Nano Girl Michelle Dickinson: The science of car airbags

Following the recall of faulty airbags by Japanese manufacturer Takata, it’s timely to look at the science behind these livesaving devices — and what can go wrong.
Airbag systems consist of an accelerometer, circuit and heating element combined with an explosive charge and a porous nylon bag. Photo / 123RF
Airbag systems consist of an accelerometer, circuit and heating element combined with an explosive charge and a porous nylon bag. Photo / 123RF

Airbags are installed in cars to help protect the head and body of its occupants in a crash.

Research shows that when combined with a seatbelt, airbags can reduce the probability of passenger fatalities in a crash by up to 70 per cent.

Japanese airbag manufacturer Takata issued a recall which now covers millions of vehicles worldwide after reports of malfunctioning airbags that fired out shards of metal when they inflated in a crash.

The recall involved so many vehicles that the wait time for installing a replacement airbag kept growing.

In Japan, customers became concerned that they owned vehicles containing potentially lethal airbags so as an interim solution, a modification was offered to disconnect the airbags in their cars.

This process involved altering the vehicle's electronics by adding a chip which caused the car's self-diagnostic system to register the disabled airbag as functional.

This week the New Zealand Transport Agency (NZTA) learned that some of these vehicles have been imported into New Zealand without the new owners being aware that their airbags had been deliberately disabled.

Although the NZTA urged the owners of vehicles affected by the global Takata airbag recall to have their front passenger airbags checked to see if they are disabled, the only way to do this is by physically dismantling the vehicle to sight the airbag connection.

Limited to those cars imported after 2015, it is not known how many vehicles are affected in New Zealand, although around 300,000 vehicles have been recalled here as part of the Takata airbag malfunction.

In 1968, chemist John Pietz pioneered the first nitrogen-generating solid propellant using sodium azide which was initially used for airbag inflation systems.

Although toxic if ingested in large doses, the chemical was carefully sealed inside a metal container within the air bag system.

Airbag systems consist of an accelerometer, circuit and heating element combined with an explosive charge and a porous nylon bag.

When the speed of your vehicle changes suddenly, the accelerometer triggers the circuit which heats up the heating element.

This heat ignites the solid wafers of propellant which explode, producing nitrogen gas.

The gas quickly fills the airbag and then starts deflating due to tiny holes in the bag.

The whole sequence happens in less than the blink of an eye (200 milliseconds) and the bag should be deflating by the time your head hits it which absorbs much of the impact energy and reduces the chance of head and neck injuries.

As the solid propellant is crucial to the airbag's successful inflation, it is housed in a metal cartridge fitted inside the steering wheel or above the glove compartment.

In 2001 Takata changed its propellant compound to ammonium nitrate after looking for a chemical which released fewer toxic emissions and was easier to source.

There is still no official cause to the Takata airbag fault, however the US National Highway Traffic Safety Administration has determined that when ammonium nitrate based propellants are used without chemical drying agents they can ignite with too much force.

This excessive force could cause the metal cartridge to rupture rather than open, sending metal shards flying through the bag in the same direction as it inflates in.

Although no reports of incidents have been reported in New Zealand, the airbags have been blamed for 11 deaths and over 150 injuries worldwide, with cases still being investigated.

In light of this new information, from Wednesday all used vehicles from Japan must now be verified for present and connected front passenger airbags and worried owners are being advised to contact the NZTA or their local vehicle manufacturer's representative.

- NZ Herald

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