Charles C. Roberts, Jr.


Supplemental individual restraints (air bags) are installed on most present-day vehicles. The modern air bag control computers (sensing diagnostic modules) store pre crash vehicular information such as speed and brake application. The deployment or non-deployment of these devices raises several claims related issues, such as: should the device have deployed in such a minor accident; why didn’t the device deploy in such a severe accident; and why was the insured injured from the air bag. These issues can often be explained by vehicle inspection and diagnostic testing.


The first patent issued regarding a deployable frontal impact protective device was issued in 1935. An example of the forerunner to the airbag is shown in Figure1: a drawing based on C. L. Straith’s 1937 patent # 2091057. This was essentially a spring loaded cushion that would be freed at the release point during an accident, theoretically restraining the occupant.

Figure 1 Drawing based on Straith patent #:2091057 August 24, 1937.

In 1953, John Hetrick and in 1958, H. A. Bertrand (Figure 2) patented the concept of the inflatable protective air bag system, which resembles present- day air bags. In 1964, Dr. Carl Clark published experimental data, which was funded by NASA, on an inflatable device called the “Airstop” system, significantly enhancing understanding of the technology. In 1966, the National Highway Traffic Safety Administration was created to develop automobile safety standards and, in the early 1970’s, promoted air bag technology as a means of reducing the severity of frontal crashes to vehicle occupants.

Figure 2 Drawing based on Bertrand patent #:2834606 May 13, 1958


Figures 3 and 4 are drawings of the two major system designs: the multi-point sensor system and the single point sensor system. The multi-point sensor system incorporates crash sensing devices placed at a distance from the diagnostic module that control the air bag deployment based on calibrations unique to the vehicle. The sensing devices are acceleration measuring sensors called accelerometers and are typically located at the front of the engine compartment for steering column airbag deployment. The accelerometer information is analyzed by a computer program in the sensing diagnostic module and makes the deployment decision within 50 milliseconds. The single point sensor system has the accelerometers inside the control module with no remote wiring to sensors. The principal direction of force of impact plays a significant role in deployment. A frontal barrier crash in excess of about 10 miles per hour will result in the steering column air bag deployment, since the principal direction of force is along the long axis of the vehicle and toward the rear. In a “T- bone” type side crash where the principal direction of force is 90 degrees to the long axis of the vehicle, the steering column air bag will most likely not deploy, since the cushioning effect of the air bag is not necessary because the driver is moving laterally. This reduces the chance of injury from an unneeded deployment. It should be noted that an occupant of a vehicle can be injured by an air bag deployment, especially if that person is small of stature and out of position, hence the warning on the visor to remain in a driving position.

Figure 3 Multi-point sensor system

Figure 4 Single point sensor system

Figure 5

Figure 5 shows a typical frontal impact with the principal direction of force along the long axis of the vehicle. Figure 6 shows the deployed driver’s airbag indicating crash sensors saw sufficient deceleration to deploy.

Figure 6

Figure 7

Figure 7 is a view of an impact to the left front fender of a vehicle. The principal direction of the force is to the right of the vehicle and at nearly a right angle to the long axis of the vehicle, which is outside of the range of allowed deployment for a frontal airbag. Although there was considerable damage to the front of the vehicle, the airbag did not deploy (Figure 8).

Figure 8

Figure 9

Figures 9 & 10 are views of a side impact to the front of a vehicle. Figure 10 shows the interior and the deployed side air bags. Note that the steering column air bag did not deploy since the principal direction of force was lateral to the long axis of the vehicle.

Figure 10


Since 1974, General Motors airbag equipped vehicles have recorded data on deployment and near deployment of air bags in the memory of the on- board sensing diagnostic modules (black box). Recently, software and hardware interface modules have become available to collect this data as shown in Figure 11. The US Government has encouraged the acquisition of crash data for research purposes and has encouraged all vehicle manufacturers to make this data available through the standard vehicle electronic interface. Another aspect of this crash data that has not gone unnoticed is its usefulness in accident reconstruction. In Figure 11 a laptop downloads data from the air bag sensing diagnostic module through the vehicle interface, using an interface processor.

Figure 11

Examples of data that can be recovered are shown in Figures 12 through 15. Figure 12 is a graph of the data from a non-deployment event (a deceleration of the vehicle that is not sufficient to deploy the air bag.) The sensing diagnostic computer stores vehicle data from up to 5 seconds before an electronic logic decision is made to deploy.

Figure 12

Figure 12 shows the brakes being applied and the vehicle decelerating from about 39 MPH to 36 MPH from 5 to 1 seconds before algorithm enable (the computer program decision to deploy.) Engine RPM and throttle position are also graphed. Figure 13 shows additional information retrieved. The air bag warning light was not lit, indicating no faults found in the diagnostic circuitry. The driver’s seat belt was buckled. The passenger air bag suppression switch circuit was not suppressed, and 6815 ignition cycles had occurred at the time the data was recorded. This data suggests that the system operated properly by not deploying the air bag.

Figure 13

Figure 14

Figure 14 shows a similar data graph from an accident where the air bag was deployed. After nearly full throttle operation, the brakes are activated and the throttle closed to the idle position. Vehicle speed is characteristic of severe braking. The impact speed is near 30 mph. Since the airbag was deployed, the velocity change data is recorded, as shown in Figure 15.

Figure 15

The velocity or speed change is the speed at a particular time minus the initial speed. If a vehicle strikes a barrier at 30 MPH and stops after the impact, the velocity change is 30 MPH. If a vehicle strikes a barrier at 30 MPH and bounces back at 3 MPH, the change in velocity is 33 MPH. In Figure 15, the total velocity change during the accident was about 43 MPH, a severe accident where the air bag should deploy. Since the striking velocity was at about 30 MPH and the change in velocity was about 43 MPH, this indicates that the subject vehicle was pushed backward at about 13 MPH after the accident with the other, larger vehicle.


When claims issues revolve around reconstructing automobile accidents, an inspection and testing of the vehicle can yield valuable information that can act as a basis for acceptance or denial of a claim. The measured speeds and speed changes stored in the on-board computer (black box) will trump most calculated estimates. A claimant may state that his vehicle was traveling at 30 MPH prior to impact when on-board data shows clearly that the vehicle was traveling at 62 MPH. Injuries sustained by a claimant may be explained by the buckled and unbuckled status data stored in the computer. Claims of alleged defects in the air bag system can be investigated. As always, permission to download the on-board computer data may be required before proceeding with an investigation.