|
Safety in Avionics:
Breaking the Electrical
Arcing Cycle
By David Evans
Imagine a cockpit without the phalanxes of
button-type circuit breakers. The breakers have
been distributed to where they are needed
throughout the aircraft. In fact, the breakers
are of a new arc fault programmable circuit
protection technology that can prevent dangerous
electrical arcs at the first hint of a power
spike.
Known as solid state power controller (SSPC)
technology, the new approach offers a three-way
punch: increased safety, less weight from wiring
and reduced maintenance. This is the promising
judgment of engineers at the Boeing Phantom
Works in Huntsville, Ala. They are developing
new circuit protection technology that could
revolutionize the design of aircraft electrical
systems and alter the appearance of the cockpit.
Phantom Works engineers envision a technology
that would place the circuit protection devices
throughout the aircraft.
"The breakers are located where they’re
needed," says Tom Jobes, an
electronics-packaging engineer at the Phantom
Works. In a recent interview, Jobes explained
the impact of these power controllers on the
pilots. (For purposes of this discussion, the
power controller technology includes circuit
protection, and the terms will be used
interchangeably.)
In the concept known as "distributed power
architecture," the breakers would be transferred
out of the cockpit, while control of the devices
would remain at the flight crew’s fingertips.
During flight, should the SSPC sense an imminent
arcing event, the flow of power on the affected
circuit would be cut before developing into the
lightning bolt of a full-blown arcing event.
"The pilot will get a message on the [screen]
that [a circuit breaker is] tripping. The
message will tell him it’s an arc event," Jobes
explains. "On a touch screen, the pilot will
have the ability to reset the device a certain
amount of times." On a flight critical system,
the SSPC might be programmed to allow just one
reset in flight.
In this regard, it is useful to refer to a
critical tidbit contained in the documents
released by the National Transportation Safety
Board (NTSB) as part of its still-ongoing
investigation of the fatal flight Jan. 30, 2001,
of Alaska Airlines Flight 261. The crew lost
control of the horizontal stabilizer; actually,
it flipped up like a whale’s tail just before a
dive and then broke off. The plane tumbled
helplessly into the Pacific waters off Los
Angeles. Buried in the reams of documents lies
what may be a significant contributing factor:
the pilots had reset the breakers for the
electric motors driving the stabilizer jackscrew
some eight or nine times. Had the Flight 261
pilots reset the breakers to the balky pitch
trim system just once or twice, they might have
been able to retain sufficient pitch-trim
control to execute an emergency landing.
The computerized arc fault protection devices
also would serve a new function before flight.
"My vision for the future is the pilot gets a
go/no-go for the wiring in the aircraft and gets
a diagnostic on a small screen," Jobes says. The
preflight checks of wiring integrity might not
attend to all circuits but would apply as a
minimum to flight critical circuits.
For example, the Aging Transport Systems
Rulemaking Advisory Committee (ATSRAC) is
calling for enhanced zonal inspections of
aircraft wiring. It recommends focus on cockpit
wiring, wiring in the electronics and equipment
(EE) bay, and power feeder cables. These three
areas do not include all flight critical
circuits, such as those to engines and flight
control surfaces, but the new circuit protection
devices could be installed per specification to
cover the 100 or more miles of wiring in a
modern jet.
The preflight check would exploit the time
domain reflectometry (TDR) that is integral to
the Boeing SSPC design. Should the TDR detect a
suspected short circuit, power would not be
applied. If an arc occurs on the wire, the TDR
records the point at which it occurred.
These power controllers could serve the
functional equivalent of built-in test equipment
(BITE) to check wiring integrity. Anomalies
could be further validated or checked by a
ground support test device.
The circuit protection project has its roots
in Boeing’s role of developing SSPCs for the
International Space Station. The controllers,
Jobes explains, "reduce 120-volt DC power to
28-volt DC power and distribute it to
experiments, which are arrayed in racks."
About a year ago, he says, "we started to
hear a lot about arc fault circuit protection
and of the need for this kind of protection in
commercial aircraft. We looked at the time
domain reflectometry to help locate the source
of any wire damage."
The goal is to license the technology for
production. "We see its application in new
aircraft designs, and in aging aircraft to
replace existing circuit breakers," Jobes says.
For new aircraft, electrical power
requirements are increasing. In the "electric
jets" of today and tomorrow, more functions are
being turned over to computers and
electromechanical devices. Eliminating hydraulic
systems—another trend—introduces the need to
supply, control and protect high-power
electromechanical actuators. In the cabin,
passenger entertainment systems are being
integrated into aircraft seats; this trend means
more power cabling placed close to passengers.
Both inside the cabin and throughout the
aircraft, more electrical system safety is
needed. That’s where the new power controller
technology also could play a role. Simply put,
demand is up—electrical system safety must keep
pace.
Furthermore, Boeing’s programmable circuit
protection technology could reduce the amount of
wiring in the aircraft. With the circuit
protection devices located closer to where they
would be needed, "you can shorten the cable
runs," says John Maxwell, a Phantom Works
engineer. The wiring would not have to be run
100 feet (30 meters) or so to breakers located
in the cockpit. Rather, the distributed wire
lengths would be closer to 20 to 25 feet (6 to
7.6 meters). "We see a 10-percent reduction in
the weight of wiring," Maxwell maintains.
The maintenance benefits might be even more
significant. The ability to locate the damage on
a wire to within 2 to 3 feet (0.6 to 0.9
meters), would simplify hunting down the source
of damage. And only the single damaged wire,
rather than a dozen or more wires, would have to
be replaced.
"Whoever gets there first with this
technology is going to save the maintenance
folks a ton of time," Jobes asserts. He
envisions repair times being cut by about 50
percent.
Since the breakers would be located closer to
where they would be needed, relays could be
eliminated. The new circuit protection device
would function as both a switch and a breaker.
"In a galley, when used in power management and
distribution systems, these devices would
automatically balance the loads among the ovens,
the coffee pots and so forth," says Jobes.
The new technology also can be applied to old
jets. "The wiring in those aircraft is getting
old, it’s easy to damage, and we are having too
many arcing events," Jobes says.
There is no question that wiring degrades
over time. For example, Eric Petersen and David
Veecks at General Dynamics’ Airborne Electronic
Systems Division use the term "salt bridges" in
a new paper to describe the degradation:
"Conductive salt bridges are a result of the
constant wetting and re-wetting of the harness
environment. Ionically contaminated condensation
water is an electrolyte that conducts current
across any gap where arcing may occur. Current
produces heat, and the heat evaporates the water
each time, leaving behind molecular islands of
salt that eventually form a kind of archipelago
of larger conductive islands. This is a
repetitive process and leads to an eventual
breakdown whose potential is proportional to the
sum of the distance between conductive islands."
For old aircraft the new power
control/circuit protection devices would not
necessarily by installed as one-for-one
replacements of the circuit breakers currently
installed. Rather, an entire panel of breakers
would be replaced with the new solid state
technology. Jobes explains that the lugs on the
backside of the panel "would be in the same
location, so the aircraft wiring would not have
to be rerouted."
Beyond efficiencies in the amount of wire and
in wiring maintenance, the biggest gain is seen
in safety. The circuit protection devices offer
quicker response to an arcing event. Numerous
cases exist where dangerous arcing events have
occurred on legacy aircraft and the breakers did
not trip. The arcing damage occurred before the
heat built up sufficiently to trip the more
conventional mechanical breakers, which are
thermally activated.
Challenges remain to developing the
technology. Reducing the size of the devices is
one hurdle. Electronics create heat, and in
shrinking the size, "you have to get the heat
off," Jobes explains.
"We also need to test more against different
[electrical] loads and different types of arcs,"
he says. These technical issues will be
addressed in coming months.
How ready is the technology for deployment?
On a scale of zero to 10, "we’re at about a
five," says Jobes—about halfway to production
and deployment. He is nonetheless confident that
the new technology "will be on the market by
2005." Meanwhile, those conductive salt bridges
keep building up. |
