Air bag Inflator Explosion
BY LEIGH T. HOLLINS
On September 19, 1995, Cedar Hammock Fire Department Engine Company No. 211 was dispatched to a vehicle fire that on arrival appeared to be a routine call. The passenger compartment of a 1994 Ford Aspire was heavily involved with fire, which was quickly knocked down with a 134-inch preconnect. What the engine company found after extinguishing the fire was anything but routine. It may have been the first or the only explosion and fragmentation of the inflator assembly of an air bag supplemental restraint system. This proves once again that there is no such thing as a routine emergency call.
Although no one from the engine company was injured or experienced anything unusual prior to discovering what had occurred, the results could have been much different. The driver`s side air bag inflator assembly exploded with such force that the top of the steel assembly was ripped in half. A piece of the assembly was propelled through the air and was found on the ground approximately 15 feet in front of the vehicle. It left a bowl-sized impression in the vehicle`s roof. Two rivets, each approximately one-quarter inch wide by two inches long, pierced the roof like bullets. One rivet protruded through the roof, while the other rivet completely passed through the roof and could not be located.
AIR BAG SUPPLEMENTAL RESTRAINT SYSTEM COMPONENTS
Most systems consist of four major components: an air bag module, an electronic diagnostic module, crash sensors, and a backup power supply. The driver`s side air bag module is the component that exploded; therefore, we will concentrate on it.
The air bag module consists of three major parts: trim cover, air bag, and inflator assembly. The trim cover is in the middle of the steering wheel hub and covers the folded air bag. The air bag is a woven nylon bag that is folded into a container within the steering wheel hub center. The inflator assembly is mounted in the air bag module, under the air bag container, within the steering wheel hub.
The driver`s side air bag inflator is a round, two-piece, steel assembly, approximately four inches in diameter and one inch thick. It consists of a diffuser and a closure and contains solid chemical gas generants (sodium azide and potassium nitrate) in the form of pellets or disks and an oxidizer (such as copper oxide), which are sealed in the assembly to keep moisture out.
When sensors detect a sudden deceleration, they trigger the inflator, igniting the generant. The generant burns rapidly in the assembly, producing nitrogen gas and dust, which are directed into the air bag, splitting the trim cover and inflating the air bag in less than one-twentieth of a second. This is the normal process during a front-end collision.
FIRES IN AIR BAG-EQUIPPED VEHICLES
What happens during a vehicle fire? In the written guidelines from Ford, General Motors, and the U.S. Department of Transportation, there is a common message concerning fires: The air bag should self-deploy when temperatures reach 300 to 3507F.
The following is quoted from the various guidelines:
Q. Is the sodium azide canister likely to explode during a car fire?
A. No. The air bag is designed to inflate normally in the event that a vehicle fire causes the canister to be heated above 3007F. Consequently, it is possible that the air bag will deploy in a car fire, but there should be no fragmentation of the inflator.
Q. If there is a fire in an air bag-equipped car, can water be used to extinguish it?
A. Yes. Any effective firefighting medium, including water, may be used to extinguish a fire in an air bag-equipped car. (“Emergency Rescue Guidelines for Air Bag-Equipped Cars,” USDOT-NHTSA, DOT HS 807 579 Rev. August 1990)
VEHICLE CRASH FIRE PROCEDURES
Use standard fire extinguishing procedures first. Use any type of firefighting agent, including water. The sodium azide is sealed in a watertight container.
While air bag modules may deploy in the rare case of a fire involving an interior occupant compartment, the inflator will operate normally. In an intense fire, the air bag module is designed to self-deploy if its internal temperature reaches approximately 3507F. The gas generant rapidly burns, without fragmentation of the inflator. NOTE: AIR BAGS WILL NOT EXPLODE. (“Rescuer`s Guide for Ford Motor Company Air Bag Supplemental Restraint System,” Ford Motor Company, October 1990)
CONCERNS
The concern to firefighters is rather obvious–a vehicle fire with flying shrapnel and large rivets shooting like bullets. Add to this possibility the fact that more and more air bag-equipped new cars are on the road every day and that there are more air bags per car than ever before, and the concern intensifies. Although firefighters should take the new knowledge and use it to their benefit, overreacting would not be prudent. According to safety representatives from General Motors, Ford, and TRW (the manufacturer of the inflator assembly involved), they are not aware of any other explosion incidents. This appears to be an isolated incident or the only incident to date.
RECOMMENDATIONS
Following are my recommendations when fighting vehicle fires:
Follow accepted vehicle fire tactics.
Wear full protective gear, including SCBA.
Look around. Don`t get tunnel vision.
Attack from a 45-degree angle.
Cool critical components.
Add the steering column and dash area to the critical components to be cooled.
Continue to cool for several minutes after knockdown.
Study the following Fire Engineering articles:
–“New Fire Tactics for New-Car Fires,” Bill Gustin, April 1996.
–“Car Engine Fires: Tactics for Quick Knockdowns,” Brian G. Anderson, February 1994.
Notify the vehicle`s manufacturer if the assembly explodes.
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It does not matter whether your response is to a vehicle fire, a trash fire, an automatic alarm, or a “nothing showing” structure fire; complacency will kill or injure you and your company. Knowledge, education, and experience will afford the best protection. Use them to your advantage. Accept the risks, but be safe. n
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The steering wheel area of the Ford Aspire involved in the fire.
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Note the remaining portion of the inflator assembly.
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dented the car roof
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The explosion ripped the diffuser in half, propelling it so that it landed on the ground 15 feet in front of the vehicle
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and caused a rivet to become lodged in the roof.
(Photos by author, reproduced from a videotape.)
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A complete inflator assembly. The assembly that exploded ripped at the “windows” of the diffuser–not at the seam of the diffuser/closure.
RESOURCES
The following will provide free information pertaining to fires involving air bag-equipped vehicles and extrication:
Carl Farris, c/o GM-Atlanta Training Center, (404) 767-2628.
Ford Motor Co., Colin Earl, (313) 845-8639.
Chrysler Corporation, (800) 992-1997.
National Highway Traffic Safety Administration, (800) 424-9393.
LEIGH T. HOLLINS, a 20-year veteran of the fire service, is training director for Cedar Hammock Fire Control District and Southern Manatee Fire & Rescue District, in Florida, and director of Starfire Training Systems, Inc. of Manatee County, Florida. He is a state-certified firefighter, EMT, fire officer instructor, and inspector instructor.
