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Aerospace Careers: Aircraft Escape Systems
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| F-16 ejection seat (ACES II) |
One of the most important safety features on a NASA research aircraft is also one of the most complex systems.
It is the aircraft escape system, more commonly called the ejection seat.
The job of an aircraft ejection seat is simple. When a crewmember must leave the aircraft in an emergency, it
is the ejection seat that lifts him or her safely out of and away from the aircraft. The ejection seat system
then deploys the parachute that will slowly return the crewmember to Earth.
Ejecting from an aircraft takes less than two seconds from the moment the crewmember
pulls the ejection handle until a parachute begins to unfurl overhead. For the
ejection seat to save a crewmember's life, nearly 500 parts in the seat's system
must function properly and quickly in sequence.
At the Dryden Flight Research Center, where more than two dozen ejection
seats are used in a fleet of NASA research and support aircraft, the essential
job of aircraft escape system maintenance is part of the mission of the
Life Support Branch.
Different Models, Common Sequences
The five types of ejection seats used in NASA aircraft flown at Dryden are made
by three different companies. All five types of seats function in a similar
manner. When the ejection handle is pulled, an explosive cartridge is fired
that triggers a chain of events, including activation of other explosive
devices. These devices release the cockpit canopy, position the crewmember
upright for a safe exit from the cockpit, move the seat up the rails and
out of the aircraft, separate the seat from the crewmember, and deploy a
small drogue parachute followed by the main parachute.
This sequence of events is engineered so precisely and occurs so fast that
all new aircraft escape systems are now rated as "zero zero" no
altitude and no movement. "Zero zero" means a crewmember could
eject from an aircraft which was sitting motionless on the taxiway and be
boosted high enough to safely parachute back to the ground.
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| Zero zero ejection seat test. |
The F-15's & F-16's
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| F-15 cockpit |
The ejection seats used in Dryden's F-15 and F-16 aircraft, including two
F- 16XL's, are called ACES II, the acronym for Aircraft Common Ejection
Seat, Model II. They were made by McDonnell Douglas. Although both types
of aircraft use the same basic ACES II system, the cockpits of the F-15's
and F-16's are different enough to require engineering variations between
the two seats. This requires different maintenance and technical orders
for each version and they are considered separate models.
ACES II seats are designed to deploy a five-foot ribbon-style drogue parachute
immediately after the ejection seat leaves the aircraft. The drogue helps
stabilize the fall of the crewmember and seat until reaching an altitude
of 15,000 ft, where the main parachute is released by an altitude sensor.
If the ejection is below 15,000 ft there is no delay in the release of the
main parachute.
The complete ACES II system weighs about 200 lb.
The F-18's
Dryden's F-18 aircraft use ejection seats made in England by Martin-Baker. Two models of the
Martin-Baker system are found in the F-18's: one is used in the single-seat F-18's and in the rear seat of the two-place
F-18's, the other is used in the front seat of the two-place aircraft.
The first parachute to deploy from a Martin-Baker system is a 22-in. drogue.
It is used to pull out a 5-ft drogue that stabilizes the crewmember and
seat until the main parachute is deployed at 13,000 ft by an altitude sensor.
If the ejection is below 13,000 ft there is no delay in the release of the
main parachute.
Fully equipped Martin-Baker seats weigh about 400 lb.
The SR-71's
Lockheed manufactured the ejection seats used in the two NASA SR-71 aircraft flown at Dryden. The system
is a modified version of the Lockheed C-2 ejection seat used in F-104's called the Lockheed ADP F-1.
The parachute used on the SR-71 seat is a back style chute. The sequence of events when the ejection process
is initiated is similar to the ACES II and the Martin-Baker systems. The aircraft, which can cruise at altitudes up
to 90,000 ft at speeds of more than 2000 mph, has a crew of two. Each person wears a full pressure suit and
helmet, much like a space suit. The pressure suits and helmets are required to provide the SR-71 flightcrew
protection from the possible loss of cockpit pressure. The cockpit is normally pressurized to a cabin
(cockpit) altitude of 35,000 ft. Should this pressure be lost the suit will protect the crewmember from the loss
of pressure.
When an ejection from an SR-71 takes place, regardless of altitude, a drogue parachute is instantly deployed
to stabilize the fall of the crewmember and seat. The main parachute is timed for release at 15,000 ft, the
same altitude at which the seat is automatically separated
from the crewmember. In ejections above
15,000 ft, crewmembers remain strapped to the seats until a moment before the
main parachute is released.
The completely equipped SR-71 ejection seat system weighs about 400 lb.
Maintenance
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| The chutes of Martin-Baker system. |
Maintenance on all models of ejection seats is controlled by technical orders,
some as much as three inches thick, with instructions and specifications
covering every part of the mechanical and pyrotechnic systems in each seat.
Precise checklists must also be followed to remove, disassemble, reassemble,
and reinstall any part of the system.
At Dryden, the job of overhauling an ejection seat requires two people; one who performs the work while
the other follows technical order instructions and monitors each step in the overhaul process. Positions can
be exchanged from task to task, or from system to system, because only experienced and knowledgeable
Life Support Branch technicians are authorized to work on aircraft escape systems.
Periodic inspection and overhaul for all ejection seats follow the same process, regardless of the model.
Once the seats are removed from the aircraft, explosive devices are disarmed and every explosive, pressure,
and mechanical system is disassembled and carefully checked. Parts that show signs of wear are replaced,
while others may be replaced because the manufacturer's time limitations have expired.
Components that function under explosive and gas pressure, tension or tension release, atmospheric pressure,
or have a timed-release mechanism are thoroughly tested before reassembly takes place. Devices that fire
drogue parachutes must be tested. Allowable tolerances for pull levers and handles are measured and dozens of
"O" rings that hold gas pressure are changed.
The ACES II seats use a gyroscopic stabilizer to help keep the seat from tumbling during ejection. The
stabilizer is also disassembled with the rest of the seat systems so that bearings and gear teeth can be checked and
cables inspected.
An ejection seat overhaul and reassembly requires approximately eight hours to complete. An overhaul can
take much more time if there is a higher-than-normal number of parts to be replaced.
ACES II ejection seats are removed from the aircraft and undergo the complete inspection and overhaul
cycle every three years, while the Martin-Baker units receive a complete inspection and overhaul every 448
calendar days, the equivalent of 14 months. SR-71 seats are removed from the aircraft every six months for
inspection and replacement of items according to the seat manufacturer's time-limit requirements.
NASA's B-52 launch aircraft is equipped with four ejection seats. They are maintained and inspected on
a regular schedule by the Air Force Life Support and Test Parachute unit at Edwards AFB.
Air Force life support personnel at Edwards also inspect and repack all parachutes
at Dryden, including those integrated into ejection seats.
Experience
Considerable training and experience are required before a person is qualified
to work on aircraft escape systems. The principle sources of this training and
experience are the military services and the ejection seat manufacturers.
A person already working in the life support field can be trained and qualified for aircraft escape system
maintenance, but it requires attending either formal military schools or schooling provided by the seat manufacturers.
The individual would then work at an apprentice level with experienced personnel until he or she reached
a level of competence to be certified.
As modifications and improvements are made to existing ejection seat systems, training courses are
conducted by the military services or by the manufacturers to keep personnel fully qualified.
Introduction of a completely new ejection seat by a manufacturer would also require an extensive
training effort.
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| SR-71 cockpit and pressure suit. |
Technicians repair ejection seat. |
Document Number: IS-2000-10-018-DFRC
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