Development of a Side-Impact (Gy) Neck Injury Criterion for Use in Aircraft and Vehicle Safety Evaluation

OCCUPATIONAL APPLICATIONS In this study, a lateral impact (Gy accelerative input), upper-neck, multi-axial, neck injury criterion is developed. This criterion could be used to evaluate the safety of U.S. Air Force ejection systems incorporating helmet-mounted displays or other applications where neck injury risk from side impact occurs (e.g., automotive and civilian fixed-wing or rotary-wing aircraft). Development of the criterion was motivated by military aviation, which increasingly incorporates head-supported mass and an expanded pilot population, including smaller pilots (to 47 kg), raising concerns for pilot safety during ejection. Neck injury criteria based on risk functions constructed with human subject and postmortem human subject data allow system designers to quantify the injury risk and design acceptably safe systems. A maximum allowable value for the criterion applicable to U.S. Air Force ejection aircraft is proposed to limit neck injury risk to a 5% probability of moderate injury. However, the underlying risk function allows practitioners to customize the criterion for their desired level of injury risk and injury classification. TECHNICAL ABSTRACT Background: Neck injury remains a concern in ejection seat–equipped military aircraft with the growing use of helmet-mounted displays (HMDs) worn for entire mission durations, as well as in the automotive and civilian aircraft domains. Existing U.S. Department of Defense criteria are overly complex and not clearly tied to injury risk and thus require improvement. Purpose: This study developed a lateral (Gy) impact, upper neck injury criterion for use in the design and test of U.S. Department of Defense escape systems and HMDs. Methods: A multi-axial lateral impact risk function, referred to as MANIC(Gy), was constructed using the survival analysis of a data set that combined human subject (N = 56) and postmortem human (N = 9) data. The human subject data were analyzed to assess criterion sensitivity to anthropometric factors. Additionally, a risk function was applied to quantify the risk associated with changes in HMD mass and acceleration input. Results: A lateral impact (Gy), upper neck injury criterion is proposed, which yields a 5% risk of Abbreviated Injury Scale (AIS) 2 or greater injury at a criterion value of 0.48 (95% confidence intervals of 0.28 and 0.67, respectively). At an injury level of AIS 3 or greater, a risk function was generated that yields a 5% risk at a criterion value of 0.53 (95% confidence intervals of 0.24 and 0.82, respectively). Conclusions: This initial multi-axial risk function could be applied to quantify the risk of neck injury posed by lateral acceleration. Criterion values were correlated with body mass and related anthropometric factors, indicating that the critical values incorporated in this study may require improvement. This same criterion may be useful for analyzing side impact safety in other vehicle systems (e.g., automotive and civilian fixed wing or rotary wing aircraft).