Vertical Impact Tests of a Modified F/FB-111 Crew Seat to Evaluate Headrest Position and Restraint Configuration Effects

Abstract : A test program was conducted to evaluate the effects of changes in (1) fore-aft headrest position, (2) upper extremity bracing, and (3) restraint harness configuration on human response to vertical impact. One-hundred and fifteen human tests were performed on the Vertical Deceleration Tower up to an impact level of 10.5 G mean (std dev = 0.14), 26 ft/sec velocity change. Subjects were restrained in either a proposed, modified F/FB-111 crew seat and restraint system or a conventional double shoulder strap - lap belt restraint harness and were exposed to comparable impacts in different upper extremely bracing conditions and at different fore-aft headrest adjustments. Measured data included seat acceleration and velocity, head and chest translational acceleration components, tri-axial forces acting on the seat and footrest, forces acting in the restraint harness attachments, and displacements of various body segments. Parametric analysis of the test results was conducted using the Wilcoxon paired-replicate rank test. With the headrest 2 1/4 inches forward of the plane of the seat back, there was increased forward and downward head rotation compared to the headrest 1 inch aft of the seat pack plane. With the headrest 1 inch aft of the plane of the seat ack, there was increased forward- translation of the head compared to the headrest 2 1/4 inches forward of the seat back plane. Subjects utilizing the hands-on-knees bracing position were found to carry a greater proportion of the load through the extremities to the footrest, thereby unloading the vertebral column, than subjects using the hands- in-lap position.