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|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.
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.
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.”
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.”