Mon 29th Aug | 2016

How Airbags Work

Accidents Auto Defect BY

Today’s airbags are not designed to inflate in every crash, inflate at certain impact speeds, or even prevent all injuries. The way airbags inflate is not always ideal, and in some crashes may do more harm than good.

When might an airbag be defective?

An airbag can be defective if:

  • It did not inflate when it should have and failed to prevent an injury.
  • It did inflate when it should not have and caused an injury.

When should airbags inflate?

Whether an airbag should or should not inflate depends on the specific vehicle, specific seat position, and the specific crash. There is no “One Size Fits All” or one set of crash conditions applicable to all vehicles for which an airbag must inflate. Instead, government standards expect airbags to inflate when necessary to limit injurious forces on the body to levels deemed acceptable by the National Highway Traffic Safety Administration (NHTSA).

Crash Tests

To pass the required government tests needed to legally sell a vehicle in the U.S. (Federal Motor Vehicle Safety Standard – FMVSS Certified), all vehicles must be crash tested in the same government specified tests conducted with instrumented dummies inside the vehicle. The instruments in the dummies record forces on various parts of the head, neck, and body that can be compared to known force levels at which a given injury severity will likely occur to a human body. The dummies used are different heights and weights to represent men, women, and children.

Crash tests for many vehicles are available on the internet at The Insurance Institute for Highway Safety is a non-governmental organization that also conducts crash testing and makes the tests available on the internet.

In order to be FMVSS certified and sold, test dummies inside the vehicle must register forces below limits set by the NHTSA. It is common for vehicle manufacturers to conduct crash tests and fail the test by producing dummy injury levels that exceed the acceptable government limits. Before selling the vehicle, the manufacturer will then often tune or tweak its airbag inflation threshold and then retest to make sure that dummy forces are at acceptable levels.

The forces generated on a dummy vary from vehicle to vehicle. For example, dummies in a large SUV may experience forces much different from the same dummies in a small two-door passenger car when subjected to the same crash test. The deployment threshold is often different even for different seat positions in the very same vehicle. As a result, the airbag deployment threshold may be different for different vehicle sizes and types.

“Smart” Airbags

Airbags are often referred to as “smart” airbags because sensors throughout the vehicle continuously observe and consider a series of conditions, sending this information to a computer or airbag control module (ACM) as part of the decision to inflate or not inflate in a given crash.

One example of the sensing process is that the system determines when and if the seatbelt is in use. The system is typically programmed to inflate the airbag for an unbelted occupant at a lower crash severity than it will inflate for a belted occupant. The theory is that, for certain crash severities, the seatbelt will adequately protect the occupant and airbag inflation would not be necessary. On the other hand, an unbelted occupant who can be freely moved by accident forces will benefit from an inflated air bag.

If you have ever had the seatbelt bell ring when you had a couple of bags of groceries or some work files on the front passenger seat, that is your onboard system sensing from the weight as a possible person in the seat and considering if the inflation of an airbag will potentially be needed if a crash occurs. If you had an accident with groceries on the front seat the airbag might inflate if the groceries were heavy enough or the airbag might not inflate if the system sensed that the weight in the seat was equivalent to a small child who could possibly be hurt by an airbag deploying. Once again, whether the airbag inflates or not depends on the specific vehicle, seat position, and crash.

The deployment decision is different vehicle to vehicle, seat position to seat position, and depends upon a variety of factors including whether the occupant is seat belted, what seat the occupant is in, where the seat is positioned on the seat track or whether the occupant is a child or adult.

The decision whether to inflate or not is programmed into the ACM and is largely based on the acceleration/deceleration in the crash, and how quickly this acceleration/deceleration occurs. Vehicle manufacturers measure acceleration/deceleration in terms of gravitational force (“G’s”). The vehicle manufacturer programs the system for each vehicle model to specify when the airbag will inflate and when it will not and sets the lowest “must inflate” threshold. The thresholds are considered proprietary and vehicle manufacturers typically do not publish the airbag “must inflate” thresholds.

If the G’s in a crash are equal to or greater than the “must inflate” threshold, then the airbag should deploy. If the G’s are less, then the crash is deemed a “no deploy” and the airbag does not inflate.

Airbags Should Reduce/Prevent Injuries, Not Cause Them

In the final analysis, an airbag should inflate if it can help reduce or prevent an injury, and should not cause additional injury. Even though a vehicle manufacturer may have designed the airbag to inflate at a specific accident severity, it could be a design defect if, in your real-world collision, a severe injury occurred due to non-deployment that could have been prevented if the air bag inflated.

If you have a question about an airbag defect, we invite you to contact Clark, Fountain, La Vista, Littky-Rubin & Whitmantoday for a free consultation!