On March 26, 1977, the Pan Am Boeing 747 registered N736PA left Los Angeles Airport (LAS) for Spain, with a scheduled re-fuel and crew change at John F. Kennedy Airport (JFK) in New York.
As the Pan Am flight made its journey to Las Palmas de Gran Canaria, a KLM Boeing 747, registered PH-BUF, left Amsterdam Airport Schiphol (AMS) on March 27, 1977, for the same final destination.
At the time of these flights, a terrorist group known as the Canary Islands Independence Movement (CIIM) was attempting to force the Spanish government to cede control of the islands.
On March 27, 1977, the group planted a bomb in a passenger terminal florist shop in Las Palmas Airport and detonated it, injuring eight bystanders.
Members of the terrorist group then threatened to explode a second bomb. With police fearing the worst, the airport was closed and all flights suspended.
Flight crews on both the Pan Am and KLM 747s were told to divert to Los Rodeos Airport, a predecessor of Tenerife North Airport (TFN), to keep all passengers safe and away from the dangers in Gran Canaria.
Arrival in Tenerife
The two 747s were not the only aircraft that were asked to divert to Tenerife due to the dramatic events that were unfolding in Gran Canaria.
According to the Spanish accident report , once both jets had landed, they were among four other aircraft that had been parked due to the congestion caused by the number of flights being diverted to Tenerife.
At the time, Los Rodeos was a small regional airport with one main runway and was not accustomed to handling the volume of traffic it was suddenly experiencing.
Located 2,077 feet (633 meters) above sea level, Los Rodeos was also prone to rapid weather changes with fog and low-lying cloud sometimes causing operational issues.
The site was now crowded with aircraft parked anywhere they could fit, from the taxiway to the various aprons.
When the airport reopened, the Pan Am 747 was ready to leave promptly, but its pathway was blocked by the KLM 747 which had also elected to refuel with 55,500 liters of jet fuel.
As explained in the subsequent Spanish investigation report, once refueled the KLM flight was told by air traffic control (ATC) to taxi down the main runway to the approach end where it should proceed to backtrack.
At 17:02, around five minutes before the fatal crash, the Pan Am flight crew called the airport tower to request confirmation that it should taxi down the runway.
The tower confirmed this and stated that they should leave at the third taxiway to the left to clear the runway for the KLM 747’s takeoff.
At this point the weather around the airport was changing rapidly, with dense clouds rolling over the runway and visibility dropping from 500m (1600ft) to less than 100m (330ft).
There were also no taxiway markings and both crews were told that the runway center line lights were out of service.
As the KLM flight reached the runway end, it completed a 180-degree turn to face down the runway for takeoff.
As the KLM captain began to advance the throttles, the first officer advised that they had not yet been given take-off clearance. The captain responded with “No, I know, go ahead, ask.”
The KLM crew contacted the control tower, confirming they were ready and waiting for ATC clearance. The controller then gave KLM its departure clearance and told them to stand by for take-off clearance.
However, according to Dutch investigators, before the controller could inform the pilots of their full clearance, the KLM Boeing 747 had already started its take-off run.
‘We are now at take-off’
“The [KLM] captain asked him to request it, which he did, but while the co-pilot was still repeating the clearance, the captain opened the throttle and started to take off. Then the co-pilot, instead of requesting take-off clearance or advising that they did not yet have it, added to his read-back, ‘We are now at take-off’,” the investigation report stated.
The report continued: “The tower, which was not expecting the aircraft to take off as it had not given clearance, interpreted the sentence as ‘We are now at take-off position’ and the controller replied: ‘O.K., … stand by for take-off… I will call you’.”
The Pan Am’s crew heard the conversation and informed ATC that they were still “taxiing down the runway”. The controller confirmed that he heard the message and asked the pilots to “report runway clear”.
When the Pan Am crew replied “okay, will report when we are clear,” the KLM crew then began discussing their reply as the message was audible up in the Dutch cockpit.
While the first officer and flight engineer discussed whether the Pan Am had exited the runway, the captain emphatically declared “Oh, yes,” continuing the take-off.
The investigators noted that “perhaps influenced by his great prestige, making it difficult to image an error of this magnitude on the part of such an expert pilot, both the co-pilot and the flight engineer made no further objections”.
Moment of impact
With around nine seconds to impact, the Pan Am crew, who had overshot the third taxiway turning due to the low cloud, saw the KLM 747 hurtling towards them and tried their best to take evasive action.
Desperate attempts to accelerate off the runway were sadly in vain. 100 meters from impact, having spotted the Pan Am 747, the KLM flight crew applied full back pressure, prematurely rotating.
However, it was too late. The fuel-ladened KLM struck the Pan Am 747, ripping the US jet apart.
A violent explosion engulfed the KLM aircraft further down the runway, so much so that when firefighters arrived, they focused their efforts on the Pan Am 747, believing that the other plane was unrecoverable.
A total of 234 passengers and 14 crew died on the KLM flight. On the Pan Am aircraft, 317 passengers and nine crew died. There were only 61 survivors, most of whom were sat towards the front of the aircraft.
A single KLM passenger, Robina van Lanschot, chose to remain in Tenerife and never reboarded the flight. She became the sole survivor of KLM Flight 4805.
Investigation conclusions
Nationaal Archief – Netherlands The accident investigators later concluded that the fundamental cause of the disaster came down to the fact that the KLM captain took off without clearance and did not obey the tower’s order to “stand by for takeoff”.
The KLM captain also failed to interrupt take-off when the Pan Am flight reported that it was still on the runway, and when replying to the flight engineer’s query as to whether the plane had already left the runway, “replied emphatically in the affirmative”.
The report also questioned why a pilot who had the technical capacity and experience of the KLM captain could “commit a basic error in spite of all the warnings repeatedly addressed to him”.
According to the report, the captain was aware of strict rules in the Netherlands around limitation of duty time, and that if he did not takeoff within a relatively short period of time, might have needed to interrupt the flight.
The report said that the weather conditions “undoubtedly make a pilot’s decision regarding takeoff and landing operations much more difficult”.
Also highlighted was the fact that two transmissions took place at the same time.
“The ‘Stand by for take-off, I will call you’ from the tower coincided with Pan Am’s ‘We are still taxiing down the runway’, which meant that the transmission was not received with all the clarity that might have been desired,” the investigation report stated.
The report also recognized that other factors had played a role, such as the fact that when the controller was told “We are now at takeoff,” he did not understand that the aircraft was actually taking off, because to his mind permission had not been given.
The fact that the Pan Am 737 had failed to leave the runway at the third turning was also considered to be a contributing factor, along with the unusual circumstances of aircraft being forced to taxi on the runway.
Following the disaster, changes were made within the industry, enforcing the use of standardized, concise and unambiguous language to improve safety.
Through the establishment of crew resource management, overall safety in aviation greatly improved too, with an emphasis on all flight crew members actively voicing their safety concerns and being acknowledged.
