- driven by a pneumatic control system.
- -the firing handle is pulled,
- - small explosive charge is detonated which sends a charge of hot expanding gases through pipe?work around the seat. --- expanding gas first initiates the retraction of harnesses before a sear pin is removed from the top of the gun, firing the primary cartridge. The gun has a piston inside allowing the hot expanding gases to force the seat up the rails breaking the seat mounting shear bolts.
Simultaneously, the canopy of the aircraft must be removed in some way to clear the ejection path.
In ADF aircraft, this is achieved in one of three ways:
a) Through canopy ejection, in which knife?like canopy breakers mounted to the top of the seat shatter the polycarbonate as the seat accelerates upwards. This is the method used in the Pilatus PC?9.
b) Canopy detonation is used in the BAe Hawk, and other British aircraft such as the Harrier. Miniature detonation cord explodes and shatters the canopy, clearing the ejection path, but increasing the risk of injury from burns and tattooing of any skin which is not covered.
c) Canopy separation occurs in the F/A?18 Hornet. The canopy is jettisoned from the aircraft intact.
- seat accelerates to about 50kts by 1 metre
- - reaches a peak of about 20G.
- -human body can withstand about +25Gz at an onset rate of 300G/sec before major vertebral damage is experienced.
As the seat rises, the hot gases ignite secondary and tertiary cartridges.
- As the seat leaves the aircraft, rockets (if fitted) are fired for additional acceleration.
- At the same time, the emergency oxygen bottle, Barometric Time Release Unit (BTRU) and drogue gun are fired.
As the seat goes up the rails, gravity swings the legs down with gas?driven restraints pulling the legs toward the seat and holding them firmly until the crew separates from the seat. Once clear of the aircraft, the BTRU controls the rest of the ejection sequence.
FIGURE 11-1. EJECTION SEAT -SCHEMATIC?
A small drogue parachute is deployed as the seat leaves the aircraft. This small drogue (60cm) pulls out the main drogue (150cm) which decelerates and stabilises the seat until the following conditions are met
- a) Altitude is less than 10,000 feet,
- b) Deceleration is less than about 4G (at about 250kt), and
- c) 1.25 sec has elapsed.
The BTRU is a timer that controls the sequence of events following egress.
- Approximately 2.5 sec after egress and the altitude/G criteria are met,
- the BTRU allows gas flow to open a scissor?shackle on top of the seat.
- The opening of the scissor shackle releases the harness and leg restraint then the drogue deploys the main parachute by pulling it from the head box of the seat.
- The occupant will now separate from the seat and the seat simply falls away.
- The survival pack, which was stowed beneath the seat, now drops below the ejectee suspended on a lanyard.
a) Separation from the aircraft,
- b) Descent and parachute opening, and
- c) Landing.
SEPARATION FROM THE AIRCRAFT
- Back Injuries: There are a number of factors involved with injuries (especially back injuries) during ejection. The seat geometry is such that, during acceleration, there is a force tending to push your body away from the seat (Figure
- 11?2). To counter this, the harness must be done up tightly, preferably with the inertia reel locked. As the handle is pulled, it is imperative that the head, shoulders and back are pushed hard back into the seat. Muscular tension at the time of firing is critical and will be assisted by the adrenaline secreted into the blood stream. It is possible for the neck to be injured as it is flexed forward forcibly with the chin striking the chest, resulting in fractures to the jaw or
Figure 11?2 represents a straight back.
- The natural curvature of the spine positions the vertebra essentially square to each other.
- If the back is not straight on ejection or the chin is down (Figure 11?3), the front of the vertebral bones are closer together and the risk of compression fracture is increased.
- The most common area for these fractures is the middle (thoracic) to lower (lumbar) back area.
- The risks are diminished for a premeditated ejection where the harness is tight and the correct posture is adopted
- statistically, there is a 1 in 3 chance that a vertebral fracture may occur.
- -do not have very thick padding and it
- -must not be too compressible.
- -Sitting on a soft cushion greatly increases the risk of legs fractures and/or spinal damage due to dynamic overshoot.
- - if the thighs are not pressed flat on the seat pan, femoral fractures can occur due to dynamic overshoot as the seat rapidly accelerates and closes the air gap under the thighs.
Figure 11?4 shows the normal acceleration curve compared to one where a compressible seat pad is used
- Initially, there is little or no body movement then followed by a very rapid acceleration.
- The oscillation in the curve is due to the impact of the seat and the occupant.
- The impact accelerates the occupant away from the seat;
- the seat then catches up followed by another impact and so on.
- The result is that body tolerances (onset rate of 300 G/sec and peak G of 25 G) are exceeded.
In addition to the risk of vertebral injury, other injuries can occur when initiating the ejection.
- It is reasonable to expect some form of injury in 75% of ejections.
- This is most likely to be bruising and abrasions.
- Chin and sternum injuries are also possible as the head may be forced forward and downward so that it strikes the centre of the chest.
- Remember that the head and helmet weigh 14?16 times their normal weight due to the acceleration forces.
- The likely result is bruising of the chest and possible winding. In some cases fractures of the sternum have occurred.
- - causes flailing of limbs t
- -may result in multiple fractures or dislocations.
- -Air may be forced into the lungs and stomach causing rupture of the tissues (barotrauma).
Barotrauma is minimised if the helmet and oxygen mask are retained during the ejection sequence.
- Essential that the helmet
- -fitted properly
- -both visors down,
- - chinstrap securely fastened and
- -the oxygen mask on with the ?toggle? (if fitted) down.
- -Due to the weight distribution of the helmet, there is a considerable force tending to roll the helmet off the head during an ejection (Figure 11?5).
- -The nape strap is critical in helmet retention as it prevents the helmet rolling forward and must be done up ?comfortably tight?.
DESCENT AND PARACHUTE DEPLOYMENT
- Forces acting on the pilot during this phase are largely
- -exposure to the elements,
- -such as wind blast described above.
- The aerodynamic ?flat plate? load at 600kts is 1200lb per square foot.
- Drag will rapidly decelerate the pilot,
- -wind blast may dislodge protective equipment as well as cause physical injury.
- -Tumbling and flailing may cause limb injuries such as dislocations.
Shortly after the seat leaves the aircraft the drogue gun on the seat will fire a metal rod upwards.
- The rod is attached to the small drogue chute which is designed to stabilise the seat to prevent flail and prepare for main chute deployment so that the main chute is not fouled by the seat.
- Depending on the weight of the occupant, their sitting position, the aircraft attitude and airspeed at ejection, the seat may be in any attitude at the time of drogue chute deployment.
- Thus, the deceleration forces caused by this chute may be felt in any combination of axes.
- -deceleration force experienced by the ejectee as the main chute opens.
- - The higher the altitude, the greater the opening shock will be due to the greater velocity.
- - 25,000 feet opening shock can be as high as 16G,
- -while at 10,000 feet it is only about 8G.
- This is one of the reasons that the BTRU delays parachute opening until 10,000 feet.
- Contact or entanglement in parachute risers may also be a cause for injuries.
At altitude, the physiological risks of
- -hypoxia and
- decompression illness should also not be forgotten.
- - phase of ejection which is most likely to cause injury.
- -depend upon whether the person adopts a good landing posture,
- -what injuries have already been sustained,
- -the landing surface,
- -the condition of the parachute,
- - the weight of the person,
- whether it is day or night,
- -the presence of hazards (such as power lines, trees, etc.),
- the weather conditions (eg. haze, fog, rain, wind) and
- -where the aircraft lands, with the possibility of fire.
Flight Manuals should give information on how best to prepare for the landing phase of an ejection.
- For example, positioning and retention of certain safety equipment will be determined by the landing site.
- Different procedures are followed if landing in water than those for landing in trees. Be aware of the correct landing posture before you go flying.
- This can be practiced with the support of your ALSS.
- The idea of a proper parachute landing technique is to spread the force of impact over as large an area as possible.
- This is achieved by bending the knees on landing then dropping on to the side of the body and rolling to dissipate as much energy as possible.
- 30% chance that a vertebra will be fractured and also a risk of other fractures from a hard landing.
- - So one should always assume that fractures are present and minimise movement after landing.
- -Rescue teams will immobilise the spine as well, in anticipation of a fracture, until X?rays can be obtained.
FACTORS AFFECTING SURVIVAL
- The major factor in ejection fatalities is ejecting outside the ejection envelope, often when there has been plenty of time to avoid this. T
- 2 major reasons for delaying the decision to eject,
- -being a lack of ejection awareness and
- -temporal distortion.
- -most important factors in ejection survival is a mental preparedness to eject.
- - Pilots usually try to ?save the aircraft? by waiting until the last possible moment to eject, even when their life is in imminent danger.
- -his delay, even if short, may lead to an ejection outside of the seat design envelope.
- Successful ejections have been conducted outside the seat envelope, however the incidence of injury is much greater.
- -When under stress, perception of time alters.
- There have been many reports of situations where time has appeared to stand still.
- When hurtling toward the ground in an out of control aircraft, the time available to make a decision and then actually eject will be much less than that perceived.
The minimum ejection altitude
- is based on the aircraft being at least straight and level,
- -that is with no rate of descent and wings level.
- - Any angle of bank or a rate of descent may result in a vector directed toward the ground.
Figure 11?6 shows basic vector diagrams for straight and level flight compared to a descent.
- The vertical components of the ejection seat and the aircraft are additive.
- With a rate of descent, the vertical component will be negative and subtracted from the upward (positive) component of the seat.
From this, it is apparent that an upward vector should be achieved (if possible) to optimise survival prospects. The best possible upward vector can be obtained by converting any excess speed to height prior to initiation of the ejection sequence.
Bank Angle: Figure 11?7 shows the effect of bank angle on the peak ejection height. The peak is reduced as the bank angle increases until at 90o there is no gain in height. Bank angles greater than 90o will result in a height loss. This
implies that optimum ejection performance will be achieved with wings level (in addition to an upward vector).
-The seat?pan handle should be grasped with one hand (supporting the wrist with the other),
- -locking the elbows into the midriff and rapidly pulling with the forearms in an upward motion.
- When initiating an ejection, the handle leaves the detent and travels about 8 ? 10 cm before pulling the sear pin from the firing mechanism.
- Approximately 18kg of force is required to initiate the ejection sequence.
As the handle is pulled, the muscles must be tense with the head and body pushed into the seat.
- The head must be pressed back against the head box.
- To prevent fracture of the femurs (thighbones), the legs must be forward on the rudder pedals with the rudder pedals wound all the way out if time permits. Do not look down.
MINIMUM SAFE EJECTION ALTITUDE
The Minimum Safe Ejection Altitude
- - is based on the situation where, after ejection,
- -the seat mechanisms fail to operate and
- - manual seat separation is necessary with manual deployment of the parachute.
- This gives a safety margin for maximum survival potential.
ABSOLUTE MINIMUM EJECTION ALTITUDE
The minimum ejection altitude is the design criteria for an individual ejection seat.
PC9?A ejection seat is rated as a Zero/60 seat.
- That is, the absolute minimum ejection altitude is ground level, wings level, sink rate zero and a speed greater than 60KIAS.
- A Zero/0 seat, as fitted to most modern high performance aircraft, is capable of removing the occupant from the aircraft even when stationary.
The PC9?A ejection seat relies purely on a ballistic trajectory whereas the Zero/0 seat, in addition to the explosive charge, has rocket motors that will propel the seat to a sufficient height for full parachute deployment.