Aviation accidents are tragic events that can result in significant loss of life and provide important lessons for improving flight safety. One of the most devastating examples is the Tenerife Airport disaster, which remains the deadliest accident in commercial aviation history. The disaster occurred on 27 March 1977 at Los Rodeos Airport, now known as Tenerife North Airport, in the Canary Islands, Spain.
The Airplanes Involved in the Accident
The Tenerife Airport disaster involved two Boeing 747 aircraft. The first was KLM Flight 4805, a Boeing 747-206B registered PH-BUF, operating from Amsterdam, the Netherlands, to Gran Canaria in the Canary Islands. The second was Pan Am Flight 1736, a Boeing 747-121 registered N736PA, operating from Los Angeles to Gran Canaria with an intermediate stop at New York’s John F. Kennedy International Airport.
Situation
A bombing incident at Gran Canaria Airport resulted in the temporary closure of the airport and the diversion of incoming flights to Los Rodeos Airport. Both KLM Flight 4805 and Pan Am Flight 1736 were affected. The Pan Am crew preferred to remain in a holding pattern until Gran Canaria reopened, but they were instructed to divert to Los Rodeos, now known as Tenerife North Airport.
Los Rodeos was not designed to accommodate such a sudden concentration of large aircraft. The diverted traffic created severe congestion, and several aircraft were parked on the airport’s main parallel taxiway. With the taxiway obstructed, departing aircraft had to taxi along the runway itself, using a procedure known as runway backtracking. This unusual ground-traffic situation would become an important part of the chain of events leading to the disaster.
Visibility reduced to a minimum due to excessive fog
As low clouds moved across Los Rodeos Airport, visibility deteriorated rapidly, falling to around 300 meters in parts of the airfield. The control tower could no longer maintain visual contact with the aircraft on the runway, and Los Rodeos did not have ground movement radar that could have allowed the controller to monitor their positions electronically. Radio communication therefore became critically important for maintaining situational awareness.
The tower instructed Pan Am Flight 1736 to leave(vacate) the runway via the third taxiway exit, C3. However, in the rapidly changing low-visibility conditions, the Pan Am crew had difficulty identifying the correct exit. The geometry of C3 also presented an unusually sharp turn for a Boeing 747, and the crew discussed whether this was actually the exit the controller intended them to use. Pan Am consequently continued along the runway toward the next exit.
Meanwhile, KLM Flight 4805 had taxied to the end of the same runway, turned around and lined up for departure. The KLM captain initially advanced the throttles, but the first officer reminded him that they had not yet received their ATC clearance. The captain responded by telling the first officer to obtain it, and the takeoff initiation was temporarily halted. The tower subsequently issued KLM its ATC route clearance. Crucially, this was a departure clearance describing the aircraft’s route after departure—not a clearance to take off.
Radio Interference and Miscommunication Before Takeoff
After receiving its ATC route clearance, KLM Flight 4805’s first officer read the clearance back to the tower, ending with the ambiguous phrase, “we’re now at takeoff.” However, the aircraft had not received a takeoff clearance. During the readback, the KLM captain began the takeoff roll.
The controller apparently interpreted the KLM transmission as meaning that the aircraft was in takeoff position rather than actually taking off. He responded with “OK,” followed by an instruction to stand by for takeoff and that he would call them. At almost the same time, the Pan Am crew transmitted that they were still taxiing on the runway. The simultaneous radio transmissions produced a heterodyne, or whistling interference, which reduced the clarity of the messages received in the KLM cockpit.
Shortly afterward, the tower instructed Pan Am to report when the runway was clear. Pan Am acknowledged that it would do so. Hearing this exchange, the KLM flight engineer questioned whether the Pan Am aircraft had actually cleared the runway. When the captain did not initially understand him, the flight engineer repeated his concern. The captain then responded emphatically that Pan Am was clear, and the takeoff continued.
In reality, Pan Am Flight 1736 was still taxiing along the same runway ahead of the accelerating KLM 747. Because of the extremely poor visibility, the two crews did not see each other until the distance between the aircraft had become critically short.
Pan Am co-pilot shocked to see giant KLM plane fast approaching
Neither crew was able to see the other aircraft through the low visibility until the collision was only seconds away. When the Pan Am crew finally saw the KLM 747 approaching at high speed, the captain applied full power and attempted to turn sharply to the left, toward the grass beside the runway. However, there was not enough time to move the large aircraft completely out of the KLM’s path.
The KLM captain also saw the Pan Am aircraft and attempted to rotate early in an effort to clear it. The KLM 747 became airborne, but its lower fuselage, engines and main landing gear struck the upper fuselage of the Pan Am 747. The impact severely damaged both aircraft.
After the collision, the KLM aircraft remained airborne for a short distance before coming back down onto the runway approximately 150 meters beyond the impact point. It then slid for roughly another 300 meters before coming to a stop and being engulfed in fire. All 248 people aboard the KLM aircraft were killed. On the Pan Am aircraft, 335 people were killed, while 61 survived.
KLM Flight 4805 had refueled at Tenerife before departure, increasing the aircraft’s weight and fuel load. The additional fuel contributed to the severity of the post-impact fire, but it should not be presented as a fundamental cause of the collision. The official investigation identified the KLM captain’s initiation of takeoff without takeoff clearance as the fundamental cause of the disaster.
Aftermath
One day after the collision, the Canary Islands Independence Movement (MPAIAC), which had claimed responsibility for the bombing at Gran Canaria Airport, denied responsibility for the subsequent aviation disaster. The accident was investigated by the Spanish aviation authorities, with the official investigation report issued by the Subsecretaría de Aviación Civil.
The investigation identified the fundamental cause of the disaster as the KLM captain initiating takeoff without receiving takeoff clearance. Several other circumstances contributed to the chain of events, including ambiguous radio communications, interference caused by simultaneous transmissions, rapidly changing low-visibility conditions, unusual airport congestion following the diversions, and Pan Am’s failure to leave the runway at the instructed third exit.
In total, 583 people were killed and 61 survived, making the Tenerife Airport disaster the deadliest accident in commercial aviation history.