Windblast testing of an aircrew helmet: An approach to neck load analysis

Introduction: Modern generation fighter aircraft has expanded the escape envelope for a fighter aircrew. With the ejection occurring at very high airspeeds, windblast is a cause of major injuries and fatalities. Flying helmet, before its induction into operational usage, must be tested in simulated windblast conditions to ensure that they provide adequate safety. Material and Methods: Windblast tests were conducted on a newly designed/procured helmet in a standard windblast test facility as per Mil Std MIL-V-29591/1. A large instrumented Hybrid III male dummy was used for the tests. The test conditions were: Wind speed 600 ± 60 KEAS, rise time of 125 ± 20 ms, time at peak wind velocity of 300 ± 50 ms, and total exposure time of ≥3 s. Structural integrity, retention with the headform, and recorded neck loads were assessed for interpretation of test results. Results: Helmets could withstand the windblast conditions without any significant structural failures and were retained with the headform during the entire duration of test conditions. However, analysis of the neck loads resulted in a significant dilemma in aeromedical decision-making, there being no laid down criteria in the Mil Specification. The neck tension forces were more than the acceptable limits and found to have the potential for significant neck injuries as per the Injury Assessment Reference Values specified in AGARD-AR-330 specifically in the tests where blast was head on and outer visor in up configuration; however, these values were within the acceptable limits as per the other proposed criteria. Similarly, analysis of the neck tension extension combined effects revealed conflicting outcomes for Nij performance limits specified in various standards. This paper discusses the critical analysis of neck loads vis-à-vis the neck injury criteria to understand the neck loads generated during windblast conditions and its implication on aircrew safety. Conclusion: Neck loads assessment is critical in predicting aircrew safety during windblast testing. In the absence of a clearly defined criteria in the Mil Specification, critical ananlysis of neck loads vis-à-vis recommended standards in scientific literature be done to make meaningful conclusion.