Seatbelt Failure Caused by Removal of Critical Component


John R. Yannaccone, P.E., Principal Mechanical Engineer

Case Summary: An armored car was involved in a frontal collision with another vehicle. The guard, who was seated in the rear compartment in a forward-facing seat equipped with a lap and shoulder belt, sustained abdominal injuries similar to those seen when an occupant is restrained with only a lap belt. The guard reported that he was wearing the lap and shoulder seatbelt properly; however, during the crash his upper body was not restrained by the shoulder belt and his lower body slid forward on the seat. He was transported to the hospital where he was found to have internal abdominal injuries but little external trauma other than a contusion around his lower abdomen. The driver of the van only sustained minor abrasions and bruises and was treated and released from the emergency room. 

Expert Analysis: While the injuries sustained by the guard in the rear compartment were generally consistent with him wearing the shoulder belt behind his back rather than properly over his shoulder, the guard insisted he never put the seatbelt behind him and always wore it properly. The vehicle was inspected for evidence to help establish how the guard had been wearing the seatbelt at the time of the crash. This inspection showed two long abrasions on the seatbelt webbing as well as abrasion on the latch plate and D-ring. This pattern of abrasions was inconsistent with the shoulder belt being behind the back of the occupant; however, it was also inconsistent with a seatbelt that properly functioned in the crash. 

The driver’s seatbelt had more typical signs of crash loading, supporting that there was an issue with the rear seatbelt. Based on this information, the rear seatbelt was removed from the vehicle for further inspection. Once removed, the retractor was subjected to a simple tilt test, i.e., rotated to verify the function of the inertial locking mechanism. This revealed the seatbelt retractor would not lock when tipped. Disassembly of the retractor uncovered that the mechanism which senses accelerations was missing such that it had no means of locking the spool and preventing the webbing from paying out. 

Documents from the armored vehicle manufacturer, who had installed the rear seat and seatbelt, showed they purchased an off-the-shelf lap and shoulder seatbelt system to install in the rear of the vehicles. While the parts used were different from those specified on the engineering drawings for the vehicle, a parts department memorandum indicated the specified seatbelt system was no longer available and they switched to another supplier. Through the discovery process, it was found that the person responsible for installing the rear seatbelts uncovered a problem when they switched to the new seatbelts. When he installed the new system, the webbing could not be extended, so the seatbelt could not be put on. To resolve this problem, he removed the mechanism that sensed acceleration, resulting in the ability to extend the webbing. In the process, he also eliminated the means for the retractor to lock and provide restraining forces in a crash. 

It was also found that no engineering change was issued, nor was an engineering review of the new seatbelt conducted. Had the armored vehicle manufacturer performed a proper engineering review, they should have discovered the issue, and therefore able to either modify the mounting of the retractor so it functioned properly or specify another retractor that could function in the original mounting location.

Result: The armored vehicle manufacturer settled the case with the plaintiff for an undisclosed sum and issued a recall of all vehicles manufactured after the seatbelt change was made, to correct the problem caused by the removal of a critical component.

Categories: John R. Yannaccone | Mechanical Engineering

Tags: Seatbelt | Vehicle Inspection


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