Miscommunication and tragedy: Remembering the Tenerife aviation disaster

Misunderstanding and haste — these two factors combined to create the worst aviation disaster in history over 47 years ago. The collision of two airplanes at Tenerife North Airport still holds this infamous title.

On Sunday, March 27, 1977, the world's largest passenger aircraft, a Boeing 747 operated by KLM Royal Dutch Airlines, took off from Amsterdam's airport. It was en route to Las Palmas on 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 operated by the now-defunct Pan American (Pan Am) Airlines. The jumbo jet had taken off from Los Angeles, carrying 380 passengers and 16 crew members.

Both flights proceeded smoothly until the pilots received information about a bomb threat at the destination airport. By the decision of the Las Palmas airport staff, all flights were diverted to nearby Tenerife. At 3:38 AM Eastern Time, the KLM airplane landed there, and at 4:15 AM Eastern Time, the Pan Am plane appeared on the tarmac.

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

The flight controllers were also a problem—or, more precisely, their number on duty. Only two people were working in the control tower on that fateful Sunday. The situation was exacerbated by a broken runway lighting system and a thick fog that reduced visibility. Tenerife North did not have radar to track the planes, so pilots and controllers communicated via radio, reporting their locations.

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 litre 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-way down the runway and then exit at taxiway number 3. However, the pilots could not precisely understand which taxiway (number 3 or number 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 7:04 AM Eastern Time, the Boeing 747 from Amsterdam was ready to depart.

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 as it did not require a 135-degree turn.

At the same time, KLM's 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". Moments later, the KLM captain communicated in Dutch, "we're moving." Shortly after, both KLM and Pan Am pilots 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 toward 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 KLM Boeing's engines were now at full power, weighing hundreds of tons, ready to leave the runway and take off.

Two minutes after announcing readiness for takeoff, the question "Has Pan Am left the runway?" was asked in the cockpit of the KLM plane. 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 captured only the sound of the collision between the two giants on the runway, killing 583 people and injuring 61.

After the Tenerife disaster, the International Civil Aviation Organization (ICAO) introduced a series of changes to specific 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.