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Engineers working with NAVAIR's aging aircraft
integrated product team (AAIPT) are teaming with industry to develop
a new circuit-breaker technology to protect commercial and military
aircraft from the effects of aging wiring by 2002. This technology
will improve safety, reliability and readiness for all aircraft.
Chuck Singer, an electrical engineer in NAVAIR's
electrical-power-system division, is the "point man" for a program
to develop an arc-fault
circuit breaker to prevent aged or damaged electrical wires
from causing catastrophic problems.
"We currently use the same basic circuit protection Edison did in
1910," said Singer. "It works well and consistently for bolted
faults and overloads in a tough environment, but it isn't designed
for arcing faults." Thermal, or "bolted,"
faults occur when two wires touch each other solidly for a classic
short circuit.
Recent aviation mishaps focused investigators on the problems of
aging aircraft wiring, specifically hard-to-detect arcing faults.
The problem, according to Singer, is that current aircraft circuit
breakers, like those in residential use, are designed for bolted
faults. Arc-fault circuit breakers are designed to detect and
prevent electrical arcing caused by breaks in wire insulation before
it can lead to a fire or other catastrophe. These faults happen
where microscopic cracks, abrasions, or broken insulation occur in
old wire. They also exist when wire is improperly installed or
maintained.
"From July '95 to December
'97, the Navy had 64 in-flight electrical fires," Singer stated. "Of
those, 80 to 90 percent would have been prevented by arc-fault
circuit protection."
Modern civilian and military aircraft can contain hundreds of miles
of wire, much of it inaccessible once the aircraft is completely
assembled. This makes repair of wire damage difficult, if it's even
detected at all, according to Capt. Jim Shaw (head of the Airline
Pilots' Association's [ALPA] in-flight fire project team). "With
reduced thickness in the insulation of the modern wires, we made
them more susceptible to damage by environmental and mechanical
means," Shaw said. "So what do you do, rewire a whole aircraft?
"That is, in some cases, too expensive," Shaw explained. "But if you
could stop the arc from starting a fire, or taking out other wires
in the bundle, then the need to replace those wires becomes moot."
"The Navy does about 1,200 power-wire removals a year, each costing
about $2,000," explained Singer. "And if the damaged wire is located
in a hard-to-reach place, it's even more expensive. Wiring
inspections are only as good as how many places you can get in to
see," said Singer. "You can look in wheel wells, flap wells, pull
some panels and see some wiring, but not all of it."
The Department of the Navy awarded two contracts in December 1999,
one each to the Eaton Corporation and Hendry Telephone Company (both
experienced in producing industrial arc-fault circuit protection),
but the process is still in the research and development phase.
"There are technical challenges to the arc-fault circuit breaker,"
explained Shaw. "The technology is sound, and a workable device for
aircraft use is doable."
"The hardest part will be the size," he added. "Fighters and some
commercial aircraft, where space is at a premium, are going to be
more difficult. Residential-arc-fault circuit breakers must be
reduced in size by at least 50 percent to be used in aircraft,"
Singer explained.
Reducing the size on the electronic side isn't the biggest
challenge; it's making the
mechanical part of the breaker smaller. Leverage actually "pops"
open the circuit once a fault has been detected by the electronic
side. Miniaturizing that leverage reduces the force available to
overcome the electromagnetic force generated by the current that
keeps the breaker closed and the circuit complete.
Another challenge, according to Bob Ernst, head of the AAIPT, is
getting the arc-fault breaker to tell the difference between a real
fault in the circuit and transient-electrical signals that pose no
threat. It is not a simple task to take technology developed for
residential use, where the electrical signals of common devices
aren't so varied or complicated, and re-engineer it for use in
aircraft. This is necessary because transient electronic
"signatures" are more varied and complex today. "For example," Ernst
said, "if your wingman turns on his radar or makes a radio
transmission, you don't want all your circuit breakers to pop
because they interpreted those signals as arc faults.
"This isn't simple," he continued. "That's why it isn't getting done
overnight. The Navy, FAA, Air Force, ALPA, NASA, and industry are
working together to develop a common specification. It's better to
spend a little extra time up front to make sure we get it right.
"If the breaker is always popping for false alarms because we didn't
get the programming right," said Ernst, "then maintainers out in the
field are going to pull them, set them aside, and put back the old
breakers. Then what will we have accomplished? You can't just throw
something out there with a high false-alarm rate. We know the
urgency of the issue, and we are all working as fast as we can, but
we have to get it right," Ernst added.
Singer thinks flying a prototype breaker will happen soon, though.
"We should be flying a prototype breaker in October 2001, in the
Navy equivalent of a DC-9 and start to buy them in 2002," he stated.
Why is the military working jointly with industry and the airlines?
"The problems in the civilian fleet and the military fleet are both
similar and different," explained Shaw. "The civilian fleet has more
cycles and hours in a shorter period of time, and the military fleet
operates in a harsher environment. Their loading shock is much
greater. So, each will see earlier failures of different types and,
therefore, can learn from each other."
The participating agencies eagerly anticipate the new circuit
breaker. "It answers a lot of serious problems we have with wiring,"
stated Shaw. "The Navy, in particular, Dick Healing's aircraft
wiring and inert gas generator working group, has been instrumental.
Without them, this may not have gotten started for years."
The effort to develop the aircraft arc-fault circuit breaker is
being jointly funded by NAVAIR, the Office of Naval Research, and
the Federal Aviation Administration. Additional funding is being
contributed by industry.
John C. Milliman is the public affairs coordinator for NAVAIR's
AAIPT.
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