Accidental Air Bag Inflation Unlikely

<i> Times Staff Writer </i>

Will air bags only open during a serious crash--and not before?

Will they go boom when they are not supposed to?

That seems to be the biggest worry American drivers have about air bags--a concern over what engineers diplomatically call “inadvertent deployments.”

But air bag experts say they have the right answers to allay such fears.


The auto makers who are now putting air bags on some cars say they know of no cases in which their air bags have gone off at the wrong time. Meanwhile, specialists add that modern air bag systems have built-in safeguards to prevent such mistakes.

Useful in Frontal Crashes

Most air bag systems come equipped with three sensors arrayed across the front of the car, usually along the bumper, to detect a frontal impact.

The number of sensors and their location on the car vary from company to company. But most have enough sensors to pick up head-on collisions and front-end crashes within a 30-degree angle of head-on, according to Lon Bell, president of TRW Technar, an Irwindale sensor maker. Thus, air bags are really only of use in frontal accidents, and safety experts stress that drivers and passengers will continue to need the protection offered by seat belts, which will still be installed in all air bag-equipped cars.

When an accident occurs, the front sensors are designed to send an electronic flash back to inflate the air bag. But to avoid false signals, most systems are designed to override an impact detection if it only comes from one sensor. The air bag will inflate only if two out of three sensors detect the crash.

To make sure that the air bags don’t go off in minor parking lot fender-benders, most sensors are now designed to only detect impacts equal to or greater than the force of a car hitting a barrier at a speed of 14 m.p.h., according to officials at the National Highway Traffic Safety Administration.

Nitrogen Gas Expands

That industry standard seems to exceed the federal requirement that air bags protect an occupant from serious injury at an impact speed of 30 m.p.h.

When the sensors detect a crash, their electronic signals sent back to the air bag prompt the ignition of a solid, inert chemical, called sodium azide, which is stored inside the air bag system.

When ignited, the sodium azide rapidly turns into a clean-burning nitrogen gas, which immediately expands and inflates the air bag.

The bag then pops out from behind a lightweight plastic covering. On the driver’s side, the bag is stored inside the steering wheel; on the passenger side it is hidden in the dashboard. The plastic trim is usually designed to open in a way that avoids hitting the passenger. Some cars have hinged trim pieces that stay on the steering wheel and just swing open when the bag inflates.

The whole process is over in less than a blink of an eye. The air bag inflates in only a few nanoseconds, notes Rick Smith, a spokesman for TRW, parent company of TRW Technar.