Tenerife

A bomb at Gran Canaria had diverted both aircraft to a small airport not built for them. They had been waiting for hours. Fog came down to about three hundred metres of visibility.

The KLM captain began his take-off run believing he had clearance. The Pan Am aircraft was still on the runway, looking for a taxiway turn-off in the fog. The KLM lifted off and struck the other aircraft across the top.

All two hundred and forty-eight aboard the KLM were killed. Of the three hundred and ninety-six on the Pan Am, sixty-one lived - most of them from the front of the cabin, who walked out through the hole and jumped.

Every cockpit procedure in the world was rewritten because of it: standard phrases, the right of any crew member to challenge a captain, the end of the word "takeoff" in any context but clearance.