ME Lewis. Spinal Injuries Caused by the Acceleration of Ejection
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J R Army Med Corps 2002; 148: 22-26 J R Army Med Corps: first published as 10.1136/jramc-148-01-05 on 1 March 2002. Downloaded from Spinal Injuries Caused By The Acceleration Of Ejection ME Lewis ABSTRACT exposure to windblast, man/seat separation, The speed and altititude at which main parachute canopy deployment and modern military aircraft operate are parachute landing, and relates these phases such that escape can only be achieved by with the injuries the ejectees may sustain. some means of forcibly propelling the aircrew clear of the aircraft. The most Ejection Sequence common method of doing this is by use The ejection sequence varies slightly with of an ejection seat.The use of such seats, aircraft and ejection seat types; however, all whilst generally life saving, exposes assisted escape systems have broadly similar aircrew to forces that may be at the modes of operation. The emergency escape limits of human tolerance. Each phase systems fitted in RAF fast jet aircraft are of the ejection sequence is associated initiated by pulling a seat pan firing handle. with characteristic injury patterns and This fires the initiation cartridge and gas is of particular concern is the occurrence piped around the seat to: of spinal compression fractures, which are caused by the upward acceleration • The harness power retraction unit, which of the ejection seat. Thorough pulls the seat occupant’s shoulders and investigation of aircrew who eject is upper torso backward so as to improve the necessary and magnetic resonance aircrew’s posture and spinal alignment. imaging of the spines of these aircrew is • The ejection gun sear withdrawal unit, now becoming mandatory. Aircrew who which rotates the cross shaft assembly and sustain stable anterior wedge fires the primary cartridge of the ejection compression fractures usually require gun. no invasive treatment, but are prevented • The command ejection selector valve, from flying aircraft fitted with ejection which fires the command ejection cartridge (if fitted) and hence, initiates the seats for 3-4 months. http://militaryhealth.bmj.com/ ejection sequence of the second seat of a Introduction twin seat aircraft. The need for ejection systems, to provide a • The Miniature Detonating Cord (MDC) means of actively removing the aircrew from or canopy jettison firing unit. their aircraft in an attempt to overcome the The gases from the ejection gun primary difficulties associated with manual escape, cartridge initiate upward seat movement, was first considered in Sweden and Germany release the locking plunger from the top latch and then later in the UK. The increase in and thus unlock the sear from the aircraft speed of military aircraft developed during structure. As the seat accelerates up the guide the 1939-1945 war resulted in manual rails the following events occur: escape becoming an increasingly difficult on September 27, 2021 by guest. Protected copyright. task.The high speed of the aircraft subjected • The ejection gun secondary cartridges fire escapees to severe airblast loads, which could in turn to maintain the upward physically prevent the aircrew from climbing acceleration of the seat. out of the cockpit or could increase the • The static trip rods withdraw the sears possibility of failure to survive the escape from the drogue gun and the barostatic from subsequent contact with the aircraft time release unit; the command ejection structure. The development of the ejection system disconnects as does the aircraft seat could overcome these problems. portion of the personal equipment Nevertheless, ejection is still not without its connector and the main oxygen system. Sq Ldr ME Lewis dangers, and the development of escape • The emergency oxygen supply is selected MSc MB BCh systems has brought additional hazards to and the regulator is set to 100%02 DAvMed RAF which pilots bailing out from aircraft were • The leg restraint lines, and arm restraint Officer Commanding not exposed. lines, if fitted, are pulled downwards Accident Investigation The scope of this paper is to outline the through the snubber units to restrain the RAF Centre of Aviation ejection sequence, to discuss the occupant’s limbs. Medicine, accelerations imposed on the aircrew and to • The rocket motor initiator cartridge is fired Henlow, Bedfordshire, highlight the spinal injuries which may result. by the pull of a static cable. SG16 6DN. The paper describes each phase of the Rocket motor initiation is timed to occur Email:[email protected] Email:accidentivestigation@ ejection sequence: ejection initiation, escape immediately prior to gun separation, so that rafcam.mod.uk pathway clearance, initial seat acceleration, the seat continues to accelerate smoothly ME Lewis 23 J R Army Med Corps: first published as 10.1136/jramc-148-01-05 on 1 March 2002. Downloaded from away from the aircraft. As the seat separates Spinal Injury from the aircraft it will still have a high Emergency escape from combat aircraft forward speed which must be slowed down exposes aircrew to vertical accelerations that before the main parachute can be deployed approach, and may exceed, vertebral safely. Furthermore, the seat has poor compression failure limits. The firing of the aerodynamic properties which could ejection gun applies an acceleration that is potentially cause it to spin and tumble, and primarily in the upward (Gz) direction and as a consequence it needs to be stabilised on the duration of thrust is limited to the time exposure to the windblast forces.The drogue the seat is attached to the ejection gun, unless parachute system stabilises and decelerates the seat is fitted with a rocket pack. The the seat to a safe speed for main parachute objective is to attain the highest possible canopy deployment.The drogue parachute is velocity in this time and, hence large extracted from the seat by the drogue gun accelerations are applied very suddenly. The bullet which is timed to fire after the seat has problem of vertebral compression fractures cleared the cockpit structures. Following a was first observed on the early German possible short time delay (if the ejection ejection seats (1). Prototype versions of these occurs above the set Barostatic Time Release seats produced peak accelerations of 12G Unit height, usually 10,000 ft) the locks and rates of rise (jolt) of 1100 Gs-1. Although securing the occupant’s harness to the the value of 12G is known to be comfortably ejection seat are released and the pull of the within the limits of human tolerance, the drogue is transferred from the ejection seat to figure of 1100 Gs-1 would now be regarded as the parachute withdrawal line, producing extremely hazardous. main parachute extraction. As the man In the UK, work was directed towards the separates from the seat, the pull of the drogue systematic investigation of the physiological parachute continues on the main parachute limits of jolt and peak acceleration and much canopy to inflate it fully. The aircrew then effort was expended using different descends to the ground beneath the main propulsive charges and types of ejection guns parachute canopy. to produce accelerations that were subjectively tolerable to humans (2). The Ejection Injury outcome of this work was the setting of The use of ejection seats to allow aircrew to limiting values of peak acceleration at 25G escape from aircraft is generally lifesaving. and jolt to 300 Gs-1. However, their use exposes aircrew to Ejection seats manufactured by the Martin forces that may be at the limits of human Baker Aircraft Company began to be tolerance. incorporated into RAF combat aircraft The ejection sequence occurs extremely during the 1940s, but it was not until the late http://militaryhealth.bmj.com/ rapidly with the complete sequence, from 1950s that sufficient experience in their use initiation to being suspended on a fully had occurred to permit an analysis of the deployed canopy taking approximately 2.5 s. injuries sustained. The earliest ejection seats Each phase of the ejection sequence can had ejection gun velocities of 53 ft s-1 and 60 result in aircrew sustaining specific injuries ft s-1, but in later seats the gun velocity was and these injuries are summarised in increased to 80 ft s-1. An 80 ft s-1 velocity Table 1. ejection gun enabled aircrew to eject safely at zero altitude, as sufficient height was gained Table 1.Typical injuries which may be sustained during the phases of the ejection sequence on to allow main parachute canopy deployment. ejecting from a fast jet aircraft. Furthermore, the increase in gun velocity Phase of Ejection Sequence Possible Injuries Occurring During permitted the aircrew to eject at a high air on September 27, 2021 by guest. Protected copyright. Each Phase of the Ejection Sequence speed so that clearance of the tail fin was achieved. Full zero altitude, zero forward Escape pathway clearance MDC splatter burns. speed ejection, however, was not yet possible Canopy jettison rocker motor flash burns. as some forward speed was required to give Cervical spine, head, shoulder, limb injury sufficient airflow so that the main parachute resulting from through canopy ejection. canopy could deploy. Fryer (3) analysed Ejection gun firing and rocket motor burn Spinal compression fractures. RAF and RN ejections and showed there was Femoral fracture from contact with seat pan. an increase in vertebral fracture rates from 10% to 35% associated with the increase in Windblast Windblast flail injuries the acceleration of the 80 ft s-1 ejection gun. During the 1960s considerable technical Drogue parachute deployment Spinal injury from drogue parachute development of the ejection seat took place, opening shock loads. which was driven by the dual need to Main parachute canopy deployment Spinal injury from main parachute opening automate the ejection sequence and to shock loads.