The Pan Am flight had arrived after KLM and parked behind it on the apron, just short of the departure end of runway 12. On the flight deck of the KLM aircraft, it's Captain, Jacob van Zanten, a highly regarded training captain, was anxious to get back in the air as his duty hours for his crew were running low. When the tower called to inform crews that Las Palmas had re-opened, van Zanten decided that, instead of refuelling at Las Palmas which would undoubtedly be busy with the re-opening, he would refuel while waiting on the ramp at Los Rodeos. It was now Pan Am's turn to depart, but the only way to reach the departure end of the active runway, runway 30, was to enter runway 12 and backtrack.

Unfortunately, KLM had only just begun refuelling and there was no way 1736 could taxi around it with the limited space at Los Rodeos. Pan Am's First Officer Bragg called the KLM crew, asking how long it would take to refuel to which they replied "About 35 minutes." There was nothing the crew of 1736 could do but wait. While 4805 was refuelling, fog was moving onto the airport and by the time they had finished, visibility had decreased to as little as 900ft in some areas. The KLM crew finally started their engines and prepared to takeoff. As they taxied to the beginning of runway 12, the tower instructed 4805 to "...taxi straight ahead...ah...for the runway...make...ah...backtrack." At this point, 1736 had also started it's engines and was holding short of the runway.

The visibility now prevented the tower from being able to see neither the runway nor the two aircraft. Bragg then called the tower for instructions and 1736 was told to "...taxi into the runway and...ah...leave the runway third...third to your left."

Apparently the pronunciation was unclear to Captain Grubbs who said "I think he said first" to which Bragg replied "I'll ask him again." Meanwhile the tower called 4805, instructing them "...at the end of the runway make one eighty and report...ah...ready for ATC clearance." After this communication, Bragg called back and said "Would...you confirm that you want us to turn left at the third intersection?"

The tower replied "The third one, Sir...one two three...third one." The crew of 1736 was still having difficulty sorting out the taxiways as they rolled down the runway. At this point, 4805 had reached the end of the runway and was making it's 180 degree turn.

As the aircraft finished the turn, van Zanten opened the throttle and the plane began to move forward. First Officer Meurs said "Wait a minute...we don't have an ATC clearance." to which van Zanten said "No, I know that. Go ahead and ask" as he held the brakes. Meurs called for the clearance and as he was reading it back, van Zanten again opened the throttles, saying "Let's go, check thrust." After repeating the clearance, Meur, in an attempt to let the controller know what was happening, said "We are now at takeoff." The tower controller apparently took this to mean they were ready for takeoff, saying "OK...standby for takeoff...I will call you." On the flightdeck of 1736, the crew was obviously anxious about the implications of the transmission from 4805, Braggs saying "We are still taxiing down the runway!" to which the tower replied "Roger, Pan Am 1736, report the runway clear."

Unfortunately, this first transmission blocked the tower's transmission to 4805 so all the KLM crew heard was "OK." The transmission from 1736 troubled 4805's Flight Engineer Schreuder, prompting him to say "Did he not clear the runway then?" van Zanten, now focusing on the takeoff replied with only "What did you say?" Schreuder repeated himself, saying "Did he not clear the runway then, that Pan American?" to which both van Zanten and Meurs replied "Yes, he did." 1736 was still creeping down the runway, trying to find the proper turnoff, but obviously now concerned about KLM's transmissions. Grubbs said "Let's get the hell right out of here" to which Bragg replied "Yeah...he's anxious isn't he?"

A few seconds later, Grubbs spotted the lights of 4805 coming at them through the fog and said "There he is...look at him! Goddamn...that son-of-a-bitch is coming!" He opened all four throttles in an attempt to swing the aircraft off the runway as Bragg yelled "Get off! Get off! Get off!" van Zanten saw 1736 still in the runway and pulled back, attempting to climb off the runway before impacting the aircraft. The nose gear managed to clear 1736, but the rest of the aircraft slammed into the Pan Am plane's starboard side. 4805 remained airborne for a few more seconds before slamming into the ground and exploding. 1736 was crushed and quickly caught fire as well. Everyone on board 4805 was killed. The flight crew of 1736 all survived uninjured, having just missed being hit by 4805's engine. Amazingly, 66 others survived from the Pan Am aircraft. Unfortunately, 583 people died that day on Tenerife in what is still today the worst aviation accident in history.

......The biggest question on the minds of investigators was why van Zanten, a highly experienced training captain, would begin a takeoff without a takeoff clearance from the tower. Meurs was still copying the enroute clearance when van Zanten began advancing the throttles. It seems clear that van Zanten was aware that the clearance hadn't been received when Meurs checked him and he replied "No, I know that. Go ahead and ask." It is likely that van Zanten was in a rush to get to Las Palmas because of the delay on the ground and his crew's lack of extra duty hours.

However, even after the enroute clearance was given, the tower instructed 4805 to "standby for takeoff" which the crew failed to hear as well as the clear indications that 1763 was still on the runway. In addition, Meurs did nothing to further enlighten van Zanten that they were not cleared for takeoff after his initial comment. It is possible that Meurs was not comfortable challenging van Zanten due to his experience level. The efforts of the crew of 1736 were hampered by the low visibility.

They had only a small diagram of the airport and the third taxiway led backwards from their intended taxi direction, a turn of 135 degrees which would be extremely challenging in a 747. They apparently believed that the fourth taxiway, which was at a 45 degree angle in the proper direction was the one the tower intended for them to use, so they proceeded past taxiway three. None of the taxiways at Los Rodeos were marked. A final consideration was the difficulty with English of the tower controller and the 4805 crew. With the weather as bad as it was, relying solely on radio communications was already a dangerous practice, but the non-standard communications of both parties lead to the breakdown of situational awareness.