That is because all those reassuring illustrations on the safety card, the ones showing your plane floating ­gently on the ocean surface while everyone bobs off in comfy-­looking yellow rafts, are pure fiction. What really happens when a large aircraft hits the water is very ­different.

Invariably one wingtip strikes the water before the other one, and even at the slowest possible speed of descent, the impact causes the plane to cartwheel and break up. The various bits then sink. As in the Comoros islands crash in 1996, the surviving passengers are likely to be those who are not wearing life jackets and can get out of the wreckage most quickly.

Indeed, in modern aviation history there have been no cases of large airliners putting down on water and remaining intact long enough for anything like the orderly evacuation shown on landing cards to take place. (It is different with small planes, which have greater structural integrity - here, water-kit makes sense.)

Life jackets and escape rafts are a vestige of the long-­vanished golden age of flying boats and propeller planes. Today their only real function is as a prop in the absurdist theatre of aviation safety. First the cabin crew frightens you by reminding you of dangers you probably weren't even thinking of, such as drowning; then it reassures you that the airline has thought of everything by providing a life jacket.

It's a show that comes at the cost of the genuine safety margin that would be provided by extra fuel in the tanks. You're quite right to ignore it and to read your in-flight magazine instead - if there is one.