Business & GA, Commercial

Safety in Avionics: Breaking the Electrical Arcing Cycle

By David Evans | November 1, 2002
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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.

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