It is a bleak and uncomfortable irony that history's worst 'air' disaster happened to two fully servicable aircraft, one of them not even in the air. Both were 747's, PH-BUF operated by KLM, and N736PA by Pan American. Both were carrying sun-seekers to Las Palmas airport on Grand Canary Island on 27 March 1977.

The local residents, however, were less interested in their tourist popularity on this day than in a bomb blast set off in the airport terminal by activists wanting independence from Spain. This publicity manoeuvre became one link in a chain which was to bring world-wide publicity to the Canary Islands – if not to the purpose desired by the independence movement.

With the terminal damaged and fears of further explosions, traffic was diverted to Los Rodeos airport on Tenerife, some twenty minutes away. Los Rodeos lies among mountains, 2000ft above sea level. Cloud and sudden weather shanges can cause major visibility variations here, though the weather in the early afternoon of this day was fine. Normal weekend traffic and numbers of diverted aircraft soon filled the usual parking areas.

PH-BUF landed at 1338 hrs. and after keeping his passengers on board for twenty minutes in case Las Palmas reopened, the KLM captain agreed to unload. The Clipper captain also hoped for Las Palmas to open, but his request to hold was not granted, and at 1415hrs he too landed at the now congested Los Rodeos airport, parking on a holding area behind the KLM B747, a B737, a B727 and a DC-8.

Ten minutes later, Las Palmas was reopened, and the B737, B727 and DC-8 were able to move in turn past the now empty KLM B747 and leave Los Rodeos. N736PA, however, after its F/O and F/E had disembarked and checked distances, was judged not to have space enough either to move out past the other B747, or to exit in the other direction. There was nothing to do but wait.

KLM passengers then began to return to their flight when word was received of more traffic congestion, now at Las Palmas where no gate was free. The KLM captain decided to save half an hour at Las Palmas by refuelling at Los Rodeos for the return flight to Amsterdam.

This move displeased the American crew, still hemmed in, and apparently not understanding the developing problems ahead of the KLM crew. To complete the return trip to Amsterdam and adhere to strict flight time limitations, the KLM crew needed to leave Las Palmas by 1900hrs at the latest. The alternative to a rapid turnaround at Las Palmas was an overnight delay fro 250 passengers and consequent disruption for the company at home. The crew had begun duty in Amsterdam 9 hours ago. The Pan Am crew were in their eleventh hour on duty and were to rest in Las Palmas.

It can hardly have been pleasing to either crew that the weather began at this stage to deteriorate. Clouds swept across the aerodrome with a 15kt wind, and fears arose of being closed in indefinitely.

But by 1650 hrs. both aircraft were preparing for startup. They were to taxi the full distance to the end of runway 30, there to make a 180 for take off. After some misunderstanding and an amended instruction from the Spanish-speaking controller, the KLM flight proceeded along the runway. The Pan Am crew also experienced a language difficulty before they too began to backtrack along the runway with an instruction to move off at the third exit.

Visibility at times was now down to 100m, making it difficult to identify the exits, but PH-BUF passed what was thought to be the fourth exit, and confirmed it would turn 180 degrees at the end of the runway and report for clearance. Clipper 1736 was searching for the third exit to the taxiway. Both aircraft were advised that centreline lights were unserviceable. Both also checked minimum visibility requirements with the controller. Neither could be seen from the tower, now could they see each other.

At 1703 hrs ATC asked Pan Am to report vacating the runway. Two minutes later they had not yet done so. With cloud swirling around them, they had missed their exit.

By 1705:28, the KLM Captain was set for departure. Aged fifty, and head of training for his company, he spent much of his time instructing in the simulator, often supervising consecutive takeoffs and landings. No need to worry about ATC for those. Although his recent line experience was therefore less than the average captain's, he had coped with the frustrations of the afternoon. They had been stuck here for nearly three and a half hours, and he was keen to avoid wasting any more time. As he completed the turn to line up, he spooled up the engines. The F/O called, "Wait a minute – we don't have an ATC clearance." The captain responded "No, I know that. Go ahead – ask." He closed the throttles.

Pan Am was reporting still being on the runway, while ATC was simultaneously instructing KLM to standby for takeoff. The messages were obscured on the KLM radio by the high squeal. The KLM crew did not register either transmission, or the immediate further exchange that Pan Am was to report leaving the runway.

By now, as the huge aircraft accelerated along the runway, the F/E must have sensed that all might not be well, as he asked, "Is he not clear, then?" The captain replied "what did you say?" the F/E persisted: "is he not clear, that Pan Am?"

Captain and F/O were heard together: "Oh yes."

When the Pan Am crew saw the lights of PH-BUF in the distance, they tried instantly to turn left off the runway. Still unaware, the KLM F/O was calling V1.

In a desperate attempt to get airborne, the KLM captain pulled the stick fully back, scraping the tail hard on the runway. It was no use. Although the nose wheels passed safely over the Pan Am fuselage and the belly slid along the roof, the main gear slammed into its side. All 248 people on the KLM 747 were killed, as well as 335 of the 396 aboard the Clipper, only 61 surviving.