Recommended But Not Required
From Advisory Circular (AC) 25-9A, Smoke Detection, Penetration and Evacuation Tests and Related Flight Manual Emergency Procedures, Jan. 6, 1994 (extracts):

Continuous smoke in the cockpit: Although the FAR [Federal Aviation Regulations] does not require the consideration of continuous smoke generation/evacuation, the FAA recommends that the airframe design address this situation.
On-board smoke sources: Fires in inaccessible areas should be assumed to be continuous, i.e., capable of continuously generating products of combustion until it can be visually verified that the fire has been extinguished. (ASW note: This statement embodies the lesson of Swissair 111.)

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New Fault Interrupter Could Prevent Dangerous Electrical Arcing

An electrical system fault interrupter in development promises not only to forestall dangerous arcing events but also could provide diagnostic data of declining component reliability. With such warning of degraded performance, the high cost of unscheduled maintenance might be reduced significantly.

The device, known as a universal fault interrupter (UFI), could provide a greater enhancement to aircraft wiring and electrical system safety than arc fault circuit interrupter (AFCI) technology now in development. AFCIs are envisioned as replacements for thermally tripped circuit breakers (CBs) now commonly installed in cockpits. AFCI technology now in development is seen as the great hope for wiring system safety, as it acts more quickly than standard CBs to prevent dangerous arcing events (see ASW, Sept. 16, 2002). However, one of the great challenges in adapting AFCI technology to aircraft is shrinking the device sufficiently to fit in current CB panels, a necessity to provide a one-for-one replacement.

However, breakthrough UFI technology may be available sooner and would provide a step-function increase in electrical system safety. A member of the National Transportation Safety Board (NTSB) familiar with the UFI approach said the "cat is creeping out of the bag" regarding the great promise it offers.

"It has AFCI, GFCI [ground fault circuit interruption], conductive path and power monitoring," the NTSB official said. "Unplug an existing relay, install this device and plug the relay back in. You're done. Cheaper than the AFCI breaker, faster [fault] detection ... and closer to the electrical load."

His brief summary neatly captures the promise embodied in the UFI approach. The product presently is in development by TDG Aerospace, a San Diego, Calif.-based avionics company. "We have a lot of interest from the airlines," said Jerry Bench, chief executive officer of TDG. For one thing, flight delays, cancellations and unscheduled maintenance caused by unexpected electrical problems are costly. For another, consider the continuing flow of airworthiness directives (ADs) coming from the Federal Aviation Admin-istration (FAA), as the agency strives to ensure electrical system safety. Of 16 recent regulatory actions, nine dealt with aircraft electrical systems - notably wiring and connectors - in which the dominant terms were chafing, overheating, arcing, smoke and fire (see ASW, March 15). These regulatory initiatives typically result from reports of failures in the field; in other words, after problems have been reported. The UFI approach can shift the direction of effort from reactive (complying with an AD) to proactive (taking corrective action before outright failure occurs).

Bench explained that his company's first experience with electrical system safety dealt with MD-80 series aircraft, where ice could accumulate on the wing from the cold-soaked fuel in the wing tank. The company developed the NOFOD(tm) over-wing heater blanket to solve the problem. For the heater controls, wiring harnesses and other devices, an electronic protection device (EPD) was included in the installation..

After that experience, Bench recalled, "We decided to look at more advanced technology" for circuit protection. The goal was to provide enhanced protection unit-by-unit, for such things as fuel boost pumps, galley ovens, auxiliary hydraulic pumps and such (all of which, it should be noted, have been the subject of recent ADs).

Thus was born the UFI concept, to constantly monitor a circuit and to instantly cut power should a fault be detected, thereby preventing dangerous electrical arcing before it occurs. By shortstopping arcing before it occurs, there is no heat buildup and the potential for fire and/or truly serious damage to the aircraft can be avoided. "The time from fault detection to cutting the power to the circuit is on the order of a few microseconds, so there really isn't time for any heat buildup," Bench said.

Dave Lyle, a TDG engineer involved in the UFI development project, explained that the device is inserted between the relay socket and the relay. The UFIs will be installed in the electronics and equipment (E&E) bay, avoiding the challenge of having to fit in circuit breaker panels located in the cockpit.

In layman's terms, he explained how the UFI functions when connected to protect a fuel boost pump. "We track the pump when it starts and record the power," Lyle said. "So we capture a normal limit."

"If there is an exceedance, the UFI will not allow the pump to start," he said.

Perhaps of greater interest, the UFI contains "fair size memory" with a downloadable data capability for diagnostics and prognostics. Thus, Bench said, "We can track pump wear." With this tracking capability, component repair or replacement can be more accurately scheduled. Not just for fuel pumps, but for any device to which the UFI is connected.

As Bench explained, one of the worst situations for an airline is an airplane sitting on the ramp with 130 passengers aboard, unable to depart because of an electrical fault. The technology can also help to avoid unscheduled landings caused by smoke/fire events in flight.

Fuel boost pumps for the Boeing [BA] B737 are seen as one of the first applications of UFI technology. This focus stems from the high concern about fuel system safety and the fact that center wing tanks (CWTs) exploded on B737s in 1990 (in Manila) and 2001 (in Bangkok).

Availability for installation in B737 CWTs may be in the not-too-distant future. "The FAA wants to see this on an airplane," Bench said. Supplemental type certification (STC) is expected for the B737 CWT boost pump application in about 90 days. The goal is to provide operators with a means of complying with SFAR 88, the special federal aviation regulation mandating increased fuel system safety by minimizing ignition sources (see ASW, May 14, 2001). Fuel pumps and fuel quantity indication systems (FQIS) feature potential ignition sources and are top candidates for UFI technology.

Reluctant to discuss price at this stage, Bench nonetheless assured UFI technology would be "very economical." The real payback, he said, will be in more efficient preventive maintenance.

For more information on UFI technology, see http://www.tdgaerospace.com/prod_sol_fault_prevention.html

Overheated Wire Bundle
"At FL [flight level] 330 ... we had a foul, noxious odor in the cockpit ... The flight attendants also had a burning sulfur-like odor in the cabin. They quickly checked to be sure that the odor was not coming from the [galley] ovens or from someone smoking in the lavatories.

"The odor was overpowering, so the decision was made to divert ... The time from the first smell to touchdown was approximately 12 minutes ... After landing ... the smell was getting stronger so I gave the order to evacuate.

"(The fumes came from an overheated wire bundle that supplied current to the L2 cockpit window for in-flight heating. Apparently the circuit breakers had not opened to cut the current from the errant power source.)"

Source: Feb. 2004 Callback, the monthly bulletin of the National Aeronautics and Space Administration aviation safety reporting system (ASRS). NASA administers ASRS for the FAA.

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