by squelched radio messages (calls from both planes to the tower and vice versa cancelled each other), partly by non-standard phrases used by the tower, partly by the Dutch captain Jacob van Zanten seemingly jumping to conclusions (which later was hard to accept for the investigators, as the captain was otherwise known as a first class pilot).
Captain Jacob van Zanten, impatient because the flight had been delayed for hours and thinking that they had permission to take off, applied full power. the co-pilot uttered some hesitations about the level of clearance they had obtained, but he was immediately overruled and hesitated to further challenge van Zanten, who was not only senior in rank but also one of the most able and experienced pilots of the company.
As soon as PanAm, still taxiing along, spotted the KLM 747, the pilots tried to take a sharp turn away from the runway, but the collision was only seconds away. The KLM plane, by now already partially free from the ground, slammed the PanAm plane on the side, and part of the fuselage of the Pan Am jet was ripped apart. The KLM plane twisted around and ended up near the PanAm jet. All 234 passengers and 14 crew members in the KLM plane were killed in the resulting blaze, and 321 of the 380 aboard the PanAm flight perished too. The PanAm captain was among the survivors.
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 implanting equipment that helped planes see through fog. Cockpit procedures were also changed. Hierarchical relations were played down. More emphasis was placed on decision taking by mutual agreement. This is known in the industry as crew resource management, and is now standard training in all major airlines.