By Jeff Marcus, Chief, NTSB Safety Recommendations Division
Forty-three years ago, on March 27, 1977, two Boeing 747s, KLM flight 4805 and Pan Am flight 1736, collided on a runway at Los Rodeos Airport in Tenerife, Canary Islands, killing 583 people. It was the single greatest loss of life in aviation accident history.
The crash was the result of an unlikely series of events—and a flight crew’s responses to them.
To begin with, neither of the two aircraft was initially supposed to be at Los Rodeos Airport in the first place. Both planes had been scheduled to arrive at Las Palmas Airport, also in the Canary Islands; however, Las Palmas had just been the target of a terrorist attack, and the terminal had been evacuated and the airport closed. The two 747s, as well as other arriving traffic, were diverted to the smaller Los Rodeos airport in Tenerife, where they landed safely.
The Los Rodeos Airport had not been equipped to handle the influx of diverted flights and, because of that, on March 27, the airport was congested, and maneuverability issues arose when the airplanes were ready to depart. Pan Am 1736 was ready to depart Los Rodeos to resume its itinerary, but had to wait until KLM 4805, which was obstructing the taxiway, had completed taking on fuel from a refueling vehicle. The captain of the KLM flight was instructed to back taxi down the entire runway, then perform a 180-degree turn in preparation for departure. The Pan Am captain was instructed by air traffic control to back taxi down the runway, then exit on the third taxiway to their left, and to report leaving the runway. The taxiways at the airport were unmarked and the centerline lights were out of service.
Los Rodeos airport was subject to fast-appearing, thick fog, and as the KLM airplane lined up for its takeoff roll, fog enveloped the runway. The Pan Am airplane missed its exit, and its crew did not appear to know their position on the runway. Neither crew could see the other plane, and the tower couldn’t see either plane. The airport was not equipped with ground radar.
Having lost so many sources of information, one last source of information failed: verbal communication between the airplanes and the tower.
Immediately after lining up, the KLM captain who had a sense of urgency to depart before exceeding duty limits advanced the throttles and the aircraft started to move forward. The KLM first officer advised the captain that air traffic control (ATC) clearance had not yet been given. The captain replied, “No, I know that. Go ahead, ask.” The first officer radioed the tower that they were ready for takeoff and waiting for ATC clearance. The KLM crew then received instructions that specified the route that the aircraft was to follow after takeoff. The instructions used the word “takeoff,” but didn’t include an explicit statement that the aircraft was cleared for takeoff. The first officer then read the clearance back to the controller, completing the readback with the nonstandard statement: “We are now at takeoff.” The KLM captain interrupted the first officer’s read-back with the comment, “We’re going.”
The controller, who could not see the runway due to the fog, initially responded with the nonstandard terminology “OK,” which reinforced the KLM captain’s misinterpretation that they had been cleared for takeoff. The controller then immediately added “stand by for takeoff, I will call you,” indicating that he had not intended the clearance to be interpreted as a takeoff clearance. However, a simultaneous radio call from the Pan Am crew caused mutual interference on the radio frequency, which was audible in the KLM flight deck as a 3-second-long shrill sound. This caused the KLM crew to miss the crucial latter portion of the tower’s response. The simultaneous message from the Pan Am crew, “We’re still taxiing down the runway, the Clipper 1736!” was also blocked by the interference and inaudible to the KLM crew. Either message, if heard in the KLM flight deck, would have alerted the crew to the situation and given them time to abort the takeoff attempt.
After the KLM plane started its takeoff roll, the tower instructed the Pan Am crew to “report when runway clear.” The Pan Am crew replied, “OK, will report when we’re clear.” On hearing this, the KLM flight engineer expressed his concern about the Pan Am aircraft not being clear of the runway by asking the pilots in his own cockpit, “Is he not clear, that Pan American?” The KLM captain emphatically replied, “Oh, yes,” and continued with the takeoff.
By the time the KLM captain saw the Pan Am airplane, he could only try to fly over it. The tail of the KLM airplane struck the Pan Am airplane, tearing through the center of its fuselage above the wing. Fuel spilled and ignited on impact. Of the Pan Am passengers and crew, 335 died, mainly as a result of ensuing fire and explosions, and 61 survived.
The KLM airplane lost one engine on impact, and the wings were damaged. The airplane rolled sharply and crashed about 500 feet past the point of collision. All 248 passengers and crew died in the crash and the post-crash fire.
The Tenerife accident provided early lessons for the concept of crew resource management (CRM), which emphasizes that all flight crew members should actively voice their safety concerns, and all crew, particularly senior crew members like the captain, must acknowledge the safety concerns of any crew member. In the Tenerife accident, the captain rushed the takeoff, despite the first officer pointing out that they had not received clearance, and the flight engineer recognizing that the Pan Am airplane had not yet cleared the runway. Despite the flight engineer highlighting the dangerous situation, the KLM captain dismissed the concern and continued the takeoff, which resulted in the tragedy a few seconds later.
The Tenerife accident was a milestone in the study of human factors in aviation accidents. The pressures of the day’s events and delays; the logistics pressures in a regional airport handling a major airport’s arrivals; the communications misunderstanding; and the failure to understand and use CRM practices all led to bad decisions at various points in the accident chain.
Aviation has changed and become safer by leaps and bounds since the crash. As a result of Tenerife, there has been greater emphasis on English as the single working language of aviation, and on the use of standard, concise, and unequivocal aeronautical language.
Tenerife was influential in recognizing that all crew members should feel empowered to speak up, and captains should listen to their safety concerns—an important principle of CRM. The principles of CRM have even been extended beyond aviation to marine safety, where it is known as bridge resource management, and to medicine, where all doctors and technicians in an operating room are encouraged to voice their concerns, and senior, highly esteemed surgeons are trained to listen to and evaluate any safety concern expressed, regardless of who has the concern.
Humans are an integral part of the aviation system and the system must protect for human error. In 2017, many links of an accident chain were in place at San Francisco International Airport when an Air Canada airplane almost landed on a taxiway occupied by four airliners waiting to takeoff. There were over 1,000 people in those four airliners; the accident would have equaled or even surpassed the death toll at Tenerife more than 40 years earlier. That close call was another reminder of how much is on the line every time human pilots and passengers take to the skies—and how much of a role human factors can play in such tragedies and near misses.