Of seat belts and airbags

H. Kishie Singh

Seat belts are an ingenious invention that has saved millions of lives around the world. No two opinions about that. The debate is about airbags and that is thanks to my friend Major A.P., who opened up a Pandora’s Box. 

Question: Do airbags work if the seat belt is not fastened?

Answer:  Yes they work! But…

It is the ‘but’ that is the Pandora’s Box. 

Wherever these airbags are housed, SRS is printed on the cover. SRS stands for supplementary restraining system, meaning that it is not a standalone life saving device. So, the manufacturer has warned you and washed his hands of all responsibility in case you do something foolish like driving around in a car equipped with airbags and seat belts unfastened. In that case, it would be rarest of the rare case where the airbag could kill rather than save a life.

The airbag is controlled by multiple sensors like impact sensor, pressure sensor, brake pressure sensor and more. A small child’s weight may not activate the pressure sensor making the airbag inoperative.

There is no electronic connection between the seat belt and airbag.  Having said that, the statement is an oxymoron. There is a connection between seat belts and airbags. The airbag protects your chest, face and head. The seat belt across the torso and shoulder keeps you sitting upright and prevents body movement in case of a crash.

Here is an example: You are travelling at 100 kmph without seat belts. The car has a crash, sudden deceleration and stops. Your body will continue to travel at 100 kmph. You will hit the airbag at that speed. The airbag could be deploying at about the same speed. The outcome could be lethal. The seat belt will prevent this.

In case of a crash, the driver, without the seat belt, will hit the steering, dash board or windscreen almost instantly. The time between crash detection and complete deployment of airbag is 15 milliseconds. Slower than instantly! The front seat occupants will be a mangled mess by the time the airbag deploys. On the other hand, the seat belt, a passive safety device is at work all the time.

The airbag, an active safety device, is not always a standalone safety device. Remember SRS?  ‘Supplementary’ is the key word.

Now, how does an airbag work? 

The instant a vehicle crashes, strategically placed sensors, alert and arm the airbag. An igniter fires inside the metal case holding the propellant tablets. A small explosion takes place and fills the airbag with nitrogen. The airbag opens up, provides protection and immediately collapses. 

Sounds simple? Almost. 

The explosive material is Sodium Azide mixed with Ammonium Nitrate. There is a problem with Ammonium Nitrate. After prolonged exposure to heat and humidity, it can become unstable. Pellets disintegrate into powder. When the unstable compound explodes, it does so with much greater force than required, shattering the metal case and spraying shrapnel into the passenger cabin!

Japanese company Takata Corp, the world’s largest airbag manufacturer, has faced this problem. Over 2.9 million cars with Takata airbags have been recalled globally. 

In India, the Toyota Altis has been recalled to rectify passenger side airbag. And earlier this week, Toyota recalled 23,157 Corollas in India over airbag issue.

Happy Motoring!