A. L. Pennybaker, S.J.H. Veronneau
Aircraft Accident Research Team
FAA Civil Aerospace Medical Institute
P.O.Box 25082 Oklahoma City, Oklahoma 73125-5066

This study began with participation in the on-scene survival factors investigation of a crash-landed BAe Jetstream 31 airplane in January 1991. There were no fatalities and all 17 passengers evacuated, but a uniform pattern of seat failure was observed which contributed to the severity of sustained injuries. This seat is a current, off-the-shelf assembly available in a number of currently flying aircraft and is considered for use in new aircraft.
Each failed seat was noted and examined for mode of failure. Information collected from passenger interviews included each passenger's weight and seating location. Information from hospital records was used to document injury patterns.

Seat failure occurred on all six double-seat assemblies and one single-seat assembly. The same three (of four) attach fittings failed on all seven failed seats. The forward seat legs, having no shear or forward load carrying capacity, were detached from the floor and wall mounted seat tracks. The aft, inboard, shear-carrying seat leg fittings, being double-lugged, remained attached to the floor mounted tracks. However, on all seven failed seats, the aft, outboard, shear-carrying fittings were detached from the wall mounted tracks and had failed in the same manner. Failure of each 3/8-inch diameter track fitting stud was at its weakest point: a 1/8-inch diameter hole in the stud shank 1/4-inch from the bottom surface.

Contact with part and seat manufacturers raised the question of the failed part's suitability for use as a seat fitting. However, part performance in this accident was marginal, i. e., some seats did not fail. Passenger weight/seat failure comparisons revealed seats with less than 200 pounds total passenger weight did not fail, while those with more than 200 pounds did. A Safety Recommendation was submitted recommending issuance of an Airworthiness Directive (AD) to replace the marginal fittings on all affected airplanes.

Accidents are sometimes the ultimate test of a design or of a manufacturing process. Careful observation of failure patterns can help uncover a “weak link” in a system.