Appendix B to the Preamble - Glossary

Air Bags--In General

Air bags are inflatable restraints. Enough gas must be pumped into them to cushion occupants. Otherwise, occupants, especially large ones, could "bottom out" the air bag and hit the vehicle interior in a crash. Thus, the amount of pressure within air bags must be carefully controlled. This is done by controlling both the rate at which gas is pumped into the air bag and the rate at which the gas is released from the air bag through vents or microscopic holes in the fabric itself.

Categories of Frontal Air Bags

Advanced air bags. Advanced air bags are air bags that minimize the risk of serious injury to out-of-position occupants and provide improved protection to occupants in high speed crashes. They accomplish this either by incorporating various technologies that enable the air bags to adapt their performance to a wider range of occupant sizes and crash conditions and/or by being designed to both inflate in a manner that does not pose such risk as well as to provide improved protection. Some of these technologies are multi-stage inflators, occupant position sensors, occupant weight and pattern sensors, and new air bag fold patterns. (The inflators and sensors are explained below.)

Redesigned air bags.(1) Redesigned air bags are bag systems used in vehicles that have been certified to the unbelted sled test option instead of the unbelted crash test option in Standard No. 208. Typically, a redesigned air bag in a MY 1998 or 1999 vehicle model has less power than the air bags in earlier model years of that vehicle model. However, the power levels of current air bags vary widely. For example, the redesigned air bags in some current vehicles are more powerful than the unredesigned air bags in some earlier vehicles.


Inflators are the devices which pump the gas into air bags to inflate them in a crash.

Single stage inflators. Single stage inflators fill air bags with the same level of power in all crashes, regardless of whether the crash is a relatively low or high speed crash.

Multi-stage inflators. Multi-stage inflators (also known as multi-level inflators) operate at different levels of power, depending on which stage is activated. The activation of the different stages can be linked to crash severity sensors. In a vehicle with dual-stage inflators, only the first stage (lowest level of power) will be activated in relatively low speed crashes, while the first and second stages (highest level of power) will be activated in higher speed crashes. As crash severity increases, so must the pressure inside the air bag in order to cushion the occupants.


Many advanced air bag systems utilize various sensors to obtain information about crashes, vehicles and their occupants. This information is used to adapt the performance of the air bag to the particular circumstances of the crash. It is used in determining whether an air bag should deploy and, if it should, and if the air bag has multiple inflation levels, at what level. Examples of these sensors include the following:

Crash severity sensors. Crash severity sensors measure the severity of a crash, i.e., the rate of reduction in velocity when a vehicle strikes another object. If a relatively low severity crash is sensed, only the lowest stage of a dual-stage inflator will fill the air bag; if a more severe crash is sensed, both stages will fill the air bag, inflating it at a higher level.

Belt use sensors. Belt use sensors determine whether an occupant is belted or not. An advanced air bag system in vehicles with crash severity sensors and dual-stage inflators might use belt use information to adjust deployment thresholds for unbelted and belted occupants. Since an unbelted occupant needs the protection of an air bag at lower speeds than a belted occupant does, the air bag would deploy at a lower threshold for an unbelted occupant. (Deployment thresholds are explained below.)

Seat position sensors. Seat position sensors determine how far forward or back a seat is adjusted on its seat track. An advanced air bag system could be designed so a dual-stage air bag deploys at a lower level when the seat is all the way forward than it does when the seat is farther back. This would benefit those short-statured drivers who move their seats all the way forward.

Occupant weight sensors. Occupant weight sensors measure the weight of an occupant. An advanced air bag system might use this information to prevent the air bag from deploying at all in the presence of children.

Pattern sensors. Pattern sensors evaluate the impression made by an occupant or object on the seat cushion to make determinations about occupant presence and the overall size and position of the occupant. They could also sense the presence of a particular object like a child seat. An advanced air bag system might use this information to prevent the air bag from deploying in the presence of children. An advanced air bag system might utilize both an occupant weight sensor and an occupant pattern sensor.

Deployment Thresholds

The term "deployment threshold" is typically used to refer to the lowest rate of reduction in vehicle velocity in a crash at which a particular air bag is designed to deploy.

No-fire threshold. The no-fire threshold is the crash speed below which the air bag is designed to never deploy.

All-fire threshold. The all-fire threshold is the crash speed at or above which the air bag is designed to always deploy.

Gray zone. The gray zone is the range of speeds between the no-fire and all-fire thresholds in which the air bag may or may not deploy.

Vehicles with advanced air bags may have different deployment thresholds for belted and unbelted occupants, e.g., the deployment threshold may be higher if an occupant is belted. (See belt use sensors above.)

Crash Tests vs. Sled Tests

In crash tests, instrumented test dummies are placed in a production vehicle which is then crashed into a barrier. Measurements from the test dummies are used to determine the forces, and estimate the risk of serious injury, that people would have experienced in the crash.

In sled tests, no crash takes place. The vehicle is placed on a sled-on-rails, and instrumented test dummies are placed in the vehicle. The sled and vehicle are accelerated very rapidly backward. As the vehicle moves backward, the dummies move forward inside the vehicle in much the same way that people would in a frontal crash. The air bags are manually deployed at a pre-selected time during the sled test. Measurements from the dummies are used to determine the forces, and estimate the risk of serious injury, that people would have experienced in the crash.

Fixed Barrier Crash Tests

All of the crash tests proposed in this SNPRM are fixed barrier crash tests, i.e., the test vehicle is crashed into a barrier that is fixed in place (as opposed to moving). The types of proposed fixed barrier crash tests are shown in Figure B1.

Rigid barrier test, perpendicular impact. In a rigid barrier, perpendicular impact test, the vehicle is crashed straight into a rigid barrier that does not absorb any crash energy. The full width of the vehicle's front end hits the barrier.

Rigid barrier, oblique impact test. In a rigid barrier, oblique impact test, the vehicle is crashed at an angle into a rigid barrier.

Offset deformable barrier test. In an offset deformable barrier test, one side of a vehicle's front end, not the full width, is crashed into a barrier with a deformable face that absorbs some of the crash energy.

Figure 1

Crash Pulses

A crash pulse is the graph or picture of how quickly the vehicle occupant compartment is decelerating at different times during a crash.

Stiff crash pulses. In crashes with stiff pulses, the occupant compartment decelerates very abruptly. An example of a crash with a stiff pulse would be a full head-on crash of a vehicle into a like vehicle. The perpendicular rigid barrier crash test produces a stiff crash pulse.

Soft crash pulses. In crashes with soft pulses, the occupant compartment decelerates less abruptly, compared to crashes with hard pulses. An example of a crash with a soft pulse would be the crash of a vehicle into sand-filled barrels such as those seen at toll booths or at the leading edge of a concrete median barrier. The offset deformable barrier crash test and the 30 degree oblique rigid barrier crash test produce soft crash pulses.

In crashes involving comparable reductions in velocity, an unrestrained occupant would hit the vehicle interior (i.e., steering wheel, instrument panel and windshield) at a much higher speed in a crash with a stiff pulse than in a crash with a soft pulse.

Belted and Unbelted Tests

Belted tests use belted dummies, while unbelted tests use unbelted dummies. Despite increases in seat belt use, nearly 50 percent of all occupants in potentially fatal crashes are unbelted. Unbelted tests are intended to evaluate the protection provided these persons, many of whom are teenagers and young adults.

Static Out-of-Position Tests

Static out-of-position tests are called "static" because the vehicle does not move during the test. These tests are used to measure the risk that an air bag poses to out-of-position occupants. Test dummies are placed in specified positions that are extremely close to the air bag, typically with some portion of the dummy touching the air bag cover. The air bag is deployed. Measurements from the test dummy are used to determine the forces, and estimate the risk of serious injury, that people would have experienced in the crash.

Injury Criteria and Performance Limits--In general

In a crash test, sled test, or static out-of-position test, measurements are taken from the test dummy instruments that indicate the forces that a person would have experienced under the same conditions. Standard No. 208 specifies several injury criteria. For each criterion, the Standard also specifies a performance limit, based on the level of forces that create a significant risk of producing serious injury.

Injury Criteria

This SNPRM proposes performance limits for various injury criteria to address the risk of several types of injuries. Among these injury criteria are:

Head Injury Criterion or HIC. Head Injury Criterion or HIC address the risk of head injury;

Nij. Nij addresses the risk of neck injury; and

Chest Acceleration and Chest Deflection. Chest Acceleration and Chest Deflection address the risk of chest injury.

Test Dummies

This SNPRM proposes to use several test dummies to represent children and adults of different sizes. These dummies are:

12-month old Crash Restraints Air Bag Interaction (CRABI) dummy, representing an infant;

Hybrid III 3-year-old and 6-year-old child dummies, representing young children;

Hybrid III 5th percentile adult female dummy, representing a small woman;

Hybrid III 50th percentile adult male dummy, representing an average-size man.

1. These air bags are also sometimes called depowered air bags, second generation air bags or next generation air bags.