A new concept for a helicopter sub-floor structure crashworthy in impacts on water and rigid surfaces

The first generation of helicopters whose design incorporated crashworthiness design requirements has completed more than fifteen years in service. The analysis of their behaviour in crash events showed remarkably positive results, forecasting an increasing demand for safety considerations in future designs and promoting the reassessment of aircraft built earlier. A significant amount of research related to helicopter crashworthiness identified water as a crash environment very different from the others. Such investigations showed that the conventional sub-floor structure performs poorly with respect to energy absorption in a crash on water, when compared with a crash on a rigid surface. Firstly, the main energy absorbing components respond very differently, e.g. landing gears and sub-floor structures designed specifically for land impact load cases were less successful in absorbing energy in the case of an impact on water. Second, impacts on water pose a variety of peculiar post-crash survivability problems. Research concluded that the main occupant injuries and some of the deaths were due to excessive acceleration resulting from interaction with the helicopter interior and from insufficient structural energy absorption. In this paper the main differences between impacts on water and impacts on solid terrain are discussed. A novel design of helicopter sub-floor structure is presented. Numerical results obtained for the impacts of the sub-floor structure on water and a solid surface clearly demonstrate that the new structure fails in two different modes depending on the type of impacted surface. This proves that the concept works as initially intended. Enhanced structural energy absorption performance was achieved, when compared to conventional sub-floor structures, impacting on water, therefore improving occupant crash survivability.