Air Bags – A Vehicle Occupant Safety Device

Airbags are one of the greatest technological advances in safety products aimed at keeping vehicle occupants alive during a road collision, but what is your perception of all that happens when an vehicle airbag is deployed?

Just about everyone at some time will have seen film footage on the television showing crash tests being carried out with vehicles manned by crash test dummies, and will have watched bemused as the brightly painted vehicle glides along in slow motion and then crumples against a large block of concrete.

As this happens you will have seen artificial humanoid forms flop forward, and at the same time a white coloured balloon–like pillow explodes from the steering wheel, or dashboard. It will have seemed almost tranquil how the heads of the figures flop onto them, just like a child at play, falling face forward onto a feather–filled cushion. How comfortable it appeared and how safe it made you feel.

Being someone who has had first–hand experience of being saved by an airbag deploying in anger I can tell you the reality is far from the fantasy the televised crash tests will suggest. The experience is one that is hard to explain well enough to give you the full flavour for it, but to try and help you, we’ll use a once popular child’s toy.

Ever Been Smacked in The Face by a Spacehopper?

Did you ever have one of those Spacehopper things when you were a kid? You know, one those big plastic balls inflated with air and with a handle on the top? Now imagine a Spacehopper covered in sand paper, instead of the nice smooth vinyl, and an athlete, who is a power–lifter, taking it by the handles swinging it with full strength around his head and then slamming it into your face.

Now you are getting closer to the reality of being on the receiving end of a vehicle airbag going off in your face during a collision.

Whilst this device may well have saved your life, or reduced the severity of your injuries, you don’t get off scott–free. What you come out of it with is chemical burns around your eyes, nostrils and mouth, and a face covered with patches of gravel rash. If you wear spectacles, these become rammed into your face, so it is best to make sure you have plastic lenses.

With my own experience behind me, and thereafter attending road collisions as a traffic police officer, I would give a wry smile when I would hear a disgruntled driver complaining that his or her airbag had not deployed during their crash. If those people had ever had the experience they would actually be very thankful that it hadn’t — certainly not in the case of a crash where the forces involved allowed the vehicle occupants to be left walking around complaining about the state of their car.

Thankfully, the amount of collision energy that is required to activate an airbag system has to reach a substantial level before it will be deployed. The design of these devices is such that it should only come into play in the type of impact that would otherwise leave the vehicle occupants dead, or at the very best, critically injured.

How a Vehicle Airbag System is Activated

When you look at the construction of a vehicle airbag system you will learn that it is activated by an electronic device, acting as a switch, and which is triggered by a pre–determined rate of deceleration. The switch completes an electrical circuit, which in turn triggers an explosive charge within the airbag stowage compartment. When detonated the airbag explodes from the steering wheel centre, dashboard, or wherever it is fitted. Yes, that’s right, you and your passenger are sitting in the car looking at an unexploded bomb, right there in front of your eyes.

Many cars now being produced have multiple airbags, these being fitted to B–pillars, seat backs and in various overhead positions within the roof lining. This is in addition to the more common locations, such as the steering wheel boss and dashboard panels. In fact, in a modern car, they can be all around you, and in a big impact, they could all be deployed together.

When the explosive charge is detonated this rapidly inflates the airbag envelope with nitrogen gas, which causes it to burst from its hiding place and into the appropriate position to do its job.

The envelope itself is made from a nylon type fabric which is quite abrasive in texture. It actually has vent holes in it, and these are present as it is not intended the airbag should remain inflated once deployed. The idea is for the vehicle occupant to fall onto it, and as all the gas is expelled from the envelope through the vent holes, it provides a cushioning effect to
disperse the energy.

An Airbag System Should Not be Relied Upon on Its Own

An airbag can not do its job properly without the assistance of the seat belts, as the two systems are designed to work in harmony with one another. In the early days of airbags there was a big problem in that the safety belt allowed the vehicle occupants to move too far forward and so causing too great an impact between head and airbag. What used to happen in that situation the head of the vehicle occupant would collide with the airbag before that airbag was fully inflated, so instead of landing onto the airbag, the airbag punched the driver or passenger in the face.

To overcome this, modern seat belt systems have a device built into them known as a pre–tensioner and this is connected to the same trigger mechanism that fires the explosive charge within the airbag system. The pre–tensioner takes in an slack in the seat belt, pulling it tight into your body, and so effectively pinning you to the back of your seat. This allows the airbag to do its job more effectively and ensures that your head is in the right place to connect with the airbag at just the right time during the deployment process.

A 50mph Impact Into a Solid Object

The damaged police vehicle in the folowing photographs is a 1998 Vauxhall Omega police traffic patrol car, and when it crashed I was in the front passenger seat.

The speed was 50–miles per hour when it hit a solid immovable object in the form of a steel structure deeply concreted into the ground. To say it was a solid object, if it was capable of being asleep, the 50mph impact would not have even caused it to stir, let alone wake it up! There was absolutely no give in it whatsoever.

The collision speed of 50mph was determined, not by way of an estimate, but through being scientifically calculated by collision investigators.

The make of the car, as far as this article is concerned, is not important. It is the fact the vehicle was fitted with industry standard airbags for both driver and front seat passenger, as well as seat belt pre–tensioners that is the point — and the fact both airbags deployed upon impact.

When the impact occurred there was a deafening bang, after which everything fell silent. The first thing I became aware of was the interior of the car was filled with was appeared to be smoke, but this was in fact a talcum powder type substance that came from within the stowage compartments of the airbags, and which is used to protect them from dampness. Nonetheless, it is easy to see how this could be confused for smoke, and for the crash victim to worry the car was about to burst into flames.

The second thing I was aware of was that I had a very sore throat and by whole face felt as though it was on fire, with particular soreness around by eyes, nostrils and the corners of my mouth. We’ll come pack to this later.

The First Instinct After a Crash is to Get Out of The Car

After quickly checking that both arms and legs were still attached and working, and that I was not leaking blood from anywhere obvious, I reached for the seat belt clasp so as to release the seat belt. Now this is the weird bit, because I couldn’t readily find it.

What happens when a seat belt pre–tensioner is activated is the part of the seat belt system into which you locate the tongue of the actual belt will move rearward and downward. This is how the pre–tensioner tightens the belt. The thing is, when you get in and out of the same car many times a day, and on many days of a week throughout the years, the process of applying and removing the seat belt is as easy to perform as putting your finger on the end of your nose with your eyes closed.

Imagine that one day, when unbeknown to you, someone had moved your nose to another part of your face, so that when you attempted to touch it with your finger, an act that is so very familiar to you, now you can’t find it. The fact that you have been in a substantial crash, and you are in a cloud–filled car, can be confusing enough, but when reaching for the button to release your seat belt, only to find it has been moved, is really baffling.

Seat Belt Pre–Tensioners Fitted to Modern Cars

If you have a fairly modern car the chances are it is fitted with seatbelt pre–trensioners. Next time you are in the vehicle, take peek down the side of the seat nearest the middle of the car, and if you can see what appears to be a small black gas canister with a yellow label on it, that will be a seatbelt pre–tensioner.

So, just remember this story, as it may prove useful should you be unfortunate enough to be involved in a crash.

At the time of my collision I was wearing a pair of sunglasses, and when the passenger airbag struck my face, the nose rest pads of the shades were rammed into my face on either side of the bridge of my nose. after that they were pushed upward, over my forehead. Those sunglasses, whilst everything else was being thrown forwards during the impact, ended up on the rear parcel shelf!

Make no mistake about the power of the airbag. Such is the violence of the explosion during activation, these things are capable of throwing a child from the front seat of the car and into the back.

A Face Covered in Gravel Rash and Chemical Burns

When I got out of the car my face was covered in what I have already described as gravel rash, and there were droplets of blood standing on the surface of my skin in the affected areas. I had two furrows running up my forehead from my nose, these caused by the sunglasses, and my eyes, nostrils and lips were stinging from the effect of the chemicals from the airbag detonator.

Sodium Azide (NaN3), and Potassium Nitrate (KNO3), were reacting with the moisture in these areas of my body, but apart from that I was intact, and the mild discomfort of a stinging face was a small price to pay for still being in one piece.

You will note that I have so far not mentioned the driver of the police car, and for the purpose of sharing this account with you I shall introduce him now as Dave. His situation, as a result of the crash, was different to my own.

Just before the main impact, and whilst fighting for control of the car, Dave managed to get his right forearm across the centre of the steering wheel. When the driver’s airbag deployed it smashed his arm into his face, breaking and displacing his nose. However, his arm didn’t stop there.

The force of the airbag pushed it over the top of his head, and the seat head restraint, before it ended up behind the middle of his back, but behind the seat. You might at first think this would not have been a problem, but if you are sitting in a chair whilst reading this page, try the following to help you understand.

Try Doing This With Your Arms – Bet You Can’t Get There

First, put the back of your forearm against the bridge of your nose. Now move the arm upward to the top of your head before trying to get it over the top and down your back. You can’t do it can you? Now imagine what it would be like for someone or something to force it over your head and down your back.

For the longer term, the broken nose was not too much of a problem, as that kind of injury will usually heal well, but the effect of the arm being forced into an unnatural position was something more serious. As a result of this Dave’s shoulder was pulled out of its socket and all his muscles and ligaments attached to that area were torn to ribbons.

That crash happened in 1999, and when I retired from the police service, late in 2005, Dave was still being treated for that shoulder, and the long–term prognosis was that it will never be fully fit again. However, in spite of his injury, he too was still able to walk away from the crash, and like me, he owes his life to both the seatbelt and airbag systems in that car.

The Headlamp Ended Up at The Back of The Dashboard

The photographs may not accurately convey the degree of force involved during that impact. If I tell you the bodywork at the back of the off–side headlamp mounting ended up hitting the bulkhead at the rear of the engine bay, and that bulkhead was moved backwards, pushing the dashboard further into the cockpit, you might begin to get some idea of the forces involved.

When talking of modern in–car safety systems there is something that every driver and vehicle passenger needs to consider. Whenever you see the initials SRS written on a vehicle dashboard, piece of internal trim or the centre of the steering wheel, it means there is an airbag beneath that covering. It also means that airbag is a Secondary Restraint System, as this is what SRS stands for. SRS means the airbag is intended to work in unison with a seat belt, and not instead of a seatbelt – the seat belt being classed as the primary restraint system.

Beware of The Bomb in The Steering Wheel

If you are a driver who does not wear a seatbelt, take these words as a friendly warning. A human body going forward into an airbag bursting from its recess in the dash, or in the steering wheel, will probably cause more injury than it will prevent. In the United States, where the trigger threshold of airbag systems is more sensitive than ours, they have been known to kill.

One more thing to consider here too. If you are like 98% of drivers who have abandoned the traditional style of steering in favour of the rotational style (cross arms), just spare a thought for what lies beneath your arms. Then spare another thought, this time for Dave.

Julian Smith
Ride Drive Limited

Related Topic – Airbags
Legislation relating to seatbelts an child restraints
Fitting Child Safety Restraints Advice

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This page was last updated
Thursday, 27-Jan-2011

Air Bags – A Vehicle Occupant Safety Device