Dual-Stage Airbag Systems

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SUBJECT: Airbags
TOPIC: Dual-Stage Airbags
OBJECTIVE: Understand the design and operation of dual-stage or dual-threshold airbag systems.
TASK: Explain what a dual-stage or dual-threshold airbag system is, how it operates, and what influence these systems can have on fire, EMS, and rescue operations if present at an incident scene.

Airbags can deploy a second time. If that’s news to you, then you aren’t familiar with the technology of dual-stage or dual-threshold airbag systems. Over the past few years, this system has become so commonplace that it is actually advertised by automakers as a selling point for their vehicles and is commonly listed as a safety feature on the window sticker of a new vehicle.

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Photo By Ron Moore
The entire passenger frontal airbag unit has been removed from a late-model vehicle. Nothing appears unusual about the airbag. Its airbag ID is typical. The blow-out panel is a common design. It is, in fact, a dual-stage airbag with two separate inflator modules.

A dual-stage airbag has two inflator modules connected to one airbag. Each inflator module has a different power rating; say for example, a 70% charge in one inflator and a 30% charge in the other.

Depending upon conditions such as speed, crash severity, occupant seating position, and seatbelt use or non-use, the airbag may deploy by firing only one of these two charges, leaving a second “live” charge. Generally, the more severe the crash, the greater chance that both inflator modules will deploy in rapid succession. Some dual-stage airbags deploy both inflator modules, one after the other, in every crash situation. From a responder’s point of view, this is the safest system design for fire, rescue and EMS personnel.

With a typical dual-stage airbag system, the “first” firing utilizes one charge of propellant and initially deploys the airbag. If needed and called for by the airbag “brain,” the “second” firing utilizes the other propellant charge and more fully inflates the bag during the few milli-seconds of the collision.

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Photo By Ron Moore
A view of the rear of this same frontal airbag clearly shows the two separate inflator modules. This airbag is designed to fire only one inflator in a low-speed collision, leaving responders faced with a “dead” airbag with a “live” charge remaining.

The way the system is supposed to work, in a minor impact with a properly seated and belted occupant, the airbag system may deploy the airbag using only the lower power charge. Mercedes-Benz is one of the manufacturers who does this. In some Mercedes cars for example, they only fire one stage, leaving the second stage active. This minimizes the chance of an occupant being injured in any way by the deployment of the airbag. It is this desire to prevent airbag-induced injuries that has brought about the creation of dual-stage airbags in the first place. If just one charge fires off, then it is possible for a deployed airbag to fire off a second time.

Dual-stage airbags can have two chemical inflators or may actually be dual-stage stored-gas units containing two newer stored gas inflators. The two inflator modules can deploy with a varying time delay one after the other, depending upon accident severity.

How does dual-stage airbag technology affect our fire, rescue and EMS actions at vehicle incidents? The first and obvious difference is that now, with dual-stage airbags, a deployed airbag can deploy a second time while we are at the scene. The deployed airbag hanging from the steering wheel or dashboard – which until now has been considered a “dead” airbag – must now be thought of as “live.”

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Photo By Ron Moore
The backside of a dual-stage driver’s frontal airbag unit shows two connector plugs. One wire has been removed. One power wire would run to each inflator, allowing them to both fire at the moment of a collision or only one charge to deploy while the other charge remains “loaded.”

We need a renewed respect for the airbag inflation zones: 10 inches for the driver’s frontal airbag, 18 inches for the passenger’s frontal airbag, and the five inches of thickness for any roof airbag or a new knee airbag system.

We must review our EMS and extrication practices and procedures. Now, even with a bag hanging out of the steering wheel or drooping over the dashboard, we cannot become complacent and place ourselves, our medical equipment and rescue tools, or our patient within an airbag inflation zone. Working from the side and always respecting the airbag inflation zones is now more important than ever before.

The importance of taking away a vehicle’s electrical power should now be considered an essential action if medical or rescue work will take place. Early in an incident, efforts to simply disconnect or double-cut the battery cables must be initiated. Taking away the power begins the airbag capacitor draining process. The sooner this is started, the sooner the capacitor loses its power and the less likely a deployed dual-stage airbag is to deploy a second time during extrication.

Rescuers continue to dream up unorthodox procedures for dealing with loaded “live” or the newer generation of “dead” airbags. One question that is frequently asked is, “Can’t I just cut the nylon bag off the deployed airbag unit? That way if it deploys a second time, the airbag can’t inflate and hurt me.”

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Photo By Ron Moore
The inflated passenger front airbag on this Mercedes SL-Class convertible actually has a warning about undeployed second charges printed on the airbag itself. Clearly posted in five languages – English, French, German, Italian and Spanish – the printed warning states:
Risk of Injury!
Two-stage airbag system! Airbag could trigger a second time.
Use the same safety precautions as if the airbag had not deployed.

Automakers continue to discourage this action or any other effort that would cause the rescuer to tamper with, cut, restrain or otherwise alter the airbag system. If you did cut away a dual-stage airbag with a live second charge and the second inflator ever fired, it would be like being in front of a sawed-off shotgun blast of heated gases.

Emergency response personnel who deal with vehicle crashes or fire situations can study this new technology to better their understanding of the systems that are available. The best place to start is a body shop or repair service center. Ask permission to rummage through their junked airbag units.

Look on the back of the unit or the end where the wiring connects to the inflator. If you find one with two plugs, two connectors on the same inflator unit, or two hockey puck-sized inflator modules side by side, then you’ve got yourself a great training prop: your own dual-stage airbag. Show this to all your personnel so they too can become familiar with dual-stage airbag systems and can better understand the reality of this new technology.

The nylon airbag on the Mercedes SL-Class convertible has a warning about undeployed second charges printed on the airbag itself. Clearly posted in five languages – English, French, German, Italian and Spanish – the printed warning states:

Risk of Injury!

  • Airbag could trigger a second time.
  • Use the same safety precautions as if the airbag had not deployed.

That’s good advice for us all. Welcome to the world of dual-stage airbags!

Ron Moore will present “University of Extrication: Ron Moore’s ‘Top Ten’ Vehicle Rescue Challenges” and “Safe Parking” at Firehouse Expo 2004 in Baltimore, July 13-18.


Ron Moore, a Firehouse® contributing editor, is a battalion chief and the training officer for the McKinney, TX, Fire Department. He also authors a monthly online article in the Firehouse.com “MembersZone” and serves as the Forum Moderator for the extrication section of the Firehouse.com website. Moore can be contacted directly at Rmoore@corp.firehouse.com.

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