The basic design features of seats include an ejection gun, guide rail, seat frame structure, adjustable seat pan, parachute container, drogue, seat pan and/or overhead seat-firing handle. Some seats are provided with armrest firing handles. A barostatic control is incorporated to initiate separation of seat and man, when high altitude ejections are made. The seat is stabilized by a drogue parachute. A time-release mechanism is designed to disconnect the drogue from the seat and unlock the safety harness automatically at a tolerable preset altitude. The present day ejection seats have a ‘G’ switch; sensitive to loads imposed by deceleration which delays the deployment of main parachute till the forward speed is reduced. An automatic leg restraint mechanism is also fitted, which on egress automatically withdraws and secures the occupant to the seat pan. Similarly arm restraints or arm guards are provided to prevent flailing of the arms.
An Important Concept to Remember: Minimum Safe Ejection Altitude (MSEA). The minimum safe ejection altitude quoted for an ejection seat is only applicable to the aircraft in straight and level flight. For an aircraft in descent, the stated MSEA is no longer applicable and has to be proportionately increased. A rule of thumb is to divide the rate of descent by 12 and add the result to the basic limitation of the seat, e.g. in aircraft fitted with zero altitude seat diving at 6,000 ft/min, the MSEA would be:
MSEA = Manufacturer defined Seat Altitude + (Rate of descent/12) = 0 + (6000/12) =500 feet
Remember, it is NOT ZERO feet, but the Minimum Safe Ejection Altitude (MSEA) is 500 ft.
Sequence of Operation of an Ejection Seat
On pulling the firing handle, the canopy gun fires immediately and jettisons. Within a second after the canopy jettisons, the ejection gun fires and the harness locking mechanism gets actuated, which locks and tightens the pilot into the seat harness. As the seat rises, the leg restraint cords pulls the feet close to the seat pan and hold them in place. The Oxygen, anti-G suit, and R/T connections are automatically severed at the Personal Equipment Connector (PEC).
Half a second later, the drogue gun fires and extracts the controller drogue, which pulls out the main drogue parachute. The drogue parachute stabilises the seat pan combination. After a certain time delay, determined by the barostatic time delay unit (BTRU) and ‘G’ switch, the drogues are released, seat harness opens and the parachute is released from the seat. When the drogues are released, they pull on the lifting line in turn deploying the parachute.
Manual Release. In the event of the damage to the seat mechanism, resulting in failure of the seat to eject or failure of the automatic gear to operate, provision is made for the occupant to bail out. However, the latest trend in seat design is to annul this facility, due to weight penalty, as well as, a very high reliability of modern systems. After ejection, in case of failure of the seat-man separation, manual override handle is available to separate and to deploy the parachute.
The sequence of events in assisted escape differs from seat to seat. It is important to be familiar with the sequence of events during ejection for the aircraft type one flies in order to take prompt action in emergency, which is vital for survival. To this end one must read and understand the pilot notes and technical notes on the ejection system thoroughly.
The sequence of events in general can be enumerated as follows: –
- Pulling of firing handle
- Occupant pulled into seat with inertia reel system
- Canopy jettisoning/ MDC initiated fragmentation/separation of canopy
- Ejection gun fires
- Rockets activated
- Seats start moving up
- Leg restrainers (Garters) and Arm guards activated
- PEC aircraft seat portion separate
- Emergency oxygen starts (from seat mounted/ PSP contained Oxygen bottle
- BTRU activated
- Drogue gun activated
- IFF (Indication Friend/Foe) disabled
- Deployment of drogue shoot
- BTRU operates
- Unlocks seat harness
- Leg restrainers released
- Scissors shackle opens
- PEC seat man separation
- Parachute deployment
- Parachute descent
- Escape from an Aircraft
- Biodynamics of Ejection
- Potential for Ejection Injuries
- Current Ejection Systems
- Human Factors in Delayed Ejection
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