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Eject! Eject! Eject! – Biodynamics of Ejection

The force moving an ejection seat must be sufficient to enable it to clear the tail of the disabled aircraft. To achieve this, the seat must accelerate from zero velocity to about 12.2 to 24.4 m/sec (40 to 80 ft/sec). This gives a rapid rate of rise of acceleration or jolt factor, which must remain within the physiological tolerance of the human spine. The maximum acceptable limit for jolt factor is 300 G/sec with a peak acceleration of not more than 25 G for a period not exceeding 0.1 sec. The peak jolt factor and thrust in current ejection seats does not exceed 240 G/sec with a peak of 20 G. This allows sufficient margin of safety for variables like the pilot’s weight and the ambient temperature.

 

Parachute Opening

The time to open the parachute depends primarily on the altitude of escape. At altitudes below 152 m (500 ft) there should be a minimum delay, just sufficient enough to clear the aircraft. A delay of about one second is usually adequate. The minimum altitude for safe parachute opening in bail-out would depend on various factors such as rate of descent of disabled aircraft, parachute diameter. It must be remembered that at 30 m (100 ft) and below, bailing out would invariably end up in a fatality.

At medium altitudes of 610 – 4572 m (2000 – 15000 ft), a pilot should wait for 3-5 seconds before pulling the ripcord. Estimation of time can be roughly done by counting “one thousand one”, “one thousand two”…, where each of these count represent approximately one second. This delay is also essential when the aircraft is traveling at speeds beyond 250 Knots, so that the aircrew’s velocity reduces sufficiently before safe opening of the parachute.

Parachute Descent

Although the sequence of events in assisted escape is automatic and rapid; in case of bail out it is necessary and possible to adapt correct body posture prior to the opening of the parachute. The ideal position is head out, torsos slightly bent at the waist and feet held together. This position will prevent the head being stuck by parachute lift web and the entanglement of rigging lines between the legs.

Minor complications may sometimes follow the deployment of the parachute. Lift webs and rigging lines may get twisted. These can be untwisted by manually spreading them apart. A rigging line may be thrown over the canopy making two bulges. This can be corrected by pulling the rigging lines of the smaller of the two bulges. Oscillation and pendulum like movement of the body can occur prior to landing. These can be dangerous during landing and can be controlled by pulling down on two adjacent lift webs and slowly releasing them to their normal position once the oscillations are dampened. During descent, it is necessary to release the survival pack from its attachment to the body.

Parachute Landing

A large number of non-fatal injuries resulting from emergency escape occur on landing. It is, therefore, important to know the basic rules of parachute landing. Pre-landing position should be assumed approximately 305 m (1,000 ft) above the ground. Both arms should be stretched over the head, firmly grasping the lift webs. The knees should be bent and feet held together. The line of vision should be directed at an angle of 45°to the ground and not straight down. The landing should be on the balls of the feet and the body rolled in the direction of moving parachute. This distributes the impact over a large area of the body. Tolerance of a trained man (trained paratrooper) for parachute touch down on ground (parachute landing) is a descent rate of approximately 7.9 m/sec (26 ft/sec). Normal touch down with a 28 feet diameter parachute canopy and for a 70 Kg man is approximately 6.19 m/sec (20 ft/sec), which is considered safe.

After landing, injuries can still be caused by dragging of the parachute, which must therefore be collapsed immediately. The best method is to operate the quick release box and release the harness. Another one is to pull on the lower rigging lines, which will cause the canopy to collapse.

At night the landing position should be assumed from the moment the parachute canopy has been deployed. If landing on trees cannot be avoided, the arms should be folded in front of the face and feet kept together.

Before landing over water, pre-landing preparation is important. During such landing, the oxygen mask should be removed, the flotation stole (Mae West) be inflated on touching the water, and the parachute canopy release operated immediately. As soon as parachute has been released, one must swim upwind from it to avoid entanglement in the rigging lines.

Personal Survival Packs (PSP)

Most PSPs are carried in the seat pan of the ejection seat. PSPs must be packed correctly or else it may alter the ejection characteristics of the ejection seat. Ideally, a PSP should be as hard as is compatible with sitting comfort. Spring like physical characteristics of the pack increases the injury potential to the spine of the pilot by giving rise to acceleration overshoot. Personal survival packs are evaluated for safe ejection characteristics before introduction in service and their contents should not be altered thereafter. If any alteration is considered necessary, further test of the modified packs should be made. The PSP can be separated from the man, during descent either automatically or by pulling on a special ‘D’ ring.

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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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