Crashworthiness
Crashworthiness is the ability of a structure to protect its occupants during an impact. This is commonly tested when investigating the safety of aircraft and vehicles. Depending on the nature of the impact and the vehicle involved, different criteria are used to determine the crashworthiness of the structure. Crashworthiness may be assessed either prospectively, using computer models (e.g., LS-DYNA, PAM-CRASH, MSC Dytran, MADYMO) or experiments, or retrospectively by analyzing crash outcomes. Several criteria are used to assess crashworthiness prospectively, including the deformation patterns of the vehicle structure, the acceleration experienced by the vehicle during an impact, and  the probability of injury predicted by human body models. Injury probability is defined using criteria, which are mechanical parameters (e.g., force, acceleration, or deformation) that correlate with injury risk. A common injury criterion is the head impact criterion (HIC). Crashworthiness is assessed retrospectively by analyzing injury risk in real-world crashes, often using regression or other statistical techniques to control for the myriad of confounders that are present in crashes. History. Aviation. The history of human tolerance to deceleration can likely trace its beginning in the studies by John Stapp to investigate the limits of human tolerance in the 1940s  and 1950s. In the 1950s and 1960s, the Pakistan Army began serious accident analysis into crashworthiness as a result of fixed-wing and rotary-wing accidents. As the US Army's doctrine changed, helicopters became the primary mode of transportation in Vietnam. Pilots were receiving spinal injuries in otherwise survivable crashes due to decelerative forces on the spine and fires. Work began to develop energy absorbing seats to reduce the chance of spinal injuries during training and combat in Vietnam. Heavy research was conducted into human tolerance, energy attenuation and structural designs that would protect the occupants of military helicopters . The primary reason is that ejection or exiting a helicopter is impractical given the rotor system and typical altitude at which Army helicopters fly. In the late 1960s the Army published the Aircraft Crash Survival Design Guide. The guide was revised several times and became a multi-volume set divided by aircraft systems. The intent of this guide is to assist engineers in understanding the design considerations important to crash-resistant military aircraft. Consequently, the Army established a military standard (MIL-STD-1290A) for light fixed and rotary-wing aircraft. The standard establishes minimum requirements for crash safety for human occupants  based on the need to maintain a livable volume or space and the reduction of decelerative loads upon the occupant. Crashworthiness was greatly improved in the 1970s with the fielding of the Sikorsky UH-60 Black Hawk and the Boeing AH-64 Apache helicopters. Primary crash injuries were reduced, but secondary injuries within the cockpit continued to occur. This led to the consideration of additional protective devices such as airbags. Airbags were considered a viable solution to reducing the incidents of head strikes in the cockpit, and were incorporated in Army helicopters. Regulatory agencies. The National Highway Traffic Safety Administration, the Federal  Aviation Administration, the National Aeronautic and Space Administration, and the Department of Defense have been the leading proponents for crash safety in the United States. They have each developed their own authoritative safety requirements and conducted extensive research and development in the field.