ROTA, Spain -- The AV-8B Harrier II+ is by all standards a highly sophisticated machine, and so are its individual systems. The aviation egress system, commonly known as the ejection seat, is certainly no exception. Only the people who maintain them, the aviation equipment safety mechanics, or "seat mechs", as they call themselves, exceed its degree of precision and efficiency.
These Marines are responsible for maintaining the aircraft's oxygen, environmental control, and cabin pressurization systems in addition to the seats themselves, according to Staff Sgt. James Anderson, chief aviation equipment safety mechanic for the 24th Marine Expeditionary Unit (Special Operations Capable). The environmental control system, which runs the heating and air conditioning in the cockpit, works like a car's cooling system, he said. The cold air turbine spins and compresses the air, which goes through a heat exchanger, which will alter the temperature depending upon the desired setting. The aircraft's main engine will "bleed" air, letting it pass over the heat exchanger and into the cockpit. The same basic process fuels the oxygen system and maintains cabin pressure.
One of the many things that sets the Harrier apart from other aircraft is the design of its ejection system, beginning with the canopy. Unlike other planes, the Harrier's canopy does not open or jettison itself when the pilot pulls the ejection control handle -- instead, it explodes. "There's a detonation cord vacuum-sealed to the canopy," said Anderson. It blows the acrylic plexiglas to bits, allowing the pilot to exit the cockpit. Should that fail, the seat has "canopy piercers" on its top that will achieve the same effect.
New mechanics must learn the system and its various components, and how they function in the egress process overall. "It can be frustrating for the new guys - there's quite a bit to learn and remember," said Anderson. The MEU (SOC)'s seat mechanics agree. "You could work on these aircraft for twenty years and never learn everything about them. There are changes and updates every year," said Cpl. Clayton Judd, a seat mechanic from Springfield, Ill.
One of the fundamental tasks to master is an understanding of what happens upon egress.
If a pilot ejects in mode four, or a high-altitude mode, a complex sequence of events takes place in a matter of seconds, beginning with the destruction of the canopy. "Ballistic gas goes through the system," said Anderson, starting the process. Catapult tubes, driven by an explosive device that resembles a stick of dynamite, raise the seat 20 to 22 inches off the floor of the cockpit. Then the seat's two-foot long rocket motors fire, launching the pilot roughly 150 feet from the aircraft. Since the pilot may be injured or unconscious, a drogue chute, tether line and a WORD rocket (wind oriented rapid deployment) helps stabilize the seat and slow it down, while leg restraints automatically retract to keep the pilot stable and sitting upright.
The pilot and the seat will fall to an altitude of 14,000 feet before the main parachute deploys. According to Anderson, this is critical, because if a pilot ejects at a significant altitude, like 30,000 feet or more, he or she may drift for too long if the chute opens at that height. "The pilot only has about five minutes of emergency oxygen," he said. Sensors called "aneroids" detect when the seat reaches the appropriate altitude, and then the main chute is deployed by a spreader gun, which ensures its opening. At 7000 feet, the pilot separates from the seat and continues the descent.
Overall, the ejection system is 98 percent gas-activated and has few mechanical parts. This makes for a very reliable mechanism, but it still requires a fair amount of maintenance and daily inspections. Before every flight, "seat mechs" will inspect the canopy, detonation cord, the parachute and its risers, and recharge the emergency oxygen tanks. Every 448 days, they will conduct a total rebuild of the seat and all its components.
Anderson finds his job very rewarding, and has more than nine years experience in the field. He worked in a related field for two years due to a lack of boatspace in his military occupational specialty, and is thrilled to be back. " I stayed in touch with all my friends (in the community) to get back in. I'm glad to have this job," he said. The most rewarding aspect of the job is seeing the equipment function in a real emergency. "We don't wish to see our equipment tested, but we're glad when it does its job, saving a pilot's life."