Patches of thick fog were drifting across the airfield, so visibility was greatly reduced for pilots and the control tower.

[2][3] The subsequent investigation by Spanish authorities concluded that the primary cause of the accident was the KLM captain's decision to take off in the mistaken belief that a takeoff clearance from air traffic control (ATC) had been issued.

Cockpit procedures were also reviewed, contributing to the establishment of crew resource management as a fundamental part of airline pilots' training.

[9] Its cockpit crew consisted of Captain Jacob Veldhuyzen van Zanten (age 50),[10] First Officer Klaas Meurs (42), and Flight Engineer Willem Schreuder (48).

At 13:15, a bomb planted by the separatist Canary Islands Independence Movement exploded in the terminal of Gran Canaria Airport, injuring eight people.

[15] Due to the threat of a second bomb, the civil aviation authorities closed the airport temporarily after the initial explosion, and all incoming flights bound for Gran Canaria were diverted to Los Rodeos, including the two aircraft involved in the disaster.

[4] The Pan Am crew indicated that they would prefer to circle in a holding pattern until landing clearance was given (they had enough fuel to safely stay in the air for two more hours), but they were ordered to divert to Tenerife.

[16] Los Rodeos was a regional airport that could not easily accommodate all of the traffic diverted from Gran Canaria, which included five large airliners.

While waiting for Gran Canaria airport to reopen, the diverted airplanes took up so much space that they had to park on the long taxiway, making it unavailable for the purpose of taxiing.

The Pan Am aircraft was unable to maneuver around the refueling KLM in order to reach the runway for takeoff, due to a lack of safe clearance between the two planes, which was just 3.7 meters (12 ft).

The tower instructed the KLM plane to taxi down the entire length of the runway and then make a 180° turn to get into takeoff position.

[19] 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.

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 (C4).

A study carried out by the Air Line Pilots Association (ALPA) after the accident concluded that making the second 148° turn at the end of taxiway C3 would have been "a practical impossibility".

Drifting clouds of different densities cause wildly varying visibilities, from unhindered at one moment to below the legal minimum the next.

"[4] The controller, who could not see the runway due to the fog and did not have any ground radar to use at the airport, initially responded with "OK" (terminology that is nonstandard), which reinforced the KLM captain's misinterpretation that they had takeoff clearance.

[4] After the KLM plane had started its takeoff roll, the tower instructed the Pan Am crew to "report the runway clear."

When it became clear that the KLM aircraft was approaching at takeoff speed, Captain Grubbs exclaimed, "Goddamn, that son-of-a-bitch is coming!

"[citation needed] Grubbs applied full power to the throttles and made a sharp left turn towards the grass in an attempt to avoid the impending collision.

In desperation, the pilots prematurely rotated the aircraft nose upward and attempted to clear the Pan Am by lifting off, causing a 22 m (72 ft) tailstrike.

The right-side engines crashed through the Pan Am's upper deck immediately behind the cockpit, instantly killing all of the passengers seated there.

Upon impact with the runway, the full load of fuel, which had caused the earlier delay, ignited immediately into a fireball that could not be subdued for several hours.

Most of the survivors on the Pan Am plane walked out onto the intact left wing, the side away from the collision, through holes in the fuselage structure.

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 hundreds of metres away in the thick fog and smoke.

The investigators suggested the reason for this was a desire to leave as soon as possible in order to comply with KLM's duty-time regulations (which were put in place earlier that year) and before the weather deteriorated further.

[56] These included: The extra fuel taken on by the KLM added several factors: 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.

Hierarchical relations among crew members were played down, and greater emphasis was placed on team decision-making by mutual agreement.

This course of action was later expanded into what is known today as crew resource management (CRM), which states that all pilots, no matter how experienced they are, are allowed to contradict each other.

[16][69] A Dutch national memorial and final resting place for the victims of the KLM plane is located in Amsterdam, at Westgaarde cemetery.