Accident Analysis : Tragedy at Tenerife

Sunday, March 27, 1977. 1:15 pm. A terrorist bomb explodes at Las Palmas, main airport of Canary Islands, an autonomous region of Spain located in the Atlantic. Eight persons are injured and the passenger terminal sustains some damage. Apprehensions of a second bomb prompt authorities to close the airport and divert incoming flights to Tenerife, another island in the Canaries, triggering a chain of events that climax in the worst aviation accident in history—two Boeing 747s collide on Tenerife runway killing more people than the militants at Las Palmas could ever have imagined.

Pan Am Flight 1736, captained by Victor Grubbs, had taken off from New York and was bound for Las Palmas. KLM Flight 4805, a Boeing 747 charter flight with Jacob Veldhuyzen van Zanten in command, had departed Amsterdam for Las Palmas. The Boeings were just two of several aircraft that, following the bomb explosion at Las Palmas, were diverted to Los Rodeos, a small regional airport now known as Tenerife North. Tenerife had one runway (orientation 12/30) and a parallel taxiway, as well as several links connecting the two. Since the gigantic airliners were parked on the parallel taxiway it was unavailable for taxiing.

Within a few hours of the bombing, Las Palmas was reopened to traffic. Since the Pan Am passengers had not disembarked, their flight was ready for departure from Tenerife. However, their taxi path was blocked by the KLM flight which was parked close to the mouth of the runway and was still refuelling. Consequently, the KLM became the first in the departure sequence. After refuelling, KLM was cleared by Tenerife tower to backtrack along the runway, make a 180 degree turn and await clearance for departure from Runway 30. Meanwhile, the weather began to deteriorate. Heavy fog drifted across the runway, reducing visibility to 300 m. After three minutes, the Pan Am flight was also asked to backtrack along the runway and instructed to clear at the third exit to the left and report leaving the runway. However, the runway exits were unmarked and in the limited visibility, the Pan Am crew appeared unsure of their exact location. They overshot the third link and were approaching the fourth when the collision took place (see sketch).

Lining up on Runway 30, KLM Captain van Zanten advanced the throttles slightly to verify the health of the engines. The copilot advised him that ATC clearance for take-off had not yet been obtained. Captain van Zanten responded, I know that. Go ahead, ask. Then the KLM reported ready for take-off and was given departure instructions. The KLM crew repeated the instructions and added, We are now at take-off. Almost immediately, as later investigation of the timeline showed, the brakes were released and KLM 4805 began the take-off roll. Aware that Pan Am 1736 was yet to clear the runway, Tenerife tower replied: OK… stand by for take-off, I will call you. This critical call coincided with a transmission from Pan Am 1736: No… uh, we’re still taxiing down the runway. The two transmissions jammed each other and only a shrill noise lasting 3.74 seconds was heard by the KLM crew. Tenerife tower then asked the Pan Am crew to report runway clear, whereupon the Pan Am replied: OK, will report when we’re clear. This alerted the KLM flight engineer who asked his captain: Is he not clear then? After he repeated the question the captain answered emphatically: Oh, yes.

Seconds later, the KLM crew saw the Pan Am Jumbo still on the runway ahead of them. Captain van Zanten apparently tried to get airborne, but as the speed was low, rotation was premature. The tail scraped the runway for 20 m. Simultaneously, the horrified Pan Am crew turned the aircraft to the left and applied full power in a desperate attempt to get out of the way. The KLM managed to get airborne but as it passed over the Pam Am, the undercarriage hit the rear fuselage and came crashing down, exploding into a fireball. The Pan Am jet also caught fire and both planes were completely destroyed. All 234 passengers and 14 crew aboard the KLM plane perished, while 326 passengers and nine crew on the Pan Am flight were killed. Only 56 passengers and five crew of the Pan Am survived. The combined death toll was a staggering 583—a figure exceeded only by the combined ground and air fatalities of the September 11, 2001 attacks.

Crash investigators from Spain, the Netherlands and the US participated in the inquiry. The Cockpit Voice Recorders, tape recordings in the ATC and Flight Data Recorders provided bulk of the evidence. A series of misinterpretations and false assumptions was soon revealed. In the final minute before the collision, key misunderstandings occurred among all the parties involved. Most importantly, the KLM captain was apparently convinced that he had been cleared for take-off, while Tenerife tower was equally certain that the aircraft was awaiting take-off clearance. The misperception seems to have arised due to use of non-standard phraseology. Some exchanges between the tower and the two planes were ambiguous at best. A couple of vital transmissions also got jammed. Had they not been, they would have been registered by the KLM crew allowing them to abort take-off. The Pan Am aircraft was still on the runway because it had missed unmarked Link 3 due to very poor visibility. In fact, the control tower and the two aircraft were not in visual contact.

Just as important were the psychological factors. All parties involved seemed afflicted by tunnel vision—focussing on the immediate task while ignoring the broader picture. Both aircraft were understandably keen to put the unforeseen delay behind them and the deteriorating weather injected a sense of urgency. The KLM captain seemed in a hurry because he was concerned his crew might violate Dutch regulations on maximum duty hours. To avoid refuelling at the next stop, he took on additional fuel at Tenerife which resulted in a longer take-off roll besides adding many tonnes of fuel to the blaze. Captain van Zanten was the most senior KLM pilot and it would not have been easy for his junior crew to point out even an obvious mistake, as their tentative interjections seem to suggest. The controller on duty was probably overwhelmed by the sudden and unexpected increase in traffic and may not have been able to remain abreast of the rapidly changing situation. From the ATC log it appears that the Pan Am captain was more alarmed by the fateful KLM take-off call than the tower was. Perhaps somewhat unfairly, the inquiry laid the blame entirely on KLM Captain van Zanten.