General Motors' recall of 2.6 million small cars is a reminder that air bags may not always deploy when drivers expect them to.
Here's what we've learned about the air bags in the recalled cars, and what federal regulators are still trying to learn:
Q. GM says 13 people died in crashes involving older-model small cars where the ignition switches fell out of the "run" position and stalled the engine. In each of those crashes, the air bags failed to deploy. Why?
A. GM says the air bags were programmed to detect a crash and fire for up to 150 milliseconds after the power to the vehicle was cut off. If the engine had stalled but the ignition was still in the "run" position, the air bags would have worked for much longer, depending on how much energy was stored in the car's battery. But they also may not have worked because air bags are designed not to deploy in certain situations.
Q. What are some reasons air bags might not go off?
A. Air bags deploy at a speed of 150 miles per hour, so they can help protect passengers in less than a second after the car detects a crash. But that force could be harmful to children or small adults or people who aren't correctly seated in the car. If the car's computer detects that the force of the bags could harm passengers, they won't go off.
Q. Do all air bags work like GM's did?
A. No. Newer GM vehicles have more reserve energy, so the air bags would work for slightly longer — but still less than a second — even if the ignition was off. But all automakers' air bags are programmed differently, and there's no common time frame in which the air bags must fire. The government says it's talking to automakers and suppliers to find out how other companies' air bags would work if the ignitions inadvertently turned off while driving.
Q. Does the government have any requirements for when air bags must work?
A. No. The government only requires air bags to meet certain levels of protection for dummies in crash tests. But the GM recalls may prompt the government or the industry to consider new standards for air bags.