Is It Time to Change Our Training Yet? Part 1: Airbag Explosions
Lee Junkins, Midsouth Rescue Technologies
Now that we see what these can do, let’s look at our training, it is pretty much the same nation wide. With a straight stream, we wash out under the vehicle from a long distance, to extinguish any spillage; we then begin our approach diagonally to one corner of the vehicle to avoid the bumpers, we cool the tire, and knock the fire down. Once we have knocked the fire down, we reach through the window or usually open the door and in a circular motion, put the fire out.
With the door open on either side, let’s look at where we are standing. Depending on the make and model, directly in front of our face is a dual stage frontal airbag, over our head is a curtain airbag, to one side of us could be a curtain airbag inflator mounted in the A post, in the middle of our legs may be a knee airbag, behind us could be a door airbag, or to our side could be a seat airbag, beside our knee is a pyrotechnic seat belt tensioner and this could be just the beginning, We have inflators along the roof rails on both sides. We have inflators in the C and D posts, some have rear seat or door airbags, some even have carpet airbags. Taking all of this into account, the worst danger of all; is that we are trained to be standing here, but we did avoid the bumper struts, that the vehicle probably does not have.
Is it time to change our training yet?
Let’s go back to my initial thoughts: How are we in more danger of airbag systems in fire than in extrication? In an extrication scenario, we can cut the battery cables and allow the capacitors to drain, shutting down the systems. We can also peel the trim to avoid cutting an inflator. We can stay out of the inflation zones, to prevent being accidentally struck.
In a fire scenario there is no way to shut them down; cutting the battery cables does nothing for us. In a catastrophic explosion, where is the inflation zone? In Fig 2, it would have been 110 feet behind the vehicle, in fig 3, it would have been two stories straight up.
When a gas felled inflator is exposed to the heat, the gas is going to expand and either the bag is going to deploy, or the canister is going to explode, there is no way to cut it off.
The same process happens with the chemical type inflators. The chemical used in these inflators is Sodium Azide, the same solid fuel that is used in rockets. Sodium Azide’s ignition point is 350 degrees, which means when the inflator is heated above 350 degrees, the bag is going to deploy, or in the case of our two examples the housing is going to eject and possibly explode into shrapnel.
In researching these chemical inflators, I wanted to find out how these housings come loose from the steering wheel, when they are bolted to the center of the wheel, with four bolts that can normally stand the pressure of the airbag deployment. I actually discovered two different situations.
1. Some inflator housings are made of aluminum, with thin mounting flanges extending from the sides. The flanges are then bolted to a bracket that mounts to the steering wheel hub. The housing itself is a much thicker metal and the back half of it sets into an indention in the hub, some what protecting it from the heat. The front half of the housing is covered by the bag, giving it some insulation. Therefore the thin flanges are the most exposed part and melt, releasing it from the bolts.
This type seems to be the ones to put off shrapnel, the housing being aluminum will melt at very low temperatures and with the flanges already melted we know that the housing is right at the point of melting, then when the azide ignites the housing is not strong enough to contain the explosion and is ripped into shrapnel.