Aircraft Crash Survival Analysis and Design - Essay Example

Aircraft Crash Survival Analysis and Design Ejection Seat D. Poojaa Kunar This paper brings out the significance of Ejection Seat as means of Air Crash Survival for fighter class of aircraft. With the expansion operational flying envelope of fighter aircraft and the increased value of human life, the requirements of ejection seats have increased manifold times. From the bail out era to the present day, ejection seats have improved exponentially. The modern day ejection seat not only provides a means of egress for the fighter pilot but also ensures that he does not suffer even the slightest of physical damage under all conditions of flight. While the paper dwells on the basics operation and construction of ejection seat, it also brings out new developments in the field. Introduction Emergency escape from a malfunctioning aircraft is of utmost importance for any Air Force. The cost of training a fighter pilot is prohibitive and time taken for him to become operational is significant. Therefore all efforts have to be made to save a fighter pilot from the malfunctioning aircraft. In the older era, the aircraft envelope was fairly limited. The speeds of the pre World War I fighters were low enough to permit manual bail out, where in, the pilot jumps out of the aircraft with parachutes on. However, the modern day fighter class of aircraft operates at speeds beyond Mach .2.0 and at altitudes from Ultra Low Levels (30m) to beyond 60,000’. The kind of maneuvers performed by the fighter pilots during operations leave little margin of error. Flying at such envelope, it would be impossible for a fighter pilot to bail out from the aircraft manually. Therefore ejection seats are a must for a safe escape from the malfunctioning aircraft. The ejection seats not only provide a means of escape from the malfunctioning aircraft, but also house the Pilot Survival Pack which contains adequate reserve equipment for the pilot to survive for about 48 hours till rescue arrives. The type of survival pack would depend on the type of terrain over which the pilot is expected to operate namely jungle, snow or water. History The first bungee assisted escape from an aircraft took place as early as 1910. Earliest example of ejection seat was a seat using compressed air, patented in 1916 by Everard Calthrop. The current design for ejection seat is attributable to Romanian inventor Anastase Dragomir. Dragomir patented his "catapult-able cockpit" at the French Patent Office (patent no. 678566, of April 2, 1930, Nouveau système de montage des parachutes dans les appareils de locomotion aérienne). This design was successfully tested on August 25, 1929 at the Paris-Orly Airport near Paris and in October 1929 at Băneasa, near Bucharest. The design was perfected during World War II. Helmut Schenk, a prototype test pilot of Heinkel He 280, became the first pilot to successfully eject on 13 January 1942. Early models of ejection seats were powered by compressed gas. Later in 1943, Bofors developed and tested a gunpowder ejection seat for the Saab 21. In late 1944, the Heinkel He 162 featured a new type of ejection seat using an explosive cartridge. Later types of cartridge ejection seats employed a telescopic tube attached to the seat. The explosive cartridge was fired from inside this tube and the seat catapulted from the aircraft like a bullet from a gun. However with increasing aircraft speed, this technique was inadequate to safely propel the seat out of danger. Any further increase in the amount of propellant in the cartridge would increase the ejection ‘G’ load resulting in damage to the pilot’s spine. This resulted in experiments with rocket propulsion. The first such seat was fitted on the F-102 Delta Dagger in 1958. The current day design of ejection seat features multi stage rocket propulsion which ensures both adequate separation from aircraft as well gradual onset of ‘G’ loads with minimal injury to pilot’s spine. Sequence of Operation of Ejection Seat The modern day ejection seat is fully automatic in nature with no intervention required from pilot till the parachute is fully deployed. For the seat to be fully automatic, the construction of the seat has to cater to the minutest of the details. The sequencing and timing of events are very critical and to the tune of milliseconds. The aircraft canopy is generally automatically jettisoned or shattered (embrittled) prior to the firing of the seat. In case of a twin cockpit / two seater, ejection sequence can be initialized by either of the pilots and the sequence remains automatic fort both the pilots. While the construction and operation of the seat may differ from seat to seat, the underlying principles and the event time frames are largely same. The broad sequence of operation is enumerated in the subsequent paragraphs. The sequence is initiated when the pilot pulls the ejection handle located on the seat. The distance handle travels is small (generally 50 mm) and force required is of the order of 20 DaN. On this action by the pilot, the primary firing cartridge is fired and the seat restraint system is activated and simultaneously the seat starts moving up along the guide rails. The seat restraint or the harness retraction system gets the pilot into correct posture for ejection by pulling back and locking shoulder harness, legs, waist restraint system and by lowering arm guards. Correct posture is critical for minimal injuries to the pilot. The entire event is over by 0.2 seconds with deceleration force of 2g. As a safety measure, the primary cartridge is backed up by a secondary cartridge which generally fires with a delay of 0.2 seconds and fully capable of initiating seat ejection on its own. The initial upward movement of the seat activates the canopy jettison / embrittlement system. The onboard IFF transponder is activated and starts sending distress signals. The emergency oxygen located on the seat is turned on since the pilots would now be separated from the aircraft oxygen supply (critical in case of high altitude ejection). After approximately 0.2 seconds, the rocket pack initiator cartridge is fired which smoothly accelerates the seat out of the aircraft. At approximately 0.5 seconds, the rocket phase ends and the pilot(s) along with the seat is well clear of any part of the aircraft. At this stage the seat may encounter diverse random forces and would have to be stabilized for safe pilot – seat separation. This stabilizing operation is carried out by a system of controller drogue / stabilizer drogue chute assembly. The deployment of drogue chute, which is much smaller in size as compared to the main pilot parachute, causes the seat to decelerate and stabilize in the airflow. Apart from stabilization, the drogue chute also ensures adequate longitudinal separation from the aircraft by rapid deceleration. While ejecting at high altitudes, parachute opening shock is a critical factor and if not taken into consideration can endanger the pilot. For example, at 40,000 feet, the shock would be four times that at sea level (approx 20 G). Therefore in a fully automated ejection system, the parachute should open only at lower altitudes even in case of a high altitude ejection. This is achieved by a safety feature namely, Barostatic Time Release Unit (BTRU). The BTRU is activated by the deceleration force provided by the drogue chute. The BTRU is a barometric capsule based release unit which senses barometric pressure and activates pilot seat separation mechanism generally below 20,000 feet. The pilot seat separation timing can be adjusted in some seats to cater for high mountainous terrain like the Himalayas, where the average terrain height is in excess of 20,000 feet. Based on the barometric altitude, the next event i.e. the pilot seat separation occurs. The BTRU works on three pronged algorithm i.e. if barometric capsule sensed height greater than 20,000 feet then BTRU operates at 20,000 feet, if the sensed height is between 20,000 feet and 7,500 feet BTRU activates on a deceleration force of 2.5 ‘G’ (caused by operation of drogue chute) and if the sensed height is lesser than 7,500 feet the BTRU operates immediately. In the event seat separation failure / high mountainous terrain wherein the BTRU prohibits separation, the pilot has an option of manual separation by help of a manual separation lever located on the seat. At this stage, 1.5 seconds have lapsed (not taking into account the free fall time in the case of high altitude ejection). After pilot seat separation has taken place, a unit cartridge fires which results in opening of pincers, allowing drogue chute to separate from the seat. The pilots parachute also gets deployed and all the restraint locks are released. The unit cartridge firing also ensures disconnection of oxygen and anti G suit connections. At approximately 2.65 seconds, the shock caused by opening of main parachute ensures complete separation of the pilot from the seat. The ejection sequence is now complete. The normal rate of descent is of the order of 20 feet per second. The only now required by the pilot in case of ejection over water is to release parachute just before hitting water, so that he does not get entangled in it. Construction of Ejection Seat The construction of ejection seat is complex in nature. All the assembly units are critically placed to operate at the exact time (to the order of millisecond). While most of components are mechanical in nature, some modern seats have electrical / electronic components for better algorithm management. A modern ejection seat can be broadly divided as follows. Ejection Control Mechanism The ejection control mechanism is primarily the ejection handles positioned on the seat. An ejection seat may have one or handles depending on the space available. Typically all seat have an ejection handle between the legs of the pilot. Some aircraft have handles on top of the seat with a face blind for protection from wind blast for e.g. Kiran aircraft of the Indian Air Force fitted with Martin Baker ejection seat. Some aircraft have additional ejection handles on the sides of the seat e.g. MiG-21 (Type 77). In modern seats, the ejection handles move on two stages – the first half of travel jettisons the canopy and the second half initiates the ejection sequence. The handle is generally secured by a safety pin on ground to prevent inadvertent operation. In more modern ejection seats like the K-36 D 3.5E seat fitted on the Su-30 MKI aircraft, the ejection handle can be lowered by 70° to prevent inadvertent operation on ground. This seat also features an electrically actuated safety squib which when switched off prevent the seat firing. Restraint System The restraint system forces-fixes the crew member to the seat during ejection by retracting shoulder, waist, elevating/retracting legs, besides eliminating flailing of crew member arms. In most European aircraft, the pilot has to wear a leg garter or similar mechanism which is connected to the cockpit by suitable mechanism. In Russian aircraft, there is no such requirement as the leg restraining lines are run along the cockpit walls and below the Main Instrument Panel. In either case the legs are pulled back towards the seat to prevent injury during ejection. All the harnesses remain locked till pilot seat separation takes place. Guide Rails The ejection seat is mounted on guide rails at an obtuse angle to the cockpit floor. The main purpose is to ensure adequate clearance of pilot from the cockpit walls during the upward travel of the seat. However this is not the only purpose of the guide rail. The movement along guide rail (in mm) determines the next event, for e.g in K-36 D 3.5E seat fitted on the Su-30 MKI aircraft, 10 – 30 mm of seat movement on guide rail would initiate BTRU and seat stabilization system is activated at a seat travel of 1070 mm along guide rail. This movement along guide rail is carefully calibrated and discrete commands for subsequent events are associated with it. Main ejection Gun The main ejection gun is the most important part of the ejection seat. The gun generally comprises of primary as well as secondary cartridges as back up for redundancy management. In a two seater aircraft, the firing of the main gun takes place in sequence after a safe time delay. The standard sequence is front canopy, rear canopy, rear seat and front seat. Wind Blast Shield Unit In the older aircraft like the MiG – 21 (Type 77), the canopy attached to the seat and formed a capsule to prevent the pilot from the effects of the wind blast. However such a system was complicated and prone to failures. In case the canopy did not separate, rest of the sequence also failed, resulting in fatalities. In more modern aircraft, especially of Russian origin, a windblast shield (WS) unit diminishes the effect of dynamic pressure upon a crew member’s head/chest. The unit is typically a aerodynamic surface deflecting the wind blast which the pilot is likely to encounter. It actuates during ejection at calibrated air speeds above 480 knots. In the initial position, the windblast shield is retained by a lock. When ejecting at air speeds above 480 knots, an igniter actuates in response to the seat automatics’ command and fires a charge to move a telescopic tube. At the end of travel, the cylinders telescopic tubes are pressed in and keep the WS in the extended position. Trajectory correction Devices Trajectory correction devices are primarily used to correct the trajectory the seat for a safe ejection. In the case of a zero-zero ejection seat (ejection possible when aircraft is stationary on ground), an ejection would result in the falling back on the aircraft in the absence of trajectory correction devices. To prevent this, the zero-zero ejection seats have rockets inclined sideways at an angle for trajectory deviation. For e.g Martin Baker Mk 10 seat fitted on the Mirage 2000 aircraft deviates to the left. In case of a two seater, the seats diverge to the opposite sides. In more advanced seats like the K-36 D 3.5E seat fitted on the Su-30 MKI aircraft, the seats are gyrostabilized and have Trajectory Divergence Motor (TDM). The seat has two TDMs which fire in opposite direction along the lateral axis to correct the trajectory of the seat. These cater for all conditions of flight including inverted flight at low levels. Drogue Parachute / Seat Stabilization System The drogue parachute system is designed to stabilize the seat as it leaves the aircraft. The wind blast along with the rocket motor forces may result in toppling of the ejection seat as it comes into the free stream airflow. Prior to seat separation / main parachute deployment, it is critical for the seat to be stabilized flight failing the main parachute can candle (not deploy correctly). The drogue chute is position along the centre of gravity of the seat such that the seat aligns to the gravity vector once the chute is deployed. The deceleration from the drogue parachute also acts an input for the BTRU to function. In more advances seats like K-36 D 3.5E, the stabilization system comprises of right and left telescopic booms with stabilizing parachutes. The telescopic booms also contain cartridges which are automatically fired for rapid stabilization of the seat. Therefore the Su-30 MKI ejection seat even caters for an inverted ejection at low levels. Pilot Separation / Parachute deployment The pilot seat separation and parachute deployment is activated by the BTRU as explained earlier. In case of ejection over high mountainous terrain or at very low levels, the pilots are generally advised to operate the seat separation and main parachutes manually, since the BTRU is a mechanical device and takes finite amount of time. Pilot Survival Pack Survival after ejection can be crucial especially in difficult terrains and over water. Therefore Pilot Survival Pack (PSP) also known as Personal Survival Pack would suitably cater for these contingencies. The contents of survival pack cater for at least 48 hours. The PSP mainly comprises of high energy fibre diet cakes, water / water purification tablets, fire producing tablets, suitable knife, fishing lines, survival charts, Air to Ground code chart, heliometers, flares, medicines etc. In case flying over water, some stores are removed and replaced with automatically inflating dinghy. Modern Trends The Advanced Concept Ejection Seat (ACES II) is currently being used in many aircrafts of the USAF. ACES II has a zero-zero capability and has a gyro-controlled vernier rocket for providing pitch stabilization. In high speed ejection conditions additional stabilization is provided by a drogue parachute. To achieve minimum-distance recovery in low-speed ejections, the recovery parachute is deployed as the seat leaves the cockpit. At high speeds, the drogue parachute is deployed immediately, quickly decelerating the seat and crewmember to a suitable speed for recovery parachute deployment. The use of multiple recovery modes permits the functions and timing of the recovery subsystem to be selected for each mode allowing optimum performance throughout the escape envelope. The recovery parachute and the drogue parachute subsystems are entirely independent. In the low-speed mode, Mode l, deployment of the recovery parachute is initiated as the seat and the crewmember are emerging from the cockpit. Thus, the elapsed time from ejection initiation to parachute inflation is minimized for the critical low-speed, low-altitude ejection conditions. In the high-speed mode, Mode 2, the drogue parachute is needed to slow the seat and occupant prior to recovery parachute deployment. The drogue is not severed until after the recovery parachute has been deployed. Mode 3 is used for high altitude ejection allowing the seat to descend or decelerate into the Mode 2 parameters prior to Mode 2 recovery being initiated. Mode selection is performed by the recovery sequencer in conjunction with an environmental sensing subsystem which determines airspeed and altitude conditions independent from aircraft systems Conclusion Fighter flying is a complicated process full of associated dangers. Ejection seat comes as a boon to the fighter pilots who can now exploit their machinery without fear. With increased complexity of flying and more advanced technologies, the ejection seats are also undergoing metamorphosis. References M/s Dassault Aviation (1996). Operating Manual M-2000 Part 1 Flight Manual M/s Sukhoi Design Bureau (2003). Su-30 MKI Maintenance Manual BK 4. Ejection Seat and Canopy: General Description. M/s Sukhoi Design Bureau (2003). Su-30 MKI Maintenance Manual BK 4. Ejection Seat and Canopy Ejection Seats Retrieved from http:// en.wikipedia.org Read More