Here's an unsettling fact about cars equipped with air bags: They don't always deploy when drivers _ or regulators _ expect them to.
Thirteen people have died in crashes involving older GM cars with defective ignition switches. In each of those crashes, and in others in which occupants were injured, the air bags failed to deploy even after striking trees, guard rails or other objects.
Puzzled by these failures, federal safety regulators told Congress last month they believed the cars' air bags should have worked for up to 60 seconds after the engine stalled. But GM has since told The Associated Press that regulators were mistaken: The cars only had enough reserve power to sense a crash and deploy the air bags for 150 milliseconds after the switch malfunctioned and cut off the car's power.
General Motors is recalling 2.6 million small cars to fix the ignition switches. The National Highway Traffic Safety Administration is now scrambling to find out from other automakers and air bag suppliers how their air bags would function in similar situations.
Regulators, lawmakers and ordinary drivers are learning what auto engineers already know: These billowing white bags are actually very complex. After a crash, a car's computer determines, in 15 to 20 milliseconds, where it was hit, what position the occupants are in and whether the 150-mile-per-hour speed of the air bag would do more harm than good. Then it deploys _ or doesn't. Every automaker programs them differently.
“It's very complicated, the logic behind it. It makes it very, very difficult for an automaker or supplier to explain why it did or didn't go off in a certain situation,'' said Joe Nolan, senior vice president for vehicle research at the Insurance Institute for Highway Safety, a group funded by the insurance industry that performs crash tests and other research.
If an occupant is unbelted or very small, or the car is traveling very slowly, the air bag may not deploy because it could cause even more severe injuries. Depending on the angle, the side air bags may deploy but not the front ones. If a car is parked and turned off when it's hit, the air bags won't work.
GM's switches created an unusual problem. Because of insufficient resistance, they moved from the ``run'' position into the ``accessory'' or ``off'' position while the car was moving, possibly due to a bump from the driver's knee or the weight of