Why Airbags Don't Deploy

By Chad Lückhoff

One of the greatest advances in vehicle safety over the last 30 years is without a doubt, the airbag system. Upon the moment of impact, within milliseconds of contact, the airbag system triggers and a canvas-style cushion is inflated, helping to arrest your body and prevent it from striking the hard surfaces in the car. All of this is done in a fraction of a second.

No one said that airbags guarantee your safety or that you will not be injured in the accident. There are too many variables at play and unfortunately, each and every eventuality cannot be catered for or voided. It's also important to note that airbags are known as SRS, or Supplemental Restraint Systems, meaning that they cannot work on their own and they require the addition of seatbelts in order to work most efficiently and stand the best chance of saving you. This is important to note for reasons we will go into briefly.

Understanding the system

You first need to understand how an airbag system works in order to understand why an airbag doesn't deploy. Our article on How Airbags Work  goes into more detail as to the system's operation.

Sensors fitted to the car, at the front, rear and in the doors, work in conjunction with the wheel speed sensors, ABS sensors as well as a host of other sensors (depending on the make and model of the vehicle and the system fitted to the vehicle) to recognise when an impact has occurred. These sensors send a signal to the Airbag Control Unit (ACU) and the ACU determines if the impact is severe enough to warrant deploying the airbags, and which airbags need to be deployed.

If the impact warrants deployment, a signal is sent to the relevant airbag units where a pyrotechnic device is triggered, starting a chemical reaction that creates a gas, usually nitrogen. This rapidly expanding gas inflates the airbag and provides some protection from the interior elements. All of this happens in under 30-milliseconds.

Why Airbags Don't Deploy.

Because of the destructive and expensive nature of airbags, the system needs to be sure that the collision is severe enough to warrant activation. The deployment of an airbag creates enough damage due to tear-away panels and the cost of the single-use pyrotechnic that a car can be deemed uneconomical to repair should one be deployed. So they want to make sure that when it does go off, that there is a valid reason for it. You wouldn't want your car written off because a shopping cart happened to bump into it, would you?

Thus, the speed of the vehicle needs to be over a certain threshold in order for deployment to take place. This is usually around 23km/h and in certain instances, twice this speed, if the object is deformable. At speeds lower than this, the airbags will not deploy.

Some systems require that the seatbelts be clipped in in order for the system to activate. The seatbelt not only helps arrest your body, it also aligns your body and positions it in the correct position so that the airbags make proper contact. If you are not wearing a seatbelt, the system may not deploy.

Striking a pole or thin/small object can also result in a failure to deploy. As mentioned above, there are several sensors that need to give the signal to the ACU to trigger an activation. If only one of these signals is missing, the system will not deploy. This can happen if the lamp post strikes an area where one of the impact sensors is not fitted ie: between two sensors.

The same argument applies to strange impacts that don't involve the bumpers (where most impact sensors are fitted). This can happen when the vehicle strikes an object that is higher than the bumper, such as hitting a truck or trailer with a high bumper.

These are the most common reasons for airbags not deploying but system error cannot be ruled out. Should one of the sensors malfunction, the incorrect signal will be sent to the ACU and the parameters for deployment may not be met.