Programmable circuit protection

Engineers at the Boeing [BA] "Skunk Works" are quietly developing an arc fault programmable solid state circuit breaker. Many in the industry believe that arc fault circuit protection devices are the answer to wiring woes in airliners and the potential they pose for in-flight fire and system failures. Basically, an arc fault circuit breaker, similar to the kind found in residences throughout the country, can cut or interrupt a spike in the current before it develops into the lightning bolt of a full- blown arc. One of the major challenges is to shrink the cigarette pack-size arc fault breakers designed for home use into a reliable package about a tenth that size for aircraft use. The Boeing effort relates to the wiring safety and separation issues recently outlined by Jim Shaw to the Aging Transport Systems Rulemaking Advisory Committee, or ATSRAC (see ASW, May 13).

According to sources, Boeing engineers are far from an arc fault breaker suitable for installation in aircraft, but the device they have developed has demonstrated that it can be programmed to trip at varying overcurrent levels, up to 20 amps AC and DC. The Boeing solution features a so-called "time domain reflectometer" (TDR) that inhibits power on a circuit with a potential short. If an arc occurs on the wire, the TDR kicks in and records the point at which it occurred. The approach may be unique in this respect: when the breaker is installed, a "footprint" of the integrity of the wire is taken. This stored value is used as a basis for comparison. Each time the circuit is powered up, the TDR determines if conditions have changed. If there is an anomaly in the waveform, the circuit will not activate, or "power on."

The breaker's TDR function only looks at the power circuit in which the breaker is installed, but if the devices were installed in all new aircraft, they could serve as built- in test equipment (BITE) to check the integrity of all wiring. Anomalies could be further validated or checked by a ground support tester that would look at the flags set by such a test. The flags would alert maintenance personnel on where to look for wire problems.

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No rush - 4,000 hours

A spate of recent airworthiness directives (ADs) affecting both Boeing [BA] and Airbus aircraft relate directly to the problem associated with insufficient wiring separation standards outlined recently by Jim Shaw, head of the In-Flight Fire Team for the Air Line Pilots Association (ALPA). Shaw suggested the dielectric equivalent of two inches of air as a minimum separation standard (see ASW, May 13). The ADs have an effective date of June 13, 2002, and allow operators 4,000 flight hours to fix the problems (see table, below). That compliance window equates to about 11 to 17 months at average aircraft utilization rates. This amount of time may be noteworthy, in that the special federal aviation regulation (SFAR) issued May, 7, 2001, by the Federal Aviation Administration was brought out with a certain sense of urgency to "fix" ignition sources in all airliner fuel tanks (see ASW, May 14, 2001). The latest ADs address actual flaws found - yet grant liberal periods in which to fix them.

Prevent Explosions of Fuel Vapors
AD Number Aircraft Affected Nature of Hazard
AD/AB3/190, Fuel System Wiring (Australian Civil Aviation Safety Authority designation). DGAC (Direction G�n�rale de l'Aviation Civile) AD 2002-172(B) refers A300 and A310 Segregation rules applied between fuel gaging system electrical power supply 'route S' and 'route M' not satisfactory. Short circuit in 'route S' could cause significant overheating and possible ignition of fuel vapor in the tank. AD requires 'route 1S' to be insulated from the fuel system.
AD/AB3/191, Fuel Quantity Indicating System Wiring. DGAC AD 2002-171(B) refers All A300-600 airplanes Sensor can overheat if 115V AC wiring short circuits. Overheating could ignite fuel vapor. AD requires separation of the 115V AC cabling from the sensor in the right-hand wing leading edge.
AD/AB3/192, Fuel Quantity Indication System Probes A300 and A310 Insufficient space between fuel quantity indication (FQI) probes and airplane primary structure. When associated with lightning strike, situation could allow electrical arcing in the fuel tank resulting in possible ignition of fuel vapor.
AD/A320/137, Fuel Probes and Fuel Tank Level Sensors A319, A320 and A321 series airplanes A 28V DC short circuit, external to the fuel tank, to the fuel quantity indication wiring that enters the tank could lead to overheating of the problems and consequent ignition of fuel vapors within the tank.
AD/B737/158 B737-100 through B737-500 series airplanes Requires installation of components to provide shielding and separation of fuel system wiring (routed to the fuel tanks) from adjacent wiring.

Point of minor correction...the Skunk Works is part of Lockheed Martin, not Boeing. I have no idea which one the author of the article wanted to refer to!

David you are right. According to someone in the know, it should read, "the Boeing "Phantom Works".

http://www.boeing.com/defense-space/military/phantom/phantom.htm