New Technology Enhances Electrical System Safety

Air Safety Week, Sept 16, 2002

Dangerous arcing events can be intercepted before they occur

Arc fault programmable circuit breakers can increase the safety of aircraft electrical systems, while shaving the weight of wiring in the aircraft and reducing the maintenance effort involved in trouble-shooting wiring faults.

This is the promising judgment of engineers at the Boeing [BA] Phantom Works in Huntsville, Ala., who are developing new circuit protection technology that could revolutionize the design of aircraft electrical systems and could alter the appearance of the cockpit. Current cockpits feature banks of circuit breakers, characterized visibly by phalanxes of buttons on the overhead panel and in panels behind the pilots. Phantom Works engineers envision a technology that would place the circuit protection devices throughout the aircraft, closest to where they are needed. The concept is known as "distributed power architecture." The breakers would literally be transferred out of the cockpit while control of the devices would remain at the flight crew's fingertips.

"The breakers are located where they're needed," said Tom Jobes, an electronics-packaging engineer at the Phantom Works. In an interview last week, Jobes explained the impact of these high-tech circuit protection devices on the pilots.

During flight, should the solid-state device sense an imminent arcing event, it will cut the flow of power on the affected circuit. Similar to the mechanical arc fault circuit interrupters found in residences throughout the country, the new device can cut or interrupt the current before it develops into the lightning bolt of a full-blown arcing event.

"The pilot will get a message on the EICAS [engine indicating and crew alerting system] screen that it's tripping. The message will tell him it's an arc event," Jobes explained.

"On a touch screen, the pilot will have the ability to reset the device a certain amount of times," he added. On a flight critical system, the device might be programmed to allow just one reset in flight. For example, during the fatal January 30, 2001, flight of Alaska Airlines Flight 261, in which the crew lost control of the horizontal stabilizer, they had reset the breakers for the electric motors driving the jackscrew some eight or nine times, according to documents produced as part of the National Transportation Safety Board's ongoing investigation. 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 successfully execute an emergency landing.

Preflight wiring health check

The new 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 said. The preflight checks of wiring integrity might not be for 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, with primary focus on cockpit wiring, wiring in the electronics and equipment (E&E) bay, and power feeder cables (see ASW, July 15). These three areas of focus do not include all flight critical circuits, such as those to engines and flight control surfaces, but it is evident that the new circuit protection devices could be installed per specification to cover any amount of 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 design. The solid state power controller [SSPC] may prevent power from being applied to a circuit should the TDR detect a suspect short circuit. The SSPC would prevent turn-on in the face of a short circuit. If an arc occurs on the wire, the TDR kicks in and records the point at which it occurred.

The devices could serve the functional equivalent of built-in test equipment (BITE) to check the integrity of the wiring. Anomalies could be further validated or checked by a ground support test device.

The circuit protection project has its roots in the International Space Station. Jobes recalled Boeing's involvement in the design of SSPCs for the space station. "These boxes reduce 120-volt DC power to 28-volt DC power and distribute it to experiments, which are arrayed in racks," he said.

"We built these SSPCs for NASA [National Aeronautics and Space Administration], and then we started looking at how to take this space technology and advance it" to other applications, Jobes explained.

Closer to earth

About a year ago, he said, "We started to hear a lot about arc fault circuit protection" and 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," Jobes added.

The goal is to license the technology for production.

"We see its application in new aircraft, such as Boeing's Sonic Cruiser, and in aging aircraft to replace existing circuit breakers," Jobes said. "The wiring in those aircraft is getting old, it's easy to damage, and we are having too many arcing events," he said. There is no question that wiring degrades over time, and General Dynamics engineers are exploring new techniques to monitor wire integrity during maintenance. A paper by Eric Petersen and David Veecks at GD's Airborne Electronic Systems Division contains a vivid description of the effect of repetitive wetting of wire.