Forty-six years ago, the deadliest peacetime aviation disaster in history took place. 

On the morning of March 27, 1977, the skies over the Atlantic Ocean near the Canary Islands were packed with planes, most of them full of holiday-goers looking to enjoy a nice vacation. That day, most of the planes in the sky were bound for the main international airport of the Canary Islands, Gran Canaria (LPA).

At 13:15 a bomb was set off at Gran Canaria airport by Canary Islands Independence Movement fighters. Shortly after this, officials decided to temporarily close Gran Canaria airport, forcing all flights inbound to Gran Canaria to divert to the much smaller, and less capable Los Rodeos airport (Now Tenerife Norte Airport) on the neighboring island, Tenerife.

Among the flights being forced to divert were Pan Am flight 1736 and KLM flight 4805. 

Pan Am 1736 

Pan Am flight 1736 was being flown by a Boeing 747-100(N736PA) named Clipper Victor. The flight had originated in Los Angeles (LAX), stopping in New York-JFK before making the stretch across the Atlantic. The crew flying the 747 to Gran Canaria consisted of Captain Victor Grubbs, First Officer Robert Bragg, and Engineer George Warns, along with 13 flight attendants. Of the 380 passengers, only four were not American nationals. 

Upon receiving the directions to divert, the crew of flight 1736 suggested that they remain airborne until Gran Canaria Airport reopened, as the aircraft still had two hours left of reserve fuel. However, authorities ordered the plane to land in Los Rodeos. 

Side Note: N736PA was the first ever 747 to enter commercial service on 22 January 1970. Later that year, the aircraft became the first 747 to undergo a hijacking on its way from New York JFK to San Juan Puerto Rico. 

KLM 4805

Flight 4805 was being operated as a charter for Holland International Travel Group from Amsterdam (AMS) to Gran Canaria (LPA). Operating the charter flight was a Boeing 747-200, PH-BUF, named Rijn. The crew of flight 4805 was extremely experienced, with the pilot, Captain Jacob Veldhuyzen van Zanten, being KLM's chief flight instructor. Also in the cockpit were first officer Klaas Meurs, and flight engineer Willem Schreuder. Of the 235 passengers onboard, 14 were crew, 52 were children, and the rest of the passengers were various European nationals — mostly Dutch, with German, French, and Austrian nationals also being present onboard. 

An Overloaded Airport

Shortly after aircraft began landing at Los Rodeos airport, the airport realized that it was incapable of handling such traffic. The airport had only one runway and one main taxiway, with a small apron area with a capacity for only small-to-medium-size jets. Because of this, there were so many diversions that planes were forced to park on the taxiways.

When Gran Canaria airport was finally re-opened by authorities, Pan Am 1736 was ready to depart but, because it had parked behind the KLM 747-200 (Flight 4805) on the taxiway, it had to wait for the KLM aircraft to finish refueling. During the 35 minutes in which flight 4805 was refueling, the passengers were de-boarded and taken to the terminal at Los Rodeos. Of the 235 passengers, only one chose to stay on the island of Tenerife, de-boarding at Los Rodeos. This would be the only survivor of KLM 4805. 

Pre-Collision & Loss of Visibility 

As a result of the traffic, aircraft were forced to utilize a procedure called "back-taxiing," which involves pilots utilizing an airport's runway to get into position for takeoff by taxiing the length of the runway, then making a 180-degree turn to face the opposite end.

Due to its proximity to the runway, air traffic control at Los Rodeos gave KLM 4805 permission to back-taxi first. Upon its entry to the runway, the crew of KLM 4805 was going through pre-takeoff checklists and missed the Air Traffic Controller's instructions to read back the clearance given to them. 

Pan Am 1736 soon followed the KLM 747 onto the runway. The Air Traffic Controller's KLM 747 would back-taxi and get into takeoff position while the Pan Am flight would taxi off the runway at runway exit C4. The Pan Am crew became confused, and the air traffic controller had to clarify which exit the Pan Am 747 was supposed to take to exit the runway. "...Third one, sir; one, two, three; third, third one..." 

Due to the airports' high altitude, clouds of different densities could result in rapid changes in visibility. Such was the case on the 27th of March 1977. When Pan Am 1736 entered the runway, the visibility at Los Rodeos airport decreased, as a high-density cloud was making its way over the airport. The visibility at the location of the Pan Am 747 reportedly decreased from 500 meters (1,600 feet) to 100 meters (330 feet). The KLM aircraft was still in good visibility, as the cloud had yet not completely covered the airport. It was not until the 747 lined up and was ready for takeoff that the cloud covered the 747 as well. 

Meanwhile, on the Pan Am 747, the pilots had successfully identified the first two exits (C1, C2). Discussions in the cockpit (recovered from the flight's CVR), indicated that the crew of Pan Am 1736 weren't sure of their position on the runway. Around this point, the plane was taxiing past runway exit C-3, which they had not seen due to the decreased visibility as well as the lack of signage. C-3 was the runway exit that the air traffic controller at Los Rodeos told them to use. According to the Air Line Pilot's Association (ALPA), it would've been a "...practical impossibility..." for the pilots of the Pan Am 747 to make the second 148-degree turn at the end of taxiway C-3 onto the main unobstructed taxiway paralleling the runway. 

Misunderstandings in Communication 

Immediately after finishing back-taxing and lining up, Captain Van Zanten of KLM 4805 advanced the throttles of his 747 to takeoff power. The first officer onboard the flight, Klaas Meurs, advised Captain Van Zanten that they didn't have clearance from ATC. "No, I know that. Go ahead, ask." responded the captain. 

When Meurs was requesting takeoff clearance from ATC, Captain Van Zanten interrupted the communication between the first officer and ATC, stating "We are now at takeoff...We're going." The controller responded to this call from KLM 4805 with non-standard aviation terminology - "OK", giving instruction to the KLM crew about winds, and their flight route. This gave confirmation to Captain Van Zanten that he had clearance for takeoff despite the controller not explicitly stating that the KLM crew had clearance. 

At this point, the air traffic controller at Los Rodeos couldn't see that there were two planes on the runway due to low visibility. The air traffic controller soon added to his statement to flight 4805 stating "...standby for takeoff, I will call you...". 

At the same time that this was occurring, Pan Am 1736 gave a radio call to ATC. A few seconds later, the crew gave another radio call further stating "...We're still taxiing down the runway, Clipper 1736!". This caused mutual interference on the frequency, which was heard in the KLM cockpit as a heterodyne (a three-second monotone beep). Had either of the Pan Am's radio calls been heard, disaster could've been avoided by the KLM crew by aborting their takeoff roll. 

After the KLM aircraft started its takeoff roll, Rodeos tower told the Pan Am crew to "...report when runway clear..." to which the crew of flight 1736 responded, "...OK, will report when clear...". In the KLM cockpit, the flight engineer was heard saying "...Is he not clear that Pan American?...". To this, Captain Van Zanten responded "Oh yes", continuing the 747's takeoff roll. It's from this point, that disaster could no longer be averted. 

Collision 

At the time and area of the collision, Pan Am 1736 was nearing runway exit C-4. According to the Boeing 747-100's cockpit flight recorder (CVR), Captain Grubbs of the Pan Am flight said "There he is!" upon spotting the KLM 747's landing lights. Upon the realization that the aircraft was heading at takeoff speed down the runway towards his aircraft, Captain Grubbs said "Goddamn, that son-of-a-b***h is coming!", applying full power to exit the runway at taxiway C4 as First Officer Robert Bragg can be heard yelling "Get off! Get off! Get off!". 

When the KLM crew saw the Pan Am 747 in the distance fast approaching, the pilot Van Zanten over-rotated his aircraft's s tail in a bid to get airborne before hitting the Pan Am aircraft. The nose and nose-wheel of the KLM jet missed the Pan Am aircraft, with the rear landing gears and back tail sections of the KLM jet plowing through the middle of the Pan Am 747's fuselage. 

The KLM jet had collided with the Pan AM 747 at around 140 knots (260 km/h; 160 mph). Most of the upper deck of the Pan Am 747 was destroyed as well as the entire middle-fuselage section of the jet above the wings. After the collision, the KLM 747 immediately stalled, hitting the ground 150 meters (500 feet) past the point of collision, with remaining non-ignited sections of the aircraft's fuselage sliding another 1,000 feet down the runway. 

All 248 passengers and crew onboard the KLM Boeing 747-200 were killed in the collision, while out of the 396 people onboard the Pan Am flight, 327 were killed, with 69 surviving the initial impact, and 8 dying in hospital of injuries sustained in the collision. In total out of the two jets (totaling 644 people), only 61 people survived the crash without succumbing to their injuries. 

After the crash, Los Rodeos airport fire crews only attended to the wreckage of the KLM 747, leaving the only survivors of the collision to fend for themselves inside the blazing Pan Am aircraft. Fire crews were unaware in the first place that there were two planes involved in the runway crash, because of the thick fog shrouding the airport in a haze of low visibility. 

The fires kept the wreckage of both the KLM and Pan Am 747s smoldering at an intense temperature for several hours, hindering fire and rescue crews' abilities to look for survivors. 

All of the wreckage and damage done to the runway from the disaster was cleaned up by March 30 and by April 3, Los Rodeos airport was re-opened to fixed-wing commercial air traffic. 

Aftermath - Investigation 

The day following the disaster, the Canary Islands Independence Movement, the driving force behind the two planes' diversions to Los Rodeos in the first place, denied responsibility for the disaster, instead blaming the Spanish government. 

The Spanish government put together a team of investigators comprising 70 investigators from the United States, Spain, and the Netherlands, and representatives from both KLM and Pan Am. 

The sudden low visibility at the airport should've been a sign to the Los Rodeos air traffic controller to stop the movement of planes, instead of moving air traffic (especially in an airport with no ground radar and partially closed taxiways). Interference from pilots talking over each other made it difficult for clear directions to be given to the KLM crew and for the Pan Am pilots to indicate they were still on the runway. Pan Am 1736 didn't exit the runway when instructed. While yes, some of these events were mistakes and some unavoidable (such as the diversion of planes to Los Rodeos due to the bomb threat), the fact still remains to this day that the captain of KLM flight 4805, Jacob Van Zanten, attempted to take his Boeing 747-200 off without proper, affirmative clearance from air traffic control. This also took advantage of the crew working with him, as Van Zanten knew he wouldn't face objection (or if so little resistance) to his decision from the others in the cockpit due to his senior position at KLM. 

The fact that the crews of both KLM 4805 and Pan Am 1736 used ambiguous, easy-to-misinterpret phrases when speaking to Air Traffic Control such as "We're at takeoff" and "OK", is what was the primary contributing factor to the disaster. Before this air disaster, there was no standard terminology (besides basic phrases such as repeating directions, saying flight numbers, and VOR (navigation beacon) names) that pilots had to use. 

What Changed After the Tenerife Airport Disaster? 

After the crash, it became mandatory for pilots and air traffic controllers in most countries around the world to use standardized, hard-to-confuse terminology. Words such as "OK" were replaced with "Roger" (meaning that the receiving person acknowledges). Phrases such as "We're at takeoff" were replaced with "[Insert Flight Number] is taking off the runway [insert number]" or "[Insert Flight Number] holding short runway [insert number]". 

In the moments leading up to the crash, both the first officer and flight engineer were hesitant to question Captain Van Zanten's decision to immediately go for takeoff roll due to his seniority at KLM. In a post-1977 world, airlines began to incorporate playing down the idea of a cockpit hierarchy, choosing to place more emphasis on pilot training based on team decision-making. This idea of "Crew Resource Management" (CRM), has become a mandatory requirement for cockpit crew training in North/South America (FAA) and Europe (EASA) since 2006. 

Despite air disasters and incidents such as this involving the needless destruction of aircraft and loss of lives, they are necessary in making the aviation industry a safer place for not only passengers but pilots, as they act as case studies in ways of how to better train pilots. Cases such as this also act as a cornerstone for the study of Human Factors Engineers, as it displays how human error plays into high-stress environments. 

If you would like a more in-depth report on the Tenerife disaster, you can find it here.