Thread: Airbags at 280 Degrees F
02-06-1999, 07:35 PM #1rmooreFirehouse.com Guest
Airbags at 280 Degrees F
A posting from Ron Moore, Moderator
Firefighter Dwayne Hunt (firstname.lastname@example.org) asked two interesting questions: one about airbags actually causing fires in vehicles. He also asked what happens when these new roof-mounted airbags are heated above that magic number of 300 degrees F.
There is a bit of confusion on this topic so I'd like to offer some insight.
Engineers at one airbag manufacturer, TRW, recently wrote me to provide details of what happens when a vehicle with a chemical gas generator airbag system burns.
You must note that there are now two very different types of airbag inflator units out there; chemical gas generator and stored pressure. Volvo 1999 MY vehicles have the only stored gas units now but NY 2000 will see more coming from other manufacturers.
The chemical gas generator, which is typically sodium azide, heats up as the passenger compartment burns. I found out that an undeployed airbag is designed to inflate in a normal manner if these chemicals sealed inside the individual airbag modules reach a temperature above 280 degrees. This intentional "auto ignition" is to keep the airbag canister from over-pressurizing.
In situations where responders arrive at the scene of a working vehicle fire, particularly one burning in the passenger compartment, the airbag gas generators, after several minutes, will reach 280 degrees and ignite, causing the airbags to deploy.
There has been at least four "rare exceptions" to the rule; one documented case recorded on videotape of an airbag module failing during a vehicle fire.
The incident occurred in the driveway of a home in Long Island NY. As the engine crew stretched their still uncharged hoseline, the airbag inflator of the burning car exploded violently, flew through the glass sunroof and landed on the driveway behind the car.
Use caution and get your gear on!
02-06-1999, 07:46 PM #2rmooreFirehouse.com Guest
A Posting from Ron Moore
Firefighter Dwayne Hunt has a second question about ....."what happens when the new head protection bags (BMW, Mercedes, Volvo) which are non-vented, are exposed to heat of 300 degrees or more. According to Ron Moore of FETN and Firehouse mag, the BMW tubular bag
stays inflated for up to two hours. Obviously, awareness would play a big part
in safety, but anyone have any info or just some ideas?....."
BMW's HPS airbag does stay inflated a fairly long time, but I did not mean that the airbag inflator unit wasn't vented. Just the bag holds its gas. I'm sorry if I mislead you.
The HPS airbag when undeployed is nylon and is tucked along the upper edge of the roofline. It would melt through as the headliner burns if the passenger compartment flashes over. The stored pressure canister to inflate the HPS bag however is low in the A-pillar area. This is another potential hazard when that cylinder is exposed to heat. Only good thing I can see is that the cylinder is INSIDE the A-pillar structure.
Little different with Volvo's new Inflatable Curtain. That bag will also melt away if exposed to fire but as to what will happen when the cylinder is heated, Volvo says a disk will rupture and the Argon and Helium gas will vent out. Original p[ressure is 1,390 psi) The stored pressure cylinder is rigidly mounted to the D-pillar unless it has been crushed in the crash.
Firefighter Hunt also stated;
"Another problem my department encountered was the dash catching on fire from
the heat of the air bag deployment. We responded to a fairly minor accident in
which both airbags deployed in a 1994 taurus wagon. We removed battery power
upon initial arrival and some 10 to 15 minutes later smoke started coming out
of the passenger side dash area behind the deployed airbag. We cut into the
dash and extinguished the fire which was in the dash and wire harness
covering. Up until this point I had never heard of this happening, but it just
proves that it can.
Has anybody else experienced this type of problem on any type of air bag
system? It is something we will be on the look out for from now on."
I have worked with several department that have had this very same experience happen to them. In one case, the crash crumpled the dash enough that plastic dash material and yellow airbag wiring harness was in direct contact with the back of the metal airbag inflator canister. The temperature of this canister was hot enough to melt and actually smoke up the inside of the car.
What has been your experiences? Any comments?
[Note: This message has been edited by rmoore]
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