 When investigating the causation of disasters, the Swiss cheese model was often brought up. I know it sounds like a strange comparison to bring up cheese, but bear with me. Imagine a block of holy cheese, sliced. Some holes only go part of the way through it. Imagine an event, say you tripping up on a child's toy. This could have been prevented in a number of ways, say telling your two-year-old child to put their toys away after playing with him or looking where you're going. In the Swiss cheese model, either of these potential blockers for your painful fool would have averted disaster. But say all of those failures or holes line up. Like how you can line up the holes in slices of Swiss cheese to see all the way through. Then disaster is inevitable. Here is a more official sounding explanation. The Swiss cheese model likens human systems to multiple slices of cheese stacked side by side, in which the risk of threat becoming a reality is mitigated by the differing layers and the types of defences which are layered behind each other. This means that one failure wouldn't result in disaster. However, more multiple lapses in safety procedures and human factors line up those holes, resulting in a bigger failure of the systems placed to prevent disaster. Now I've got you thinking about tasty cheese. You might be asking, what has this got to do with anything? Well, one such incident in Tenerife in 1977 would be the perfect example of how this model works. Today, we're looking at the collision of KLM Flight 4805 and Pan Am Flight 1736. As such, I'm going to rate it here 7 on my patented plane default disaster scale, but here 9 on my legacy scale. This is the result of an impatient pilot, multiple communications failings by using non-standard phrases, radio interference, a lost plane on a runway, a terrorist attack at another airport and poor visibility due to low clouds. Each individual situation may have not led to disaster, but when added together, the collision of KLM Flight 4805 and Pan Am 1736 almost became inevitable. It is the 27th of March 1977 and members of the Canary Islands independence movement detonate a bomb in a flora shop in Gran Canaria Airport near Las Palmas, Gran Canaria, Injuring 8. As security forces scrabble to investigate, a second bomb threat is called in. Not wanting to be responsible for a massacre, the police are forced to shut down all traffic to Gran Canaria Airport whilst they search for the hidden device. With multiple planes in the air, the decision by air traffic control is to divert many aircraft to nearby Los Rodeus Airport in Tenerife, now called Tenerife North Airport, some 75km away. Los Rodeus, by the early afternoon, was a wash with aircraft far more than usually experienced at the airport. The airport had only one runway and one main taxiway running parallel to it, with four short taxiways for connecting the two. So much was the congestion on the ground that planes were blocking the taxiway, forcing aircraft to taxi on the runway before positioning themselves for takeoff, a procedure known as back-taxing. At the beginning of the shutdown, Pan Am Flight 1736's crew would have much preferred to stay in the air in a holding pattern, as at 1315 they had around two hours worth of fuel. However, the flight was ordered to land in Tenerife, much to the disappointment of her crew and passengers. Much better for the passengers' comfort to be ready to land when the all-clear was given, especially due to the long distance the flight had covered from Los Angeles with an intermediate stop at New York. The crew on board consisted of Captain Victor Grubbs aged 56, First Officer Robert Bragg aged 39, Flight Engineer George Warnes 46 and 13 flight attendants, along with 380 passengers, mostly of retirement age. The flight crew had over 30,000 hours of experience, although not all covered on the Boeing 747, the crew were confident in the plane. The Pan Am plane was quite a historically important specimen for the 747's legacy. It had operated the inaugural 747 commercial flight on the 22nd of January 1970. Not only that, but the plane had the honour on the 2nd of August 1970 to become the first 747 to be hijacked. Another plane caught out during the disruption of services to Gran Canaria Airport was KLM 4805, which, much like Pan Am 1736, had found itself diverted to Tenerife. The KLM aircraft was also a 747 and was chartered by the Holland International Travel Group and had departed from Amsterdam Airport's Shrevepole, Netherlands. The crew were also experienced but was captained by somewhat of a KLM celebrity, Jacob Godhuzen Van Zanten aged 50. Van Zanten was KLM's star pilot and head 747 training instructor. So much was his value to the company that his face was even on advertising material for the airline. His flying career started in 1947 when he gained his private pilot's licence and by 1950 he had achieved his commercial certification and began working for KLM the same year. By 1977 he had amassed over 11,000 hours of flying time, 1,500 of which were on the 747. Accompanying Van Zanten on the crew of the KLM were First Officer Class Merz and Flight Engineer William Schrader. Again, like the Pan Am flight, the team combined had several tens of thousands of hours of experience. By late afternoon, Gran Canaria Airport slowly opened up to traffic again. This meant that the much delayed and diverted aircraft could complete their journeys. Not wanting to be delayed further at its destination and not wanting to break the laws on duty crew working time, Van Zanten took the opportunity to refuel the KLM where they were. However, this would block in other aircraft including 1736 for over half an hour. Much to the annoyance of the Pan Am, which still had enough fuel on board for the final leg of its journey, the captain and First Officer left the plane to measure the gap between the KLM and the active runway, but the distance was lower than the safe minimum. They were obliged to wait. The added delay was further compounded by a Dutch family not going back to the KLM on time. Finally, at 1656, Van Zanten's KLM radioed the control tower for clearance to taxi. Canaria Airport is at 633 metres above sea level. This has an unusual side effect. Clouds at 600 metres above ground level at the nearby coast are at runway level at Los Rodeos. With clouds being blown in from the sea, this creates variable visibilities where clear visuals one minute can almost instantly drop below minimums the next. And the 27th of March was just such a day. After three minutes of confused conversation, at 1659, the instructions to proceed down the main runway and to backtrack for takeoff were received and understood by the flight crew. These instructions meant that the KLM would turn 180 degrees once at the end of the runway in preparation for takeoff. At 1702, the Pan Am called the tower for clearance to taxi. This was granted, but with the additional instructions to turn onto the third left taxiway. Pretty straightforward you might think, but when you look at the diagram of the airport, those instructions make little sense. Only two taxiways on site look viable for a 747 to make the desired turn to reach the end of the runway. The taxiways were unmarked and by now visibility on the runway was concerningly low. The Pan Am called up the tower to confirm that it was not indeed taxiway one instead of the requested three. The ATC replied abruptly, third one sir, one, two, three, third, third one. The aircraft proceeded to taxi up the main runway, marking off the taxiways to their left as they passed. Number one and two were successfully identified, but lost in a thick fog. The crew failed to identify number three. A confused conversation broke out in the cockpit, hinting that they were now lost. Meanwhile, the KLM plane contacted ATC to confirm that they had passed a fourth exit. The tower replied OK, make 180 and report ready for clearance. Almost immediately after the plane was in a position for takeoff, Van Zanten pulled back the throttles and the plane started to move. First Officer Meurs advised Van Zanten that ATC clearance had not been given and the captain replied no, I know that, go ahead and ask. In doing so, bringing the engines back to idle. At 1705, the KLM confirmed it was ready for takeoff and awaiting ATC. Soon after ATC informed the KLM what their route was to follow after takeoff, but no authorization for takeoff was actually given. The KLM replied and confirmed the information given, but added we are at takeoff, which is a non-standard message. ATC replied OK, stand by for takeoff, I will call you back. During this message, the KLM experienced radio interference, muffling the instructions. Van Zanten interrupted the co-pilot's readback with the comment, we're going. Taking ATC's non-standard use of OK as authority to proceed. The KLM released its brakes, took power and started to move down the runway. With ATC not being able to see the two planes in the thick fog, they thought all movements were still under control. The tower then contacted the Pan Am flight, saying Papa Alpha report when runway clear. To which Pan Am replied OK, we'll report when runway clear. Unbeknownst to the Pan Am and the tower that whilst this final conversation took place, the KLM had started on a head-on collision course. Hearing the communications between the tower and the Pan Am, the KLM engineer said, is he not clear that Pan American? Van Zanten replied, oh yes, but still continued with the takeoff. The KLM loomed off in the fog as it hurtled towards the Pan Am. Captain Victor Grubb shouted, there he is, the Pan Am was now near exit number four. Grubbs yelled, goddamn that son of a bitch is coming. Captain Bragg applied the throttles and tried to turn onto the grass off the runway in an effort to avert disaster. Shortly after the KLM crew saw the Pan Am on the ground, by now it's travelling too fast to abort the takeoff. They rotated the aircraft and attempted to clear the Pan Am by getting airborne, causing a tail strike. At a speed of 160 miles per hour and at a distance of 100 metres from the Pan Am, the KLM's nose gear cleared the aircraft off the ground. But the left side engines, lower fuselage and main landing gear struck the upper part of the Pan Am's fuselage. The result was ripping apart the centre part of the Pan Am, almost directly above the wing. The right side engines crashed through the Pan Am's upper deck behind the cockpit. The momentum of the takeoff speed of the KLM led to a brief time in the air, but it had received critical damage and subsequently the plane stalled, took a sharp roll and impacted the ground 150 metres down from the Pan Am. After impact, the freshly fuelled aircraft burst into an uncontrollable fireball engulfing all within. Seeing the Inferno, rescue workers made a beeline to the site of the KLM, completely ignoring the Pan Am. In a miraculous turn of luck, the captain, first officer and flight engineer of the Pan Am 1736 had survived. This would come to haunt them for the rest of their lives. The only survivors of the disaster were 61 members of crew and passengers aboard the Pan Am. Many escaped through and now open central part of the fuselage onto the still intact wings, jumping to the relative safety of the runway. All aboard the KLM perished, most likely from the impact and fire, when the aircraft hit the ground. Although not severely deformed, the escape doors on the plane's fuselage were not operated. The 61 survivors were promptly transported to Santa Cruz hospitals. Needless to say, Los Rodeos Airport, the only operating airport on Tenerife at the time, was closed to all fixed-wing traffic. This closure would last for two days to allow some clear-up efforts. With 583 dead, 61 injured and two multi-million-dollar aircraft written off, the only questions available are how and why. Initially, KLM, upon hearing about a collision involving one of their fleet, set about assembling an investigation team. The logical person for this task would be the head of pilot training, Captain Jacob Baird-Housen Van Zanten. But the company would be shocked to find out at the helm of the disaster was their most experienced 747 pilot. The investigation into the event would gain international attention, in part due to it being a collision between a Dutch and American Airlines on Spanish soil, but also due to the entirely preventable nature of the crash. Both planes were sound mechanically and both crews had more than enough experience for operating in such conditions. However, Van Zanten did take off, disregarding all the apparent warning signs, for example, his first officer not gaining clearance for take-off and the ATC giving no explicit instructions to allow him to, which should have pointed him to stay on the ground. In the back of the captain's mind was a recently imposed law on the limits of time a crew can operate. When the rule came into force, it was relatively easy for a captain to work out how much time his crew had to work. But after December 1976, the process to work out the amount of operating time left became too complicated. This meant that crews could not work without fully knowing their remaining time and as such aired on the side of caution, as a violation could result in not just punishment from the company, but also Dutch law. This would have played on Van Zanten's mind and even on that of the crews, where exceeding the limit would result in the plane being grounded at Los Parmas and could have cost the company money and affected plans back at home. The fear of being stuck far away from home would have had an effect on the crew called fixation, an almost tunnel vision like mental state where the goal is the most important thing and all potential hazards are ignored. In the Spanish investigation into the crash, it was thought that this phenomenon along with communication areas led Van Zanten to believe that he had the authority to take off. Let's look at the communication areas that could have caused this. The Dutch investigation pointed out that during communications between the tower and both planes, background noise could be heard from a football game being listened to. This could have contributed to some confusion or distraction amongst communications. Also, the tower had to deal with a higher than normal workload. On top of that, poor visibility due to low clouds, no one could see where the other was. Also, the instructions to the Pan Am to use taxiway 3 would have resulted in a difficult manoeuvre for the crew to do, leading to confusion and eventual loss of position of the plane. During the vital clearance request stage, once the KLM had lined up, a misunderstanding led the crew to believe they had clearance. The statement by the first officer, we are ready for take off and we are awaiting ATC clearance, was responded to from the tower with your clear to Papa Alpha Beacon, which was the ATC clearance. The intention of the ATC was to inform them of their route after take off, but this was understood as you are clear to go. The second part of the control tower's message was to stand by for take off clearance, which was continued by OK followed on with a two second gap, ending with the message I will call you back. Captain Van Zanten took power halfway through the read back of the first officer in the assumption that the authority had been given, which was further reinforced by hearing the word OK. His crew on board could have questioned the assumption that his seniority and experience led them to accept his decision. The main cause of the accident was the confusion in communication, where the KLM thought it had clearance for take off, or simultaneously the tower fought the plane with stationery. The final potential diversion from disaster was thwarted by pure bad luck. As the Pan Am heard the word OK from the tower, I feared the KLM could misunderstand this. In desperation they radioed saying that they were still on the runway, but this was blocked out by the tower's two second gap between the words OK and I will call you back. This created a squeal sound in the KLM cockpit, and the warning sign was missed. Multiple failures lined up at Tenerife, which each individual one wouldn't have resulted in the crash, but when each hole was matched, the outcome was inevitable. The Dutch government initially refused to accept that Van Zanten was responsible for the accident, but with relent and KLM paid the victim's family's compensation, ranging between $58,000 and $600,000. The event shook the airline industry and helped push forward the understanding of human factors, most importantly, completion bias and fixation. These issues still remain in all industries today, but the greater of one's ability to recognise these can help prevent future disasters. Las Rodas has now been relegated to domestic and smaller airplane flights. The airport has subsequently been renamed to Tenerife North. A newer, more modern airport opened in 1978 and has taken the load of international flights named Tenerife South, which by 2018 would handle over 12 million passenger journeys per year. This is a plain difficult production. All videos on the channel are creative commons attribution share alike licence. Plain difficult videos are produced by me, John, in a sunny southeastern corner of London, UK. Help the channel grow by liking, commenting and subscribing. Check out my Twitter for all sorts of photos and odds and sods, as well as hints on future videos. I've got Patreon and YouTube membership as well, so if you fancy, check them out. And all we have to say is thank you for watching.