An invisible killer: inertial release in seat belt buckles.

No one knows how many incidents have occurred, but this insidious defect has killed for years.

Your client tells you with 100 percent certainty that she was wearing her seat belt when the collision occurred. Yet rescuers found her unrestrained and seriously injured.

Examination of your client's car reveals that the restraint system functions normally, and no part is broken, torn, or bent. In fact, you find no physical evidence that your client wore the belt.

What is going on here? Your client has been hurt by a design defect that usually leaves no trace. The defect is called inertial buckle release. During the crash, the back of the buckle struck your client's hip or the side of the seat, releasing the tongue and leaving your client unrestrained.

Inertial buckle release is more apt to occur in higher speed collisions that frequently kill people or leave them with severe brain injuries. In many cases, the victims are driving alone and police find them unbelted, leaving the family to wonder why their loved one, who always wore a seat belt, decided not to wear it on that particular trip.(1)

No one knows how many incidents of inertial buckle release have occurred, but this insidious defect has killed and maimed for years. Although these cases are difficult to win, your client can prevail if you prepare carefully.

Inertial buckle release occurs when a properly latched buckle is struck sharply on the side opposite the button, as might happen when an accident victim's hip bone hits the back of the buckle. This blow creates inertial forces inside the buckle that release the latch and allow the tongue to pop out of the buckle as though the push button has been depressed.

Most side- and end-release buckles can open inertially.(2) The automobile industry, however, asserts that inertial release cannot occur in a real accident. Because the RCF 67-also known as the Fisher or Type I-side-release buckle is the most common, this article will focus on it. RCF 67 has been used in Ford and General Motors vehicles since about 1969.

A brief history

One of the earliest known references to the inertial-release phenomenon is in a 1959 patent for an aircraft lift-lever style buckle, which is described as one that "will not open under the influence of inertia."(3)

Many patents issued since 1959 describe and propose solutions to the hazard of inertial buckle release.(4) General Motors and seat belt manufacturers, such as Allied Chemical and Tokai-Rika, own many of these patents.

In 1963, General Motors reported observing the "inertial opening characteristics of buckles" in dynamic testing.(5) In 1966, a Ford Motor Co. executive told a congressional committee that the inertial release concept was well known to Ford but was a mere "parlor trick" that could be performed in a lab but could not occur in actual crashes.(6)

In 1978, the National Highway Traffic Safety Administration (NHTSA) investigated inertial release in buckles in Chevrolet's 1975 Monza.(7) The agency also tested buckles in Ford and Chrysler vehicles made between 1971 and 1978.(8) NHTSA, concluding that the phenomenon did "not appear to be a common problem," closed the investigation.(9)

In 1992, the agency conducted another investigation.(10) When read casually, this report appears to be thorough. However, a close analysis of the data and how NHTSA reached its conclusions reveals that the investigation was superficial and its findings preordained. Anyone handling an inertial release case must study the report carefully. Plaintiffs' attorneys will also want to examine later reports on the phenomenon.(11)

In 1995, Edward Moffatt and others published the industry's apologia and "definitive" study, which was intended to lay the issue to rest once and for all.(12) General Motors paid for this testing as part of its defense of several inertial buckle release lawsuits.

Elements for release

There are several factors that affect inertial buckle release:

Acceleration, pulse duration, and webbing load. In this context, acceleration is the measure of how hard the buckle is struck; pulse duration is the length of time the blow lasts; and webbing load is the amount of tension in the seat belt webbing itself.

Laboratory tests show that an RCF 67 buckle will inertially release if three elements coexist.

* First, the peak buckle acceleration will typically range from 150 to 180 as. (One "g" is the acceleration of gravity, 32 ft/sec./sec.)

* Second, the so-called pulse duration of that buckle acceleration must be more than 3 milliseconds.

* Third, there must be very low loading on the seat belt webbing when the acceleration is applied.

Moffatt's testing showed inertial release with accelerations as low as 140 to 150 as, a pulse duration of more than 6 milliseconds, and webbing loads of 3 and 10 pounds. As the webbing load rises, the acceleration necessary to open the buckle increases dramatically. For example, more than 300 gs was necessary to inertially release an RCF 67 buckle with about 100 pounds of belt webbing tension.(13)

Using this data, manufacturers argue that inertial release cannot occur for two reasons. First, they say it is virtually impossible to achieve 180 gs of buckle acceleration in an actual crash. Second, they say the seat belt load will exceed 100 pounds a few milliseconds after an occupant begins to move in the collision. Therefore, the argument goes, since 180 gs cannot be produced by a hip-to-buckle impact, and since webbing load rises quickly in a crash requiring accelerations even higher than 180 as, inertial buckle release cannot occur.

At first blush, these arguments seem reasonable. In fact, they have carried the day in several trials won by manufacturers. However, the simple answer is not always the correct one. Unlike staged crash tests, real-life collisions can produce high buckle accelerations in the presence of low webbing loads, which will cause inertial release.

Buckle spring force. The RCF 67 buckle has a spring that holds the latch in the tongue until the button is depressed. The spring force can diminish with frequent buckle use. If that happens, the force needed to operate the push button and the acceleration needed for inertial release will be lower. Therefore, when handling these cases, plaintiffs' lawyers need to determine the push-button force required to release the tongue on the buckle. Tests by the author on properly operating RCF 67 buckles have produced push-button release forces as low as 1.7 pounds and as high as 5.5 pounds. These forces range from 3.2 to 4.5 pounds on a buckle with normal spring force.

Buckle condition. The RCF 67 buckle essentially has three moving parts--the leaf spring, the latch, and the push button. Manufacturing and assembly variables and the presence of internal corrosion can create greater friction during buckle operation and require higher levels of acceleration for inertial release.

A simple push-button force test will determine whether or not the suspect RCF 67 buckle will inertially release at 180 gs. If the force needed to release the tongue is in the 3-pound to 4-pound range, the acceleration necessary for inertial release will probably be in the neighborhood of 180 gs.

Belt slack. Several hundred pounds of webbing load will produce collision-induced marks on seat belt hardware and webbing. Because inertial release occurs with little webbing load, there is rarely any physical evidence on the restraint system to confirm whether the injured person was wearing the belt.

Seat belt slack can keep webbing loads low during occupant-to-buckle impact. Slack can exist for several reasons. The belt can be worn loosely or the occupant can intentionally introduce slack by engaging the shoulder harness comfort feature. Occupant movement, including pre-crash bracing or reflexive actions and post-crash rebound, can also cause belt slack.

The defense assumes that every occupant is cinched snugly in the center of the seat and that no impact-induced occupant movement or seat belt slack exists. The reason for the assumption is obvious. Modest crash-induced occupant movement in a tightly cinched belt can cause webbing loads to build rapidly. That makes inertial release increasingly unlikely.

The defendants ignore the fact that people are not inanimate crash-test dummies. People do not sit with their body centerline aligned with the seat centerline. People do not always cinch the belt tightly around them. Some brace themselves or react reflexively in a crash, and some collisions jolt bodies away from the buckle, creating belt slack.

In evaluating any potential case, plaintiffs' counsel must consider the occupant's pre-crash position and movement, manner of wearing the belt, and movement caused by the impact.

Investigating cases

Because inertial release usually occurs with very low restraint system loads and without leaving any visible marks to confirm belt use, the initial investigation must focus on whether the client or decedent was in fact wearing the restraint.

The best evidence is a credible client who remembers putting the belt on and having it on at the time of the collision. Equally helpful is a credible vehicle occupant who can testify to the victim's belt use. Sometimes family members, friends, or coworkers may have watched the victim put the belt on as he or she left home or work just before the accident.

Noting the post-accident position and condition of the restraint system is also helpful. If the person wore the restraint and the retractor housing was damaged, the belt may still be extended. Similarly, especially in rollovers, shards from tempered glass or mud may be found on the webbing in places that would be stowed and protected if the belt had not been worn.

A chronic problem for plaintiffs has been the failure to actually demonstrate the inertial phenomenon. At trial, plaintiffs' lawyers and experts talk about the issue, describe velocity changes, accelerations, and hip-to-buckle movements. They draw diagrams and use other evidence to show that inertial buckle release can and did occur. To win, the jury must accept this testimonial evidence.

Defense experts, on the other hand, run a crash test with a tightly cinched occupant to simulate the accident, and unsurprisingly, the buckle does not release. The jury is left with competing experts reaching opposite conclusions. Usually the defense-conducted test tips the scales, and the plaintiff loses.

Plaintiffs, however, can use a test that helped win a 1995 inertial-release case. In Wiitala v. Ford Motor Co., a Hybrid III fifth percentile female dummy pelvis was restrained in the driver's seat of a 19X2 Mercury Lynx.(14) The driver's door was removed and a pendulum was positioned to swing into the left hip of the dummy pelvis. The RCF 67 buckle inertially released in 6 of 10 tests. The tests showed that a restrained pelvis can have sufficient acceleration to cause inertial release at speeds consistent with those in actual collisions. Presenting a videotape of the tests clearly contributed to the plaintiff's favorable verdict.

Further, unpublished data from Moffatt's 1995 study support the conclusion that inertial release can occur in real accidents. The data show that hip-to-buckle impacts can produce buckle accelerations as high as 367 gs-more than twice the minimum needed to pop open an RCF 67 buckle.

The data also show that about 2 inches of hip-to-buckle space is enough to cause buckle accelerations ranging from 180 gs to 225 gs from hip impact-enough to cause inertial opening.

The defense will argue that the victim's medical records contain no information about hip bruises. Wouldn't there be a bruise if the buckle was struck with enough force to produce nearly 200 gs of acceleration? This question is also answered by Moffatt's unpublished data.

Each of the 27 people Moffatt tested was struck at least three times at four different locations in the hip and buttocks area with a buckle that was "fired" by a spring-loaded impactor from a fixed distance. Each person was to report any bruising or discoloration at each of the four points tested. The unpublished results tell the story.

One person, struck 14 times at a single point on her hip, reported a 1-by-1 inch black-and-blue mark. Another reported a red mark at the point where six impacts were performed. Nineteen people reported no bruising or only a small mark as a result of the multiple impacts. With this data, the defense of "no bruise, no inertial buckle release" should become a vestige of the past.

Trial strategy

To directly rebut the industry position that inertial releases are impossible or, at least, extraordinarily unlikely, the plaintiff must present evidence of other unlatching incidents. Preferably this would be done with testimony from witnesses whose seat belt buckles popped open in accidents.

The best evidentiary basis for getting this testimony admitted is to impeach or rebut the testimony of the defendant's engineers. Commonly, seat belt engineers know about inertial release and will eagerly describe it as a "parlor trick" and testify that inertial release is impossible in a real collision.

Once the engineers are on the record, the plaintiff no longer needs to show that the other incidents are substantially similar because they are offered to rebut and impeach the testimony of the defendant's engineers --not to prove a defect For this purpose, any accident in which an inertial release most likely occurred meets the required test of relevance.

Other incidents should be selected where there is little likelihood of inadvertent button actuation from occupants' hands or from objects on the seat.

To foreclose the argument that mechanical failure of the seat belt system or an adulterated or corroded buckle affected performance, accidents should be selected where the consumer notified the manufacturer that the belt released and where the manufacturer conducted an inspection of the seat belt hardware. With all these elements present, the only plausible explanation for the seat belt popping open is inertial release.

Most manufacturers' crash or sled tests have shown buckle and tongue separations. On analysis, these tests will demonstrate that some separations were caused by mechanical overload or inadvertent button actuation. In other tests there may not be enough information to draw a conclusion.

But careful study of all separations may, through the process of elimination, produce unexplained buckle openings that were most likely caused by inertial release. General Motors reported two crash-induced buckle releases to NHTSA during the 1992 investigation and said that one of them "may have occurred as a result of inertial unlatching."(15)

Developing these facts is not simple or quick, but where it can be done, the evidence is powerful, persuasive, and well worth the effort.

An in-court demonstration of inertial release, which can be done by striking the back of the buckle over the knee or with a rubber mallet, is vital. The defense routinely moves in limine to prevent this demonstration, claiming that it bears no relationship to what occurs in a real accident and that it is more prejudicial than probative. Many judges have accepted this argument, leaving jurors to decide if a phenomenon they have never seen can occur.

A persuasive predicate for the demonstration's admissibility can be developed during discovery. The defendant's restraint engineers will say that this demonstration formed a basis for their opinion that inertial release could not occur in a real accident. With this as prologue, the plaintiff's expert can explain to the judge and jurors why a simple mallet impact provides valid, comparative information about whether a buckle can be opened inertially.

Once the plaintiff's expert testifies that a buckle is defective if it can be opened inertially with a blow from a rubber mallet, the in-court demonstration of the inertial opening properties of different buckles becomes relevant and should be admissible.

Inertial release cases are not easy to prove. In fact, plaintiffs have lost most of them to date.(16) However, knowledge, case preparation, testing, careful reconstruction, and credible testimony can result in justice for your client.

Notes

(1.) This scenario was taken from a real incident. Bryan Gruley, Losing Jennifer, DET. NEWS, Oct. 2, 1994, at A1.

(2.) In a side-release buckle, the push-button movement is perpendicular to the plane in which the seat belt tongue moves during insertion into the buckle. In an end-release buckle, the push-button movement is parallel to the plane in which the tongue moves during insertion. An example of this is the loins Development Corp. (JDC) buckles used in late model GM pickup trucks and late model Ford Tauruses.

(3.) U.S. Patent No. 2,876,516.

(4.) See, e.g., U.S. Patent Nos. 4,065,836; 4,310,054; 4,313,246; 4,358,879; 4,380,100; 4,566,161; 4,575,907; 4,733,444; and 4,928,366.

(5.) WILLIAM G. CICHOWSKI, SOCIETY OF AUTOMOTIVE ENGINEERS, A NEW LABORATORY DEVICE FOR PASSENGER CAR SAFETY STUDIES, SAE No. 663A (1963).

(6.) Hearings on H.R. 13228 Before the House Comm. on Interstate and Foreign Commerce, 89th Cong. (1966).

(7.) ANTHONY R. BAYER ET AL., NHTSA, OFFICE OF DEFECTS INVESTIGATION, TESTS OF 1975 CHEVROLET MONZA SEAT BELT LATCHING MECHANISMS, EA7-040 (Mar. 1978).

(8.) ANTHONY R. BAYER ET AL., NHTSA, OFFICE OF DEFECTS INVESTIGATION, SURVEY OF SEAT BELT LATCHING MECHANISMS USED ON 1971-1978 PASSENGER CARS (Mar. 1978).

(9.) Letter from George Chiang, Safety Defects Engineering, to NHTSA Acting Chief, Engineering Analysis Division, Alleged Failure of Seat Belt Buckle on 1975 Chevrolet Monza 2+2, EA7-040 (Oct. 24,1978).

(10.) Petition DP92-017, Inadvertent Release of Seat Belt Buckles, NHTSA, Office of Defects Investigation (Nov. 18,1992).

(11.) Steven M. Arndt et al., Characterization of Automotive Seat Belt Buckle Inertial Release, 37th Ann. Proceedings of the Ass'n of Advancement of Auto. Med., San Antonio, Texas (1993); MICHAEL B. JAMES ET AL., SOCIETY OF AUTOMOTIVE ENGINEERS, INERTIAL SEAT BELT RELEASE, SAE No. 930641 (1993).

(12.) EDWARD A. MOFFATT ET AL., SOCIETY OF AUTOMOTIVE ENGINEERS, SAFETY BELT BUCKLE INERTIAL RESPONSES IN LABORATORY AND CRASH TESTS, SAE No. 950887 (1995).

(13.) Id. at 12.

(14.) No. 90-2895-NP (Mich., Genesee County Cir. Ct. Nov. 3,1995).

(15.) Letter from E.E. Conner, General Motors Manager, Product Investigations, to Charles L. Gauthier, Director, Office of Defects Investigation Enforcement, NHTSA (Oct. 30, 1992).

(16.) See, e.g., Johnson v. Ford Motor Co., No. 93-5133-CA (Fla., Marion County Cir. Ct. (June 27, 1996), appeal docketed, No. 96-1931 (Flat Dist. Ct. App. July 25, 1996); Studer v. Ford Motor Co., No. BC087029 (Cal., Los Angeles County Super. Ct. June 16, 1995).

Darrel Peters is a partner at Goodman, Lister & Peters in Detroit.