Pyrotechnic Based Aircraft Escape Systems: Underwater Risks
Matthew Evans
To clear aircraft for flight over water, it is necessary to demonstrate that all escape systems are fit for purpose.
To create an escape route, on crash landing, some aircraft utilise a pyrotechnic based system to fracture and blow out the windows. The Apache Attack Helicopter is one such aircraft. This aircraft has little inherent buoyancy. On ditching into water, the crew will have to affect escape from underwater.
Immersion in water changes the propagation characteristics of an explosive shock wave. Rate of rise and peak pressure is increased at the point of detonation but movement of the wave front through water attenuates the pressure more rapidly with respect to distance than in air. Further, in water, shock waves are more dangerous as the shockwave is not significantly attenuated by clothing or body tissue until an air/tissue interface is found, i.e. the lungs. At this point, there is a large acoustic impedance difference, which results in the shock wave being partly reflected, the refracted component creates shear forces resulting in trauma to soft tissues that can be fatal.
Based on review of the literature; the acceptable limit for a water immersed aircrew man wearing an aviation helmet and flight suit is 345 kPa (peak pressure) and 100 Pa.s (impulse) and a goal for the Apache escape system has been set at 100 kPa and 14 Pa.s.
Are the risks to the aircrew affected by submersion in water?
Full scale tests showed that the shock wave from the system fitted to Apache, when detonated underwater, would kill the aircrew. A small scale test rig has been developed to enable the rapid assessment of the effectiveness of modified systems for blowing out the windows and reducing the underwater blast prior to full scale trials.
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