Fail-Safe

In the stories and hearings about the on going oil spill in the Gulf of Mexico we here about the fail-safe mechanisms that were supposed to stop the flow of oil. Via a Balloon-Juice commenter I relearned the definition of fail safe today and I think it's important enough to point out again. A fail-safe device does not mean a device that will not fail. It means that if/when the device does fail it does so in a safe manner.

Two examples. A fail-safe door lock. If the lock fails the door will not swing open.

Better, the air brakes on semi-trucks. Contra what you see in the movies, if the air lines are severed the truck does not careen out of the control. Quite the opposite. The air pressure is used to prevent the brakes from engaging. If the compressor or hoses fail the brakes are applied, stopping the truck.