Countdown to Destruction: the 'Tenerife Airport Disaster'

• The 1979 American Airlines Flight 191 crash - The accident cost the lives of over 270 people and was found to have been the result of mistakes made during routine maintenance. 
• The 2003 Iranian Air Force Ilyushun II-76 crash - 275 people were killed when a military aircraft crashed into the Sirach mountains near Kerman in Iran. Poor weather and bad visibility were blamed for the crash that killed all members of the Iranian Revolutionary Guard on board, among others. 

• The 1988 Iran Air Flight 655 crash - During the Iran-Iraq war, an Airbus A300 was shot down by a surface-to-air missile from the US cruiser USS Vincennes. All 290 people on board were killed on site. It was mistaken for a hostile F-14A Tomcat fighter, and after failed attempts to hail them, the order was given to fire. 
• The 2014 Malaysia Airlines Flight 17 - One of the most famous crashes in recent times, the Malaysia Airlines Flight 17 was also brought down by a missile over Eastern Ukraine in 2014. All 298 passengers were killed.
• The 1980 Saudi Arabian Airlines Flight 163 - While not technically a crash, over 300 people were killed when Saudi Arabain Airlines Flight 163 landed in Riyadh. A fire had started in the cargo bay shortly after takeoff, and the plane made an emergency landing moments later. Instead of immediately evacuating the plane, the pilots decided to taxi the aircraft back to the airport. When the doors were finally opened, all passengers had been killed by the toxicant fumes.
• The 1985 Air India Flight 182 crash - Widely considered one of the deadliest terrorist attacks up to that time, 329 people were killed on board the 1985 Air India Flight 182. En route from Toronto to Sahar International Airport in India, the plane crashed off the coast of Ireland after an explosive detonated in the cargo hold. The investigation found many failings in the security protocol in Canada. 
• The 1974 Turkish Airlines Flight 981 crash - A design flaw in the DC-10 led to the death of around 346 people onboard Turkish Airlines Flight 981 in 1974. After taking off from Paris Orly Airport, en route to London Heathrow, the plane suddenly crashed in the ermenonville forest just north of Paris. Moments after takeoff, the left rear cargo door blew off, causing a sudden decompression in the cabin that ultimately crippled the plane.

• The 1966  Charkhi mid-air collision - in 1996, 349 people were killed when Saudi Flight 763 collided with Kazakhstan Airlines Flight 1907 while above Charki Dadri, India. The collision was observed by a passing US Air Force plane that described it as "a large cloud lit up with an orange glow". The collision was the result of a variety of factors, including the failure of the Kazakhstan Airlines pilot to follow air traffic control instructions properly. 
• The 1985 Japan Airlines Flight 123 crash - Widely considered the worst crash to involve a single aircraft, 520 people were killed when a Boeing 747 crashed into Mount Takamagahara in Japan. The plane's wing clipped a mountain ridge, flipped over, and eventually landed on its back, following an explosive decompression at the rear of the plane resulting from faulty maintenance. Amazingly, the pilots managed to keep the plane airborne for over 30 minutes before the plane finally succumbed to its fate. This feat has never been replicated in simulators since. A month after the crash, a Japan Airlines maintenance official committed suicide, leaving a note saying that "I am atoning with my death." 

What is the world's worst plane crash?

The world's worst non-terrorist plane crash is widely considered to have been the 1977 "Tenerife Airport Disaster". On the 27th of March 1977, KLM Flight 4805 slamming into Pan Am Flight 1736 as it prepared to take off leading to the death of all 248 passengers on board the KLM flight and 335 of the 396 passengers on the Pan Am flight, for a total of 583 deaths. Sixty-one of the passengers on the Pan Am flight survived.

Sadly for all involved, neither aircraft was actually meant to be there that day. Both had been rerouted following a bomb scare at their original destination at Las Palmas, Gran Canaria. 

“The magnitude of the accident speaks for itself, but what makes it particularly unforgettable is the startling set of ironies and coincidences that preceded it,” said Patrick Smith in a Telegraph article on the subject.

Heavy fog obscured visibility, and when the KLM pilots realized the danger, they attempted to "leap-frog" their plane over the Pan Am aircraft but ended up clipping its mid-section with the belly of their aircraft. 

What caused the "Tenerife Airport Disaster" crash?

Back in 1977, the Boeing 747 was enjoying its 8th year of service and was the biggest and most glamourous commercial jetliner in the air. It also had the largest passenger carrying capacity of any jetliner in the air. 

It seemed almost inconceivable that two such behemoths would ever collide, if they did, the death toll would likely be astronomical. However, sadly, this nightmare scenario is exactly what happened on the foggy runway of Tenerife Airport in 1977. 

But how could this have happened with two of the most advanced commercial aircraft of the time? 

Like many serious accidents, the cause was not a single error or failure but a chain of events that ultimately led, in his case, to the death of more than 580 people. 

We'll borrow heavily from this very informative dissection of the events that day by Patrick Smith in the Telegraph. 

One of the planes, the Pan Am Boeing 747 Flight 1736  (plane registration N736PA) was already semi-famous as the aircraft that made the inaugural 747 commercial voyage between New York's Kennedy Airport and London Heathrow seven years earlier. 

The KLM captain, Jacob Van Zanten, was the airline's top 747 instructor pilot and something of a KLM celebrity, but would be killed during the crash along with all of his passengers. 

The Los Rodeos Airport in Tenerife, normally a quiet airport, would be very hectic that day, and both aircraft sat adjacent to one another in the SE corner of the apron. At around 4 o'clock in the afternoon, Las Palmas began accepting air traffic and both planes readied themselves for taxiing and takeoff. 

At that time, the weather was relatively fine, but the KLM aircraft requested extra fuel, delaying takeoff by a precious few minutes. During the delay, a heavy fog descended onto the airport and the extra fuel on board the KLM 747 made it slightly more sluggish during takeoff. 

After fueling, and due to the positioning of both aircraft, the KLM plane began to make its way to the runway first. 

As the airport was unusually busy that day, some aircraft were required to taxi down the main runway prior to take off and then make a 180-degree turn, in a maneuver called a "back-taxi", which the KLM flight was requested to do. 

The Pan Am 747 follows shortly after but was requested to exit the runway at its 3rd exit (C3) to keep runway 30 clear for the KLM plane to take off, while the KLM flight completed its turn at the end of the runway. 

Once clear of the runway, the Pan Am crew were requested to report to the tower to wait for the green light for the KLM flight. That was the plan, at least. 

During the maneuver, the Pan Am pilots mistake their assigned turnoff (due to the poor visibility) and remain on the runway for a few precious moments longer than expected. At the same moment, the KLM plane completed its turn and stood ready for takeoff. 

The KLM's first officer, Klaas Meurs, radioed the air traffic control and was given an ATC route clearance (not a permission to takeoff). This is normally given well in advance, but it has been a busy day and the pilot's patience was being tested. 

Sadly, as these instructions were given late, the KLM crew also mistook it for permission to take off — a mistake that will soon cost many hundreds of lives. They opened the throttle and the 747 began to speed down the runway. 

This would not normally be a problem, but the runway is not clear as the Pan Am 747 is yet to taxi off it! ATC and the cockpit crews for both planes attempt to communicate with the other plane, but as they were using two-way VHF radios, it was difficult to hear each other simultaneously, losing more precious time in interference and static. 

The miscommunication continued, with the KLM crew believing the runway was clear, and the Pan Am crew believing the KLM flight was still waiting. However, the KLM co-pilot suspected this might not be the case and asked, "Is he not clear, that Pan American?" 

Van Zanten responds "Oh, yes", but continued with the takeoff procedure regardless of his co-pilot's concerns. 

Moments later, the Pan Am crew spotted the lights of the KLM plane, and attempted to throttle up and get out of the way. Suddenly the KLM pilot, Van Zanten spotted the Pan Am flight too, and he yanked back on the elevators, dragging his plane's tail along the pavement for 70 feet (21 mt) in an attempt to "leap-frog" over the Pan am plane. 

The maneuver almost works, but the belly and undercarriage slice into the ceiling of the Pan Am plane, demolishing its midsection and setting off a series of explosions. The KLM plane crashed back onto the runway, skidded hard for about a thousand feet, and burst into flames. 

The ensuing investigation discovered that very serious communication issues between pilots and air traffic control were to blame. It led to the creation of crew-resource management policies and procedures to attempt to reduce the impact of human error as well as the adoption of English as the standard language for communications in the airline industry around the world. 

The accident would shake the airline industry to its core, and some "givens," like the ultimate authority of the plane's captain, would be seriously curtailed.