The collision of two fully-loaded Boeing 747 airplanes on a runway was an unprecedented and horrifying incident - an incident which ultimately was traced back to just a few momentary misunderstandings and communication failures.
Background
The day of the crash was a busy one for Los Rodeos Airport.
This tiny island airport consisted of only a single runway, with one taxiway alongside it.
It normally saw relatively little traffic, and certainly very few large airliners.
However, on the 27th of March, 1977, Los Rodeos was inundated with flights as multiple planes were diverted from the nearby Gando Airport on Gran Canaria, which had closed due to a small bomb exploding inside a flower shop in the terminal.
Among those diverted flights was KLM Flight 4805 from Amsterdam, and Pan American Flight 1736 from Los Angeles.
Both were bound for Gran Canaria, but were forced to divert and land at Los Rodeos instead.
These were just two of many planes which had been diverted to the tiny airport in Tenerife.
In fact the taxiway was so full of parked aircraft that there was no space for planes to actually taxi along it.
Instead, any aircraft that wished to take off had to use the runway itself to taxi into position, further complicating operations for a tower crew who weren't used to dealing with so many flights.
On top of that it was a Sunday, and just two people were on duty in the tower at the time.
They were reliant mainly on sight and radio communication as the airport had no ground radar and, worse still, as the day went on a thick fog descended on the area, reducing visibility to near zero.
The controllers in the tower couldn't see the planes they were directing, and the pilots in the planes could barely see the runway or signage on the ground.
All of this only increased the delay suffered by each of the diverted flights.
For the passengers of those diverted flights the weight was an interminable one.
There was no way to know how long they would be held there in a cramped airport with limited facilities - in fact there was so little space inside the terminal that passengers on board Pan American Flight 1736 were forced to wait on board the aircraft in sweltering heat.
Rather than wait and see what happened, one passenger from the KLM flight decided they'd had enough.
Robina van Lanschot actually lived on Tenerife.
The diversion had worked out well for her.
Rather than waiting and re-boarding, she simply went home from the airport...not knowing as she did so that this decision would save her life.
For the other passengers (mainly holidaymakers who did not live on Tenerife) the only option was to wait.
And wait they did, until around 5:00pm, when their destination airport was declared safe and reopened.
Quickly, the waiting passengers reboarded and flight crews made ready to depart.
The Tenerife Airport Disaster
As the log jam of planes at Los Rodeos began to clear the KLM plane was directed to get into position at the end of the runway ready for takeoff.
At the same time the Pan American flight was instructed to taxi down the runway and leave at the third exit.
This was to prove easier said than done, however.
Navigating an unfamiliar airport in a thick fog, the Pan American pilots missed the third exit and crawled onwards at a snail's pace towards the fourth.
At this point the two Boeing 747s were essentially facing one another on the same stretch of runway.
The KLM was at the far end ready for takeoff.
The Pan American was taxiing slowly towards it in search of an exit.
The only thing between the two aircraft was a thick bank of fog.
Staring out into that fog from the flight deck of the KLM plane was captain Jacob Veldhuyzen van Zanten.
Knowing that he and his crew were getting close to KLM's limit on continuous working hours, Captain Veldhuyzen van Zanten was extremely keen to get moving...so keen, in fact, that he actually disengaged the brakes and started to roll the plane forward without waiting for authorization from the tower.
His first officer, at this point, jumped in to remind him that they didn't have clearance to take off, prompting Veldhuyzen van Zanten to bring the plane to a reluctant halt.
"No, I know that," he said. "Go ahead. Ask."
The first officer got in touch with the tower controllers, who responded with a description of the route the flight was to take once it was off the ground.
Crucially this did not include permission to take off - it was merely the tower clarifying where the KLM flight would go once it was in the air.
The first officer read back this guidance, ending with the words, "We're now at takeoff." The tower responded, "OK. Stand by for takeoff. I will call you."
At the exact same moment the Pan American flight, still taxiing down the runway, chimed in to remind everyone that they were still on the runway.
The two radio calls conflicted, resulting in a burst of static that obscured all but one word.
The only thing that the crew of the KLM plane heard was the word, "OK," something which Captain Veldhuyzen van Zanten took to be permission to begin takeoff. "We're going," he announced as he started the aircraft down the runway.
Even as the KLM began to roll there was doubt in the cockpit.
The flight engineer questioned Captain Veldhuyzen van Zanten.
"Is he not clear? That Pan American?" he asked - a question to which Captain Veldhuyzen van Zanten simply replied, "Oh, yes," and continued with the takeoff.
If there were further reservations on the part of the other crew, nobody gave voice to them.
Captain Veldhuyzen van Zanten was a senior pilot with more than 10,000 hours of experience.
Nobody else on board challenged his authority as the KLM plane gathered speed.
The Pan American pilots saw the KLM bearing down on them through the fog just moments before impact.
They had time to begin turning, but could not avert a collision altogether.
For his part Captain Veldhuyzen van Zanten did attempt an early takeoff, perhaps hoping to just barely pass over the top of the Pan American aircraft.
The move was unsuccessful.
The KLM flight smashed through the Pan American plane, and then crashed into the runway itself exploding in a massive fireball.
The Pan American flight also burned intensely, but 61 people from that flight were able to survive and evacuate, many having to jump to the ground from the aircraft's wing, some of them enduring horrific injuries in the process.
In this incident survivors were in the minority.
583 people were killed, including every single person on board the KLM flight.
The fires on the runway were so intense that they simply could not be controlled for hours after the collision.
The Aftermath
Multiple investigations were launched in the aftermath of the disaster.
All agreed that human error had played a significant part.
Captain Veldhuyzen van Zanten impatience had combined with a reluctance on the part of his fellow crew members to challenge someone so senior, so experienced, and so high profile within the company.
Added to this were the simple radio miscommunications that had taken place.
With all this in mind KLM accepted responsibility for the disaster and paid compensation to the families of the victims.
One of the most significant changes to result from the accident was the standardization of language used in radio communications.
No longer would pilots be able to rely on colliquial phrases like "OK".
Instead they would have clear protocol for communication, including always reading back the instructions that they were given to make sure they had been heard correctly, and never using the word "takeoff" unless clearance for takeoff was being given.
The hierarchy within the cockpit of a plane was also addressed.
Now the airline industry fosters an atmosphere of cooperation and mutual decision-making.
Pilots are not just able but encouraged to challenge their superiors when they think something is wrong.
And those superiors are required to actually listen to their crew and answer any concerns they have.
In the years which followed the disaster, several memorials were put in place around the world.
The disaster affected many thousands of people.
Whole families had been lost, friends torn apart, and peaceful lives transformed in a matter of moments.
And none of it would have happened at all were it not for a few seconds of impatience, a single muddled radio call.
At least as a legacy of such great loss there are now rigorous guidelines governing all air traffic control communication, designed to ensure that never again can such a small misunderstanding cause such massive devastation. (The Tenerife Airport Disaster)
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