The take-off that began without a clearance

On 27 March 1977 a bomb exploded at the airport on Gran Canaria, and many flights were sent to the small Los Rodeos airport on nearby Tenerife. Among them were two Boeing 747s, KLM flight 4805 and Pan Am flight 1736. The airport was crowded, and thick fog rolled in.

Both planes had to taxi along the runway itself. The KLM jet reached the far end and turned around to take off, while the Pan Am jet was still on the runway, looking for its exit in the fog.

The KLM captain, a senior and respected pilot, began the take-off run without a take-off clearance. His co-pilot radioed a message that the tower took to mean the plane was in position and waiting. Part of the tower's reply, telling KLM to stand by, was blocked by a simultaneous message from Pan Am. The KLM flight engineer asked whether the Pan Am plane was clear of the runway, and the captain said it was.

The KLM jet struck the Pan Am jet just after lifting off. All 248 people on the KLM plane and 335 on the Pan Am plane died, 583 in total. Sixty-one people on the Pan Am plane survived.

Several pressures stacked up. The crew were close to their legal limit on working hours and wanted to leave before the fog got worse. The radio messages used loose, non-standard wording, and two people talking at once wiped out the key words. In the cockpit, the most senior person's confidence outweighed a junior crew member's doubt.

Aviation changed after Tenerife. Controllers and pilots now use strict standard phrases, and the word "take-off" is kept for the clearance itself. Airlines also adopted crew resource management training, which teaches junior crew to speak up and captains to listen.

The idea behind this caseLesson: The four-part prompt →

1. 1For decisions that cannot be undone, confirm the exact go-ahead in clear words, and repeat it back.
2. 2Use agreed words for important instructions so they cannot be read two ways.
3. 3Invite the most junior person in the room to raise doubts, and treat a question as a reason to stop and check.
4. 4Watch for time pressure pushing you to skip a step, especially when conditions are getting worse.