A Study of Occupant Ejection Mitigation

The National Highway Traffic Safety Administration (NHTSA) has identified ejection mitigation as a top priority, issuing a notice of proposed ruling making (NPRM) for FMVSS 226, Ejection Mitigation, in December of 2009. The NPRM proposed a linear impact test that uses a featureless head-form with a mass of 18 kg to impact a vehicle’s side windows’ daylight opening at various positions. The test measures the excursion of the head-form beyond the plane of the window glazing. The intention is to evaluate the ability of a vehicle’s ejection mitigation system, such as the curtain airbag or other vehicle features, to manage the impactor energy and limit excursion. The NPRM consists of two tests conducted 1.5 and six seconds after the ejection mitigation countermeasure is deployed at impactor speeds of 24 km/h (400 Joules) and 16 km/h (178 Joules) respectively. In January of 2011, the agency issued a final rule for FMVSS 226 revising the impact speed for the higher speed test from 24 km/h to 20 km/h, thus reducing the energy to 280 Joules. This paper will present the results of a case study using computer modeling to understand the roles of the seatbelts and curtain airbags in mitigating ejections, as well as studying a representative energy level that can be employed for evaluating ejection mitigation systems considering both rollover and side impact crashes. The results of the computer modeling will be compared with the energy levels outlined in the NPRM and final rule for FMVSS 226. Furthermore, the authors will also present the results of a parameter study in which the stiffness of a curtain airbag is optimized to balance the requirements of ejection mitigation with the injury prevention targeted by other side impact regulation such as FMVSS 214: Side Impact Protection.