Is It Time to Change Our Training Yet? Part 2: Exploding Hood Struts
Sacramento, CA, Metro Firefighter Chris Marsh.
Lee Junkins, Midsouth Rescue Technologies
As I began to study; it seemed that these were the only incidents reported and that they were just freak incidents. I put out a request on many forum boards for people to send me reports of any experiences they have had with these struts. The numbers were over whelming; today I have hundreds of these reports and pictures, near misses. As macho firefighters, these near misses are not reported, they are just a moment of excitement and a good story at the fire house, but unbeknown to most people these incidents are happening on regular bases.
These Struts are felled with Compressed Nitrogen Gas. Like the Argon we seen in the airbag canisters, Nitrogen is non flammable and non explosive, but it has a very high rate of expansion when exposed to very little heat. As the nitrogen expands the cylinder will either blow the shaft out, or the side of the cylinder itself will rupture. These are very violent explosions, and have no pattern of results. Some will shoot the shaft end out like an arrow, others will shoot the cylinder end out, and some will just blow the side out of the cylinder. They also have no pattern of travel; some have been known to exit the vehicle straight forward and some at a 45 degree angle to the vehicle. Most of the reports read a lot alike: “We seen it going over the power lines and found it many feet down the road stuck in something.”
One picture, sent to me from Florida, shows just how violent and unpredictable these can be. The shaft from the left side of the hood is shown stuck in the rear rubber bumper of the same car that it came off of, after bouncing off other objects.
A personal experience gives us an idea of the force of these explosions. I was teaching this class in Aberdeen, WA. last February. One of the vehicles we burnt was a 1989 Dodge Caravan. Finding that it had these struts, on the rear full size door, we found a safe place to stand and started a compartment fire in the van. Before the van had even begun to be fully involved, the right strut violently exploded. A few minutes later the left one exploded, but not quit as violently. Upon investigating we found that the right strut had ejected the cylinder end, it exited out at a 45 degree angle to the vehicle. In doing this it bent the door outward 8 inches. This portion of the door is a round tube shape, with three layers of metal. The cylinder was found 44 feet across the parking lot.
As more and more of these reports began to come in, I tried to find any pattern or similarity that would let us know what to expect or look for. There was no pattern; there was one thing that did stand out though. It seemed that there were many more F-150 Ford pick-ups listed than any of the other vehicles. My question was Why?
We went to the wrecking yard and burnt three F-150s, starting each fire in a different location in the engine compartment, all three vehicles had the same two results. The ball and socket that mounts the strut to the hood and fender are made of nylon. These melted very quickly leaving the strut lying loose under the hood. Also the F-150 hood is made of very thin aluminum that lasted an average of 4 minutes, before melting away, leaving the hot loose struts wide open.
One thing we have determined is that when these explode, they will follow the path of least resistance. Using the hood as an example; when the strut explodes physics tells us that for every action there is an equal and opposite reaction. Like a rifle the recoil of the cylinder is pushed into the metal firewall area, which is much stronger than the metal fender that the shaft is mounted to, therefore the less resistance is either straight forward or if it blows apart in the middle the weakest end will break loose and still having a forward and upward motion it will exit diagonally. The same principle is true with the trunk lid or hatchback door.
As we seen in part one; most of us are still teaching the approach to a burning car that was developed in the early 1970s to avoid bumper struts. This method actually teaches us to approach the vehicle diagonally to the corner of the car. Reality is: Today this actually teaches the firefighter that he is supposed to be in the path of these exploding struts.