Business & GA, Commercial

Safety: Water and Electrons: Never a Good Mix

By David Evans | May 1, 2006
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Water seeping into the electronics and equipment (E&E) bay--causing terminal contactors to short out--has led to seven nearly identical electrical fires aboard Bombardier CRJ-200 regional jets, six of them within the past six months.

The most recent fire, on an Atlantic Southeast Airlines jet descending into Atlanta on Dec. 14, 2005, is a typical example. According to the U.S. National Transportation Safety Board (NTSB) account, the fire broke out at the 1K4XD Utility Bus One contactor. The contactor is a switching device for three-phase, alternating current (AC) power coming from the two engine-driven generators.

NTSB says the crew declared an emergency when the fire broke out, but that confusion reigned. "The cockpit voice recorder (CVR) transcript revealed that the flight crew was confronted with a cascade of failure events, as electrical power from the left, and then right, generator was lost, the smell of electrical smoke entered the cockpit, all EFIS (electronic flight instrument system) displays were temporarily lost, the cabin temperature temporarily became hot, and the air-driven generator (ADG) deployed."

"The crew only had use of the single standby flight reference instrument when the surrounding EFIS displays began to flicker," NTSB's description of the event continues. "The CVR transcript indicated that the flight crew had difficulty prioritizing which checklists to use to respond to the numerous visual and aural alerts and warnings ... [and] post-incident inspection of the airplane found that the captain's oxygen supply line was blackened."

Due to a lack of smoke and fire warning systems in the avionics compartment, the pilots were unaware that the fire was burning literally beneath their feet. Fortunately, as in the six previous cases, the oxygen supply line was not breached--which would have accelerated the fire--and the pilots were able to land the aircraft. The postmortem conducted by NTSB revealed that the fires were caused by rain ingress through the open cabin door when the airplane was on the ground. The rainwater seeped down into the E&E bay and shorted out the terminal contactors.

There's more to the story. The scenario was set by the manufacturer of the contactors, Tyco Hartman, which decided that a new material, Ultem 2200, met Bombardier's specifications. The specs previously allowed a melamine resin laminate, known as G-9, as an insulator on the contactor's base. The use of G-9, however, was discontinued because of manufacturing difficulties and the risk of environmental damage from the release of fibers. Ultem 2200, a polyetherimide, was substituted, although, as NTSB notes, "Bombardier's design specification did not require that the contactor be capable of operating under full electrical load while exposed to moisture or that it be tested to demonstrate this capability."

NTSB tested both the G-9 woven glass fabric laminate and the Ultem 2200 material, dripping water onto a powered contactor. In one test of Ultem 2200, the water quickly evaporated due to the heat of the short circuit, but a residual carbon char formed a semiconductive path between the contactor's terminals. Intense flames and arcing extended more than a foot from the contactors, which were destroyed. The fire lasted more than a minute and a half before power was lost as the wiring burned away.

NTSB's tests showed that the G-9 evinced no problems when the contactor terminals were bridged with water. The water steamed off with "no material breakdown and no fire."

The Safety Board notes that Tyco Hartman was unaware of Ultem 2200's arc-tracking characteristics, which are very similar to those of aromatic polyimide wire insulation (Kapton). On U.S. Navy aircraft, which obviously operate in a high humidity and salt water environment, Kapton has been banned as a general-purpose wire because of its unfortunate arc-tracking characteristics.

Not surprisingly, NTSB wants a separation of electrical power sources, protection from moisture, and replacement of the Ultem 2200 material. The seven NTSB recommendations, four of them urgent, seek to supersede a December 2005 non-mandatory All Operators Message from Bombardier. That message suggested a wiring modification that would avoid loss of all electrics and permit operation of at least one pilot's EFIS screen--even though it would not preclude a fire.

Bombardier reported in late February 2006 that most operators had not carried out this modification, and the NTSB clearly sees urgent action as being critical to flight safety.

The problem caused by fluid leaking onto vital electrical components is not unique to the Bombardier CRJ-200. Recall the infamous AirTran smoker in November 2000. The pilots were barely able to land the DC-9 in time at Atlanta to avert catastrophe. As the NTSB recounted this event, "The greatest amount of fire damage was found just aft of the electrical disconnect panel ... There was no evidence that the drip shield normally installed over the disconnect panel was present at the time of the accident." In this case, the precipitating agent wasn't rainwater, but blue fluid from the toilets "on surfaces near the disconnect panel," the NTSB noted.

Although one airline operating the CRJ-200 has installed a metal drip shield to redirect the flow of water from cabin floor leaks, the latest NTSB recommendations only imply, but do not specifically recommend, such protection. Despite its being known that water can trigger events such as on the Atlantic Southeast jet, the CRJ-200 had no history of such problems, despite entrance-door water ingress in inclement weather. The problems started only when the insulating material was changed. The lesson is obvious: materials matter.

All aircraft types are subject to the same chain of events. That is, liquids--whether rain, lavatory fluids, coffee spilling onto the center console in the cockpit, or potable water--have at various times severely affected wiring and electronics, especially if engineering or maintenance (or common sense) does not provide the necessary protections. Drip shields and potting compounds to shield electrical components from fluids are essential. The Safety Board's recommendations make the CRJ-200 issue sound brand-new. But until the day water can mingle with electrons, it's old news in a different airframe.

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