ME Lewis. Spinal Injuries Caused by the Acceleration of Ejection

The speed and altititude at which modern military aircraft operate are such that escape can only be achieved by some means of forcibly propelling the aircrew clear of the aircraft. The most common method of doing this is by use of an ejection seat.The use of such seats, whilst generally life saving, exposes aircrew to forces that may be at the limits of human tolerance. Each phase of the ejection sequence is associated with characteristic injury patterns and of particular concern is the occurrence of spinal compression fractures, which are caused by the upward acceleration of the ejection seat. Thorough investigation of aircrew who eject is necessary and magnetic resonance imaging of the spines of these aircrew is now becoming mandatory. Aircrew who sustain stable anterior wedge compression fractures usually require no invasive treatment, but are prevented from flying aircraft fitted with ejection seats for 3-4 months. Introduction The need for ejection systems, to provide a means of actively removing the aircrew from their aircraft in an attempt to overcome the difficulties associated with manual escape, was first considered in Sweden and Germany and then later in the UK. The increase in speed of military aircraft developed during the 1939-1945 war resulted in manual escape becoming an increasingly difficult task.The high speed of the aircraft subjected escapees to severe airblast loads, which could physically prevent the aircrew from climbing out of the cockpit or could increase the possibility of failure to survive the escape from subsequent contact with the aircraft structure. The development of the ejection seat could overcome these problems. Nevertheless, ejection is still not without its dangers, and the development of escape systems has brought additional hazards to which pilots bailing out from aircraft were not exposed. The scope of this paper is to outline the ejection sequence, to discuss the accelerations imposed on the aircrew and to highlight the spinal injuries which may result. The paper describes each phase of the ejection sequence: ejection initiation, escape pathway clearance, initial seat acceleration, exposure to windblast, man/seat separation, main parachute canopy deployment and parachute landing, and relates these phases with the injuries the ejectees may sustain. Ejection Sequence The ejection sequence varies slightly with aircraft and ejection seat types; however, all assisted escape systems have broadly similar modes of operation. The emergency escape systems fitted in RAF fast jet aircraft are initiated by pulling a seat pan firing handle. This fires the initiation cartridge and gas is piped around the seat to: • The harness power retraction unit, which pulls the seat occupant’s shoulders and upper torso backward so as to improve the aircrew’s posture and spinal alignment. • The ejection gun sear withdrawal unit, which rotates the cross shaft assembly and fires the primary cartridge of the ejection gun. • The command ejection selector valve, which fires the command ejection cartridge (if fitted) and hence, initiates the ejection sequence of the second seat of a twin seat aircraft. • The Miniature Detonating Cord (MDC) or canopy jettison firing unit. The gases from the ejection gun primary cartridge initiate upward seat movement, release the locking plunger from the top latch and thus unlock the sear from the aircraft structure.As the seat accelerates up the guide rails the following events occur: • The ejection gun secondary cartridges fire in turn to maintain the upward acceleration of the seat. • The static trip rods withdraw the sears from the drogue gun and the barostatic time release unit; the command ejection system disconnects as does the aircraft portion of the personal equipment connector and the main oxygen system. • The emergency oxygen supply is selected and the regulator is set to 100%02 • The leg restraint lines, and arm restraint lines, if fitted, are pulled downwards through the snubber units to restrain the occupant’s limbs. • The rocket motor initiator cartridge is fired by the pull of a static cable. Rocket motor initiation is timed to occur immediately prior to gun separation, so that the seat continues to accelerate smoothly Sq Ldr ME Lewis