Cautionary Tales with Tim Harford - Cleared For Take-Off? (Tenerife Air Disaster 1)

Episode Date: January 10, 2025

“Evacuate the airport, we’ve planted bombs,” a terrorist tells the telephone operator at the airport in Gran Canaria, in 1977. By the end of that day, 583 people will have lost their lives – but not to a bomb explosion.The planes are diverted to the neighboring island of Tenerife. Loaded with passengers, they’re forced to sit on the hot tarmac for hours. Meanwhile, the flight crews rely on air traffic control to keep them updated.Two Boeing 747s are waiting for thick fog to lift so that they can begin the journey home; they're anxious to receive clearance to take off. One of them has just taken on a hefty 15,000 gallons of fuel. What unfolds next is the most deadly aviation accident in history.For a full list of sources see the show notes at timharford.com.See omnystudio.com/listener for privacy information.

Pushkin Did you know that the most deadly aviation accident in history happened on the ground? You might if you're a Pushkin Plus subscriber because the episode you're about to hear has been available to those subscribers for a year. We thought that everyone else might enjoy it and the gripping sequel in a terrifying two-part special on the 1977 Tenerife air disaster. And if you're a Pushkin Plus subscriber already, thank you. I hope you're enjoying our special episodes about the Panama Canal and there is another exclusive cautionary tale coming very soon on the cancer-beating Therac machine. But for now, we want everyone to enjoy this terrifying but fascinating cautionary

tale about the fatal consequences of a simple miscommunication. of a simple miscommunication. Captain Jacob Veldhousen van Zanten opens up the throttles on his Boeing 747. We're going, says Captain van Zanten in Dutch. The big KLM plane starts to trundle down the runway. On board are 235 passengers, looking forward to a package holiday on the Canary Islands off the coast of Morocco. But their holidays have not got off to a good start, because they've had to land on the wrong Canary Island. They've been hanging around in the airport all afternoon, waiting to be able to take off for the short hop

over to the right Canary Island. Finally, they're on their way. The crew were keen to get going. It's been a long day. They just want to drop off their passengers, pick up the ones who are waiting, and get back home to Amsterdam. The 747 gathers pace.

The runway is shrouded in thick fog. You can't see far ahead. The flight engineer says something. Captain Van Zanten doesn't catch it. What was that? He asked. It's not off yet, the Pan American. Off the runway, the flight engineer means. There's another 747, the Pan Am, that's also been hanging around all afternoon, waiting to make the hop from one island to the other. The KLM plane taxied down the runway first, then turned around.

The Pan American plane was going to taxi part of the way down the runway behind them, then turn off onto the first available exit. But was the Pan American plane definitely off the runway now? Yavel, says Captain Van Zanten. Yes, well is the literal translation, but the sense is, yes, of course. Perhaps with a hint of annoyance,

of course the Pan Am plane is off the runway. Faster and faster goes the KLM plane. It passes 150 miles an hour, too fast now to abort the take-off. And then, through the fog, a shape begins to appear on the runway ahead of them. It's the Pan Am plane. Oh, god damn it! Says Captain Van Zanten. I'm Tim Harford, and you're listening to Cautionary Tales. The Canary Islands are a group of volcanoes poking up above the Atlantic Ocean. They've been ruled from Spain since the 1400s, a colonial history of slavery and sugar cane.

By 1977 the economy has shifted to tourism and most Canary Islanders are happy enough to be part of Spain, which is just emerging into democracy after decades of dictatorship. Most but not all. The movement for the independence and self-determination of the Canaries Archipelago doesn't have many followers, but the followers it has are zealous. On Sunday, March 27, 1977, two young supporters of the movement walk into the airport on Gran Canaria with a bomb in a suitcase. The two men leave the suitcase surreptitiously outside a florist's shop in the terminal building. Then they call the airport switchboard. Evacuate the airport. We've planted bombs.

The switchboard operator is still getting the message to airport security. And the bomb goes off. Glass shatters. Plaster falls from the ceiling. Could have been worse. When the smoke clears, only a few people have been injured. But did the voice on the phone say they'd planted a bomb or bombs? They'd better get everyone out of the airport and conduct a thorough investigation. But did the voice on the phone say they'd planted a bomb or bombs?

They'd better get everyone out of the airport and conduct a thorough search. Up in the air, Captain Jakob Veldhausen-Vanzanten is preparing to land his 747, filled with expectant holidaymakers. Vanzanten has just turned 50. At KLM, he's something of a celebrity. He's the head of the flight training department. He drives a yellow Ferrari Dino. The Airlines magazine adverts feature an image of Vanzantin at the cockpit controls, looking back over his shoulder, silver-ired, sun tanned and confident, with a gleaming

white smile. At KLM, the advert promises, you'll find your trust sincerely reciprocated, thanks to that singular Dutch ability for doing things well. Vans Anton hasn't been flying much these days. He's mostly been training others. Later in the week he's due in Ireland to oversee exams for trainee pilots on the 747, and in Zurich for an international meeting on aviation safety. But he knows he needs to keep his hand in with actual flights, so he's taken this one. It should be straightforward. From Amsterdam to

Gran Canaria, then straight back home, hopefully in time for dinner. But then comes the message from Gran Canaria Airport. Don't land, a bomb's gone off. Circle above us while we figure out what to do. Then we think there might be another bomb. We have to clear the airport and search. It'll take a while. Land in Tenerife. Wait there till we give the all-clear. Tenerife is the next island over from Gran Canaria, about 50 miles away. The airport is tiny. It's not really set up for 747s. And Van Zanten's KLM flight isn't the only one that's been diverted to Tenerife because of the bomb. They're soon followed by another 747, the Pan American, and a lot of smaller planes.

They all land, taxi off the runway, and find a place to park as best they can. The tarmac's getting crowded, and nobody knows how long the delay is going to be. For Captain Van Zanten, that presents a headache. KLM has rules on how long a flight crew is allowed to be on duty. You need rules like that. You don't want tired and overworked pilots

in charge of a plane. But the rules have recently changed. Before, they were more like company guidelines. The captain could use discretion. If they went a bit over the limit with a good enough excuse, they'd be forgiven. Not anymore. Pilot overtime rules have been written into Dutch law. That's not all.

With the old guidelines, there'd been a simple way to calculate the maximum hours. The crew could work it out themselves. With the new law, it's more complicated. KLM's headquarters in the Netherlands have to apply a formula to work it out. Van Zanten walks to the control tower and asks to use the phone. He calls headquarters, explains the situation, and says, by when will I have to leave Gran Canaria if I want to be sure I can make it back to Amsterdam without exceeding the time limit?

Good question, they say. If you can get off Tenerife in the next hour or so, you'll probably be fine. If not, hmm, we'll figure it out and send you a telex in Gran Canaria. So Van Zanten doesn't yet know how much time he's got, but he does know that if he gets home a minute too late, he'll be in trouble, maybe serious trouble. The captain now has personal legal responsibility he'll be in trouble, maybe serious trouble.

The captain now has personal legal responsibility for not exceeding the crew's maximum duty time. In the cockpit, the crew discuss what all these recent changes mean. What are the repercussions? You'll face the judge. Is it a question of fines or of prison? Van Zanten joins the conversation. At any rate, he says,

It would mean revocation of your license for quite a while, and that means money. The clock ticks on. There's still no word from Grand Canaria Airport on how their bomb sweep is going. Van Zanten really doesn't want to be stuck on Tenerife or Gran Canaria for the night. He's got a busy week ahead. And besides, where are they going to magic up last minute hotel rooms for a plane load of stranded passengers? But he can't afford to risk his licence. He starts to think about what will happen when they finally get to Gran Canaria.

How long will it take to turn the plane around for Amsterdam? He'd been planning to refuel there for the journey home. But after all the delays and diversions, the airport's bound to be manic. Maybe he should try to save time by refueling here in Tenerife instead. He calls the control tower and makes the order. 15,000 gallons. Fuel trucks arrive

and wouldn't you know it, no sooner has the nozzle gone into the airplane, the news comes through from Gran Canaria. They've finished the search, no more bombs. The airport's open again. Well, they've started refuelling now. They may as well finish. Some other smaller planes start to taxi past and make their take-offs for Gran Canaria. But further down the tarmac, there's one plane that can't get past Van Zanten. It's too big. It's the other 747, the Pan American. Its pilot comes on the radio in Van Zanten's cockpit. How much longer are you going to be with that refuelling? About 20 minutes, says Van Zanten.

The two big planes sit and wait. And as they do, the weather changes. The island of Tenerife is a 12,000-foot volcano poking up above the ocean. The airport is on the side of the mountain, 2,000 feet above sea level. Sometimes clouds form quickly higher up the slopes and roll down towards the airport. That happens now. By the time the KLM plane has been fuelled up, thick fog is all around. Cautionary Tales will be back after the break. In the 1990s, academic researchers asked their students

to imagine a hypothetical plane crash on a national border, with debris strewn on both sides. They wanted to know what the students thought should happen next. Where should the survivors be buried if they were mostly European? What if they were travelling circus performers of no fixed abode? About half the students thoughtfully engaged with the question of the most appropriate burial site. Only half said, Hold on, you asked where the survivors should be buried, you don't bury survivors, they're

still alive. The researchers were trying to learn more about a curious phenomenon known as the Moses illusion. It had been discovered a few years earlier by the social scientist Thomas Erickson and the psychologist Mark Mattson. How many animals of each different kind did Moses take onto the ark? The answer, of course, is that it wasn't Moses. It was Noah. Still, when Erickson and Matson asked experimental subjects this question,

they found roughly half gave the confident answer, two. It's not that these people didn't know who took the animals on the ark. When asked afterwards, they were well aware that Noah was the ark guy, and Moses was the one who parted the Red Sea and brought down the Ten Commandments. So why hadn't they noticed that the question was wrong? Erickson and Mattson were intrigued. Perhaps they thought it's because Moses and Noah sound somewhat alike. They're both two syllable names. They picked another two syllable name and ran an experiment. How many animals did Nixon take on the Ark?

This time, nobody answered two. Everyone said, Nixon. And they tried another Old Testament name instead, one that sounds nothing like Noah. How many animals did Abraham take on the ark? Again, a lot of people answered two. Language scientists are still investigating exactly what's going on when people make this kind of error. They run experiments, scanning people's brains as they listen to different sentences

that might trigger the illusion. But the basic problem seems clear enough. Sometimes the brain fills in the gaps. It hears what we expect to hear, rather than what's actually said. Something, something, arc, animals, how many? If another word in the sentence sounds incongruous, like Nixon, we pay attention and the illusion breaks. But when all the other words sound conceptually related,

Ark, animals, some dude from the Bible, we often don't notice the mistake. The researchers who asked students about burying survivors of a plane crash found something similar. They wanted to test how variations on phrasing the question would affect how many people fell for the illusion. But they also tested a question without that conceptual link. Instead of asking about a plane crash, they asked about a bicycle accident. Where should we bury the survivors? Nobody suggested location, apparently because survivors isn't a word you usually hear with bicycle accident. We notice straight away that something's wrong.

But survivors and plane crash, they often go together. So the brain might not flag up that the sentence makes no sense. We just hear something something plane crash, bury, where? And we fill in the blanks for ourselves. At the airport in Tenerife, at long last, the final drop of the 15,000 gallons of fuel has been deposited in the tank of the KLM plane. An airport employee comes into the cockpit

to give Captain Van Zanten the good news. Is Captain Van Zanten so stressed by the delay that he can't think straight? If he is, he's doing a good job of hiding it. The airport staff said that he was kind, polite, and easygoing. He gets on the radio to the control tower. Tenerife KLM 4805 is ready to start.

The controller instructs him to taxi down the runway. Usually a plane will use a separate taxiway to get themselves to the right end of the runway, but that's not an option here. In the cockpit, the KLM crew can also hear everything that's being said between the control tower and the pan and plane, called Clipper Victor. They're using the same radio frequency. Tenerife, Clipper 1736.

Clipper 1736, Tenerife. We were instructed to contact you and also taxi down the runway, is that correct? Affirmative. Taxi into the runway and leave the runway third. Third to your left. Third. Third to the left, okay. So Captain Van Zanten knows that the Pan Am plane, the Clipper 1736, is going to taxi down the runway behind him. The clouds are coming and going. The fog thickens and eases and thickens again. At times, you can barely see more than 300 meters ahead, less than a tenth of the length of the runway.

From the control tower, the controller can't see the planes on the runway. He has to ask where they are. And in the fog, it's not always easy for the pilots to be sure how many of those exits to the left they've gone past. KLM 4805, how many taxiways did you pass? I think we just passed Charlie 4 now.

OK, at the end of the runway, make a 180 and report ready for ATC clearance. OK, sir. ATC clearance, that's air traffic control. Before they can depart, planes need two kinds of clearance. There's clearance for the take-off itself, and there's ATC clearance. That's for where to go after you've taken off. They're separate things. In the KLM cockpit, another conversation

between the Pan Am plane and the control tower comes over the radio. Would you confirm that you want the Clipper 1736 to turn left at the third intersection? Third one, sir. One, two, three, third. Third one. Very good, thank you. Clipper 1736, report leaving the runway. Report leaving the runway. The controller is asking them to report on the radio when they're leaving the runway.

Is it possible Captain Van Zanten misheard and thought they were reporting they were leaving the runway? Maybe. Captain Van Zanten gets to the end of the runway and turns his plane around. That isn't easy. The 747's a big plane with a wide turning circle. But he manages. They're pointing the right way now. In the cockpit, the first officer is going through the last of the formalities.

Body gear disarmed, landing lights on, checklist completed. Van Zanten releases the brakes and starts to move forward. Wait a minute, we don't have an ATC clearance. No, I know that. Says Vansantan. Go ahead and ask. Hmm, did Vansantan know that? Or had he forgotten?

Vansantan, remember, was the head of KLM's flight training department. It had been 12 weeks since he'd been in charge of an actual plane. He's more used to instructing trainees in the simulator, and as the instructor, he also plays the role of the controller. He's the one who gives clearances to the trainee pilot. Has he got too accustomed to not needing clearance from anyone else? Perhaps.

The first officer gets on the radio. The KLM 4805 is now ready for takeoff and we're waiting for our ATC clearance. Remember, they need two types of clearance from the control tower. The ATC clearance for their route after takeoff and the clearance for takeoff itself. The first officer has asked for both types of clearance.

Explicitly, we're waiting for our ATC clearance and implicitly, we're ready for takeoff. In response, the controller gives him the ATC clearance but doesn't tell him that he can take off yet. KLM 8705, you are cleared to the Papa Beacon. Climb to and maintain flight level 90. Right turn after takeoff. Proceed with heading 040

until intercepting the 325 radial from Las Palmas. By the way, what's the KLM flight's number? In that last message, the controller messed it up. He said 8705, not 4805. Did you notice? Why should you? It doesn't matter. There's only one KLM plane here.

Everyone knew what he meant. Maybe that's why our brains evolved to be susceptible to the Moses illusion. Most of the time, when we hear what we expect to hear, it's also what the person we're talking to intended to say. It's our brain helpfully filtering out a misspeak. But the Moses illusion can be deadly. Captain Van Zanten was expecting to hear

that he was cleared for take-off. He got only the air traffic control clearance, but the sentence did also contain the word take-off and a bunch of other words that are all conceptually related. It seems that all he heard was something-something cleared take-off. We're going, said Captain Van Zanten. He opened the throttles and the plane began to move. The calamity could still have been averted.

We'll hear why it wasn't after the break. When Captain Van Zanten said, we're going, his first officer was just finishing reading back the air traffic control clearance to the control tower. Roger, sir, we are cleared to the Papa Beacon flight level 90, right turn out 040 until intercepting the 325 and we're now at takeoff. Is that what he said? At takeoff? That's what most of the investigators heard. Some think he said, we're now taken off. But the controller seems to have heard, at takeoff. And understood it to mean, we're in position, ready for takeoff. OK. Said the controller.

Standby for takeoff. I will call you. But then, a tragic coincidence. Both the KLM plane and the Pan Am plane, remember, are using the same frequency to talk to the control tower. In the cockpit of the Pan Am plane, they've also heard the KLM First Officer say, we're now at takeoff. And they think they'd better make sure everyone knows where they are, still moving slowly along the runway, trying to count the exits in the fog. And we're still taxiing down the runway.

But both transmissions happen at the same time, and that causes interference. In the KLM cockpit, all they hear is... Standby for takeoff. I will call you. They might just have made out what was being said if they'd been listening closely. They weren't. Captain Van Zanten was pushing the big plane down the runway, faster and faster through the fog.

But then another transmission on the radio, between the controller and the Pan Am plane, and this one was perfectly clear. Papa, Alpha 1736, report runway clear. OK, we'll report when we're clear. Captain Van Zanten doesn't respond to this, nor does the First Officer. The flight engineer has heard the exchange and he speaks up.

Is he not off yet, then? What was that? He's not off yet. The Pan American? Jawel. Of course the Pan Am plane is off the runway. Why is Van Zanten so sure? Perhaps because he mistakenly believes he's been cleared for takeoff and he knows the controller wouldn't have given him that clearance if the runway was still blocked. But hasn't he just heard that exchange on the radio with

the controller saying, report when runway clear, and Pan Am replying, we'll report when we're clear? Maybe he wasn't listening, or maybe he half-heard, and it was another Moses illusion. Something, something, runway clear. Van Zanten heard what he expected. After every accident, there are some causes you just have to file under one of those things. There isn't much of a lesson we can draw from the strange sequence of the bomb, the diversion, the refuelling and the fog.

But sometimes there are causes you can learn from, and they change how things are done. Nowadays, all members of a flight's crew are trained to speak up, respectfully but forcefully, when they think the captain is making a mistake. There's room to wonder if the First Officer and the flight engineer felt too timid to contradict Captain Jakob Veldhausen-Vanzanten, the celebrity pilot whose face beamed from the KLM adverts. Some investigators think the first officer said, we're now, uh, taking off?

In a tone of surprise, trying to alert the controller that Van Zanten had jumped the gun. Maybe. But just a few moments earlier, he had reminded Van Zanten that they didn't have air traffic control clearance. That doesn't sound like he was too overawed to point out when his boss was messing something up. If he'd realised they weren't cleared for takeoff, he'd surely have mentioned that too.

What about the flight engineer? Was he genuinely reassured by Van Zanten's emphatic Javel, assuming the captain must have access to information he doesn't know? Or did he spend the last few seconds of his life thinking, should I insist he aborts the takeoff? That'll delay us and I'll get the blame. Would he have been more insistent if he'd had the kind of training flight crews get today? We'll never know. But something else changed, too,

as a result of what happened that day on Tenerife. Let's pick up our copy of A Guide to Phraseology for General Aviation Pilots in Europe and turn to page 7. Listen carefully to make sure you understand what is said to you. It is easy to hear what you expect, rather than what is actually said. It is easy to hear what you expect, rather than what is actually said. A perfect warning not to succumb to the Moses illusion. Pilots are now trained to be aware of the risk. But apart from asking people to listen more carefully,

what can we do if we want to minimize the chance that someone might fall prey to the illusion? One approach is simply not to say the words that might lead to confusion. Turn to page 17 of our guide to aviation phraseology. To avoid confusion, the word cleared is only used in connection with a clearance to take off or land. The words take off are only used when an aircraft is cleared for takeoff or when cancelling a takeoff clearance. Pilots and controllers are now taught to use different words in different situations. Instead of ready for takeoff, say ready for departure.

Instead of after takeoff, say once airborne. If you want to make sure nobody hears something, something cleared takeoff, don't say the words cleared or takeoff at all. The KLM plane is hurtling down the fog-bound runway in Tenerife. And there's the Pan Am plane, still searching for the right exit. Captain Van Zanten is going over 150 miles an hour, too quickly to stop.

But is he going quickly enough to get into the air? He yanks on the controls. The nose points up, tail slams down into the runway. It leaves a streak of metal in the tarmac, 20 meters long. At last the plane starts to lift off the runway, but it struggles. If only it didn't have those extra 15,000 gallons of fuel on board. The nose clears the top of the Pan Am plane, but the rest isn't going to make it. Oh, God damn it!

The bottom of the plane clips the top of the other. It crashes back onto the tarmac and careens along the runway. The collision ruptures the fuel tanks. The 15,000 gallons explode into a fireball, hot enough to melt the fuselage. Flames rise high in the air. Everyone on board is killed instantly.

44 of the bodies are so badly burned, they can never be identified. Among them, Captain Jakob Veldhausen-Vanzanten. A few hundred metres back from the fireball stands what's left of the Pan Am plane. Some of its passengers have also been killed already. Others are still alive. But many of these survivors will end up being buried. They don't have long before fire will begin to consume their plane. We'll hear what decides who makes it out and who doesn't in the next episode of Cautionary Tales.

An important source for this episode was Collision on Tenerife, the how and why of the world's worst aviation disaster, by John Ziemek and Caroline Hopkins. For a full list of our sources, see the show notes at timharon.com. Corortionary Tales is written by me, Tim Harford, with Andrew Wright. It's produced by Alice Fiennes, with support from Marilyn Rust. The sound design and original music is the work of Pascal Wise. Sarah Nix edited the scripts. It features the voice talents of Ben Crow, Melanie Gutteridge, Stella Harford, Gemma Saunders

and Rufus Wright. The show also wouldn't have been possible without the work of Jacob Weisberg, Ryan Dilley, Greta Cohn, Vital Mollard, John Schnarze, Eric Sandler, Carrie Brody and Christina Sullivan. Cautionary Tales is a production of Pushkin Industries. It's recorded at Wardour Studios in London by Tom Berry. If you like the show, please remember to share, rate and review. Tell your friends. And if you want to hear the show ad free, sign up for Puskin Plus on the show page in

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