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Eject! Eject! Eject! – Potential for Ejection Injuries

There are different phases of ejection, with potential for injury to the pilot. Sequentially, these phases are – (a) Canopy separation/ fragmentation; (b) Egress; (c) Ram Air/ Wind blast; (d) Wind drag deceleration; (e) Free fall; (f) Parachute deployment, and (g) Landing. As per the phase of ejection, a pilot can sustain various injuries with spinal injury being the commonest. 

Spinal Injury

Human spine consists of 33 vertebrae arranged one above the other and giving two posterior curvatures to the spine in the erect posture. These curvatures help in weight bearing by the spine. The width/ diameter of the vertebrae varies, being maximum in the cervico-thoracic (neck and upper torso) junction and lumbo-sacral (lower back) region and least in the thoraco-lumbar (middle torso) region . The weight bearing of the spinal vertebrae increases from neck downwards and at the lower most thoracic (T-12) level, 60 % of body weight is borne by this (T-12) vertebra. Any factor, which deforms the normal curvatures of the spine, reduces its tolerance to ejection forces. The most common cause for this is the forward bending of the spine thus reducing the vertebral surface area in opposition due to any reason and hence increasing the force per unit area. Therefore, it is important that pilot should assume erect seated posture (the ejection posture), before pulling the ejection handle.

Some pilots may sustain spinal injuries during ejection. Such injuries commonly occur during the egress phase of ejection. This occurs because the ejection forces are directly transmitted to the seated pilot’s spine from the ejection seat. Besides the ejection forces, the posture of the seated pilot during ejection is an important factor which can aggravate the spinal injuries. These may result in stable or unstable fractures of the spine, most commonly in the lower part (thoraco-lumbar) of the vertebrae.

(In this picture, an X-ray film shows compression fracture of the fourth Lumbar (L-4) vertebra. L-4 is the third vertebra from the top )

Another important consideration for a proper alignment of the body is the mounting of the ejection seat in the cockpit. To avoid fouling of the pilot’s knees against the main instrument panel, the ejection seat is so mounted on the guide rails that it follows a path upward and rearward. There is an included angle between the thrust line of the seat and the pilot’s spine. Such an included angle leads to an increased forward thrust on the upper part of the torso during ejection. With a large included angle, the forward flexion components become greater, consequently, the harness becomes less effective in preventing forward flexion of the spine, thus producing spinal injuries more frequently.

Wind Blast.  During ejection at high speed, wind blast can give raise to serious injuries. About 350 Knots is the safe limit for such an exposure. Above this speed, wind blast forcibly separates the knees resulting in severe damage to the hip joints. Leg restraint prevents this from happening. Arms and shoulders are equally vulnerable to wind blast. Other parts of the body such as the abdomen and chest do not appear to suffer any ill effects from short duration wind blast up to the level of sonic speeds. Above 350 Knots, gloves, shoes, helmets and masks are frequently lost and flying clothing is torn.

Wind Drag.  As the seat enters the air stream as a blunt body traveling forward with the speed of aircraft, it is rapidly decelerated. The magnitude of the deceleration depends on the airspeed, mass of system and cross sectional area exposed to the drag. For moderate speeds the forces of wind drag in the Gx axis are within human tolerance limits viz. peak 50 G @ 500 G/s for maximum duration of 0.2 seconds.

Rotational Stresses. Although almost any type of rotation can occur after separation from the aircraft, essentially two types are of concern. The first is head over heels tumbling about a transverse body axis. The second is a flat spin in which the body is stretched in an essentially horizontal altitude, and spins about an axis passing from back to front. Drogue stabilization obviates these hazards. Tumbling and spinning in a free fall commonly cause disorientation, nausea and vomiting.

Escape from submerged Aircraft.  Escape from submerged aircraft by use of ejection seat is especially applicable to naval operations, flying in coastal regions or during transcontinental flights. It is recommended that manual escape with the help of the floatation stole be used if the canopy is open or has separated off during submersion. However, if the canopy has not separated, it is preferable to eject through the canopy. Recommended procedure for escape includes breathing 100% Oxygen until ready for ejection, inhaling normally and blowing out slowly. After activating the ejection seat, the parachute harness is released and the floatation stole inflated, pushing free from the seat and as the floatation stole carries the aircrew to the surface, he should blow out excess lung gases to avoid air embolism. If the aircrew becomes entangled and cannot separate from the seat, inflation of the dinghy will carry both him and the seat rapidly to the surface. Such escape should be carried out before the aircraft sinks too deep. Assisted escape is effective up to 100 ft under water.

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Reference

1. Ernsting’s Aviation Medicine. Rainford DJ, Gradwell DP (Editors). 4th Edition. Hodder Arnold, London 2006.

2. Fundamentals of Aerospace Medicine. DeHart RL, Davis JR (Editors). 3rd Edition. Lippincott, Williams & Wilkins, Philadelphia 2002.

Acknowledgement.  Image courtesy Wikimedia Commons

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