Over the last few years, the installation of airbags in new cars has become more common with most manufacturers now fitting at least a driver’s side airbag. A number of models also have passenger and side impact airbags as optional extras.

What is an airbag?

Airbags are known as Supplemental Restraint Systems (SRS) and are not a substitute for the use of seatbelts. For the particulars of your vehicle’s system you should consult your owner’s manual. If airbags are fitted, the steering wheel cover and/or dashboard cover will usually have the words ‘airbag’ or ‘SRS airbag’ moulded into them.

An airbag is a fabric bag that inflates rapidly when required from the steering wheel centre or dashboard. SRS airbags have been in use for about 20 years.

The SRS airbag inflator contains a solid chemical gas generator. The solid chemicals are safely stored in a metal chamber inside the SRS airbag module. Each inflator is sealed to keep out moisture. SRS airbags are designed to deploy in moderate to major crashes only and should not deploy in minor crashes.

The following four steps show how the SRS airbag works:

• In an impact, sensors in the vehicle detect the sudden deceleration. If the crash is severe enough electricity flows to the inflator and causes ignition of the gas generator.

• The gas generator then rapidly burns in the metal chamber. The rapid burning produces inert gases and small amounts of dust. The inert gases and dust are cooled and filtered during inflation of the airbag.
• The inflating airbag splits open the trim cover. The airbag then rapidly unfolds and inflates in front of the occupant.

• After inflation, the gas is vented through openings or open weave areas in the airbag. Airbags deflate in under a second and may be pushed aside for occupants to exit.

There is a general misconception that airbags provide a soft cushion and will prevent bruising or other minor injuries in low severity crashes - this is incorrect. SRS airbags are designed to reduce peak loads on the head and chest in severe crashes (those where death or long term brain injury are possible). When deploying, the airbag is firm, but it absorbs energy as the gases are released through the vents.

While airbags significantly reduce the risk of serious or fatal injury in crashes, there are some risks from the deployment of airbags in low speed crashes. For this reason modern cars use a range of intelligent sensing functions to ensure that a crash is really happening (not just a bump in the road or a minor knock in the car park) and to fire airbags at the best time. This reduces the likelihood of airbags deploying in minor crashes.

Modern design and construction methods used in today’s vehicles include progressive crumple zones in the body and frame structure to reduce the rate of deceleration in severe frontal impacts. For this reason, damage sustained by a vehicle in a head on collision may appear quite extensive and the airbags may not have deployed because the crumple zones have absorbed a significant amount of the energy of the impact. In these cases, the airbag sensors have detected that the rate of vehicle deceleration has not been sufficient to require triggering of the airbags.

Typically, the driver and or passenger airbags deploy in head-on collisions where the force of the impact is equal to or greater than striking an immovable and non deformable barrier (such as steel or concrete) at a speed of around 12-15 mph or higher. In offset collisions or in a head-on collision with another vehicle or other deformable and/or movable object, the speed would generally need to be significantly higher than 15 mph for the airbags to deploy. Airbags designed for frontal impacts usually do not deploy in rear end collisions, side impacts, rollover accidents or in most underride accidents.

In addition to the frontal impacts described above, airbags may deploy in cases where:

• A moderate to severe impact has been sustained by a vehicle’s undercarriage such as when striking an elevated median strip or kerb.
• A wheel has struck a deep (severe) hole or pothole, or when driving down a very steep slope and the front strikes the ground at the end of the slope.