Worst aviation disaster: A tragic miscommunication at Tenerife

On Sunday, March 27, 1977, the world's largest passenger aircraft, a Boeing 747 operated by KLM Airlines, took off from Amsterdam airport. It was en route to Las Palmas (the Spanish island of Gran Canaria), carrying 234 passengers and 14 crew members.

Heading in the same direction was an identical aircraft – another Boeing 747, but this one in the colours of the now-defunct Pan American (Pan Am) airlines. The jumbo jet took off from Los Angeles airport, carrying 380 passengers and 16 crew members.

Both flights proceeded smoothly until the pilots of both aircraft received information about a bomb threat at the destination airport. The Las Palmas airport staff decided to divert all flights to nearby Tenerife. At 08:38 GMT, the KLM aircraft landed there, and at 09:15 GMT, the Pan Am plane appeared on the tarmac.

The conditions at Tenerife airport were unfavourable from the start. Most notably, Los Rodeos airport was not equipped to handle multiple aircraft at once – especially such giants as the Boeing 747s, which had to fit on the Spanish airport's tarmac simultaneously.

The flight controllers were also a problem – or more precisely, their number on duty. On that fateful Sunday, only two people were working in the control tower. The poor situation was exacerbated by a broken runway lighting system and a thick fog that further reduced already limited visibility. Los Rodeos airport did not have a radar to track the planes – pilots and controllers communicated via radio, reporting their locations on the airport.

The weather in Tenerife in March 1977 was changing at a rapid pace. The sunny Sunday quickly turned into a thick fog, limiting visibility to a maximum of 300 meters, whereas the minimum required visibility for takeoff was 700 meters.

In these unfavourable conditions, the pilot of the KLM-operated aircraft, Jacob Veldhuyzen van Zanten, decided to refuel the Boeing to avoid wasting time waiting for further instructions from the controllers. While theoretically understandable, in practice it resulted in blocking the runway with the gigantic jumbo jet. More importantly, with every gallon of fuel poured into the tank, the plane became heavier, requiring a longer takeoff roll. In the next few minutes, this fact determined the fate of nearly 600 people.

The refuelling KLM Boeing occupied so much space on the tarmac that the Pan Am jumbo jet could not reach the runway to begin taxiing and takeoff. After refuelling, the crew of the blocking KLM jet was instructed to begin taxiing on the runway.

At this point, another problem arose – the controllers wanted the Dutch aircraft to taxi only part of the runway and exit at taxiway number 3. However, the pilots could not understand which taxiway (number 3 or 1). In their haste, the airport staff instructed the KLM crew to make a 180-degree turn at the end of the runway and position themselves for takeoff. At 12:04 GMT, the Boeing 747 from Amsterdam was ready to take off.

While the KLM jumbo jet stood at one end of the runway, the Pan Am jet began to taxi at the other end towards taxiway number 3 as instructed. The pilots did not see the recommended taxiway, so they decided to use the next one – number 4 – which was more convenient for them as it did not require a 135-degree turn.

At the same time, KLM's Boeing captain Jacob van Zanten grew increasingly impatient. Over the radio, he stated: "I understand, we are cleared for takeoff, we are now starting", which might have been understood by the controllers as "we are in position for takeoff" (the phrase "we are taking off" was understood by the tower as "we are at takeoff"). Moments later, the KLM captain communicated in Dutch, "We're moving". Shortly after, the headphones of both sets of pilots (KLM and Pan Am) heard, "Okay. [a few seconds pause] Be ready for takeoff, I'll call you".

Tenerife air traffic control may have misunderstood the message conveyed by the KLM pilot. Unaware of anything, the crew of the taking-off plane heard in response to the information about the commencement of the takeoff procedure a single word - "okay". They did not know they were speeding straight towards the other jet.

The Pan Am crew taxiing towards taxiway number 4 tried to inform the controllers that their aircraft was still on the runway, but overlapping radio waves meant no one heard the warnings. The engines of the KLM Boeing, weighing hundreds of tons, were now at full power, ready to leave the Tenerife runway and take off.

Two minutes after announcing readiness for takeoff, in the cabin of the taking-off KLM, the question "Has Pan Am left the runway?" is asked. The captain confidently answered yes. Five seconds later, the Pan Am crew, still on the runway, saw the speeding jumbo jet approaching them. "Get out! Get out! Get out!" – one of the Pan Am pilots screamed. After that, the flight recorders only captured the sound of the collision between the two giants at the Spanish airport, killing 583 people and injuring 61.

After the Tenerife disaster, the International Civil Aviation Organization (ICAO) introduced a series of changes to certain procedures for communication between aircraft crews and air traffic control towers. As a result of the 1977 incidents, the term "takeoff" was mandated to be used exclusively when granting direct permission to take off. In other situations, pilots use the word "departure".

Pilots were also required to repeat the full message delivered by the tower. Thus, short responses like "OK", which could lead to misunderstandings, were banned.

ICAO introduced these decisions due to investigators' findings that because of radio wave interference (overlapping waves), pilots and air traffic control towers did not receive full messages. When the KLM crew reported the start of takeoff (understood by the controllers as readiness to take off), they heard only "OK" in response. They did not hear the next part of the message, "Wait for takeoff, I'll call you", because of interference in the radio transmission. Similarly, the KLM pilots did not hear the warnings from the Pan Am pilots who were still taxiing on the runway.