Alternative Designs of Energy-Absorbing Seat Legs for Certification of Commuter Aircraft Seats 971458
The Federal Aviation Administration (FAA)'s analysis of commuter aircraft accidents and ongoing research has indicated that the crashworthiness capabilities of smaller aircraft may be questionable. The small size of these aircraft results in a stiff structure and consequently higher impact loads experienced by the occupants. In 1993, the FAA issued a Notice of Proposed Rule Making (NPRM) 93-71 to increase the deceleration pulse amplitude of the sled tests under the Test-1 conditions (60-degree test) to 32G for the commuter type aircraft. To meet this condition, the seat design must exploit the energy absorption potential for its structural components. Energy absorbing components may include the seat legs, seat pan, and seat cushion. The intent is to design the seat so that it strikes well beyond the elastic limit to absorb the energy of the impact. To date, no seat has yet been able to pass the proposed criteria with an acceptable limit on the lumbar load (1500 pounds). At the National Institute for Aviation Research (NIAR), alternative seat legs have been designed and analyzed to meet the 32G requirement. Two types of energy absorbing seat legs, S-shape and Y-shape seat legs, are presented in this paper. Nonlinear finite element models for the seat legs were first constructed using the MARC code to study the effect of static loading on the seat leg designs. The MADYMO code was then utilized to model the Hybrid II anthropomorphic test dummy (ATD), restraint system (lap belt), seat back, seat pan, and seat legs (finite element model). The alternative energy absorbing seat leg designs resulted in the peak lumbar load of 1300 and 1370 pounds respectively for the S-shape and the Y-shape seat legs as compared to 1850 pounds obtained for a straight leg seat design.
In order to consider the response of a wide range of occupants seated on these new designs, a small 5th percentile female ATD and a large 95th percentile male ATD were also studied. The resulting lumbar loads from the crash analysis showed that the peak lumbar load decreased for the small subject and increased for the large subject, yet all fell below the 1500 pounds for the S-shape seat leg design, although no definitive injury criteria yet exists for the limit lumbar loads on different size ATD's. The analysis has shown that the energy-absorbing seat leg designs could reduce the lumbar load as mush as 40 percent compared to the seats with straight legs. Further research in this area is necessary in analyzing the stability of these structures as they undergo large plastic deformations. Further sled and drop tests also need to be conducted on these alternative energy-absorbing seat leg designs to support the analytical results.