Avionics Etc.

Aerospace Materials--Never Immaterial

After seven almost identical electrical fires aboard Bombardier CRJ200 jets, six within the last six months, the National Transportation Safety Board (NTSB) suggests strongly that the Federal Aviation Administration (FAA) should mandate some corrective action. The fires were caused by rain ingress through the open main cabin door seeping down into the avionics compartment and shorting out terminal contactors. The outcome was usually a cascading system failure accompanied by multiple visual and aural alerts, with pilots forced to revert to a standby attitude indicator for control. Due to a lack of fire and smoke warnings in the avionics compartment, the pilots were unaware that a fire was burning literally beneath their feet. Neither did they have an emergency checklist that could deal with the situation. An oxygen line ran precariously through the area of the fire.

The scenario was set by the contactor's manufacturer deciding that a new material (Ultem2200) met Bombardier's specification and, unbeknown to Bombardier, starting to utilize it in the 1K4XD Utility Bus One contactor. The contactor is a switching device for the three-phase AC coming from the two engine-driven generators. The NTSB is concerned that, following a moisture induced short circuit, the new material arc-tracks by forming a carbon char, much as does Kapton wiring's outer insulation. The resulting failures caused both pilots' screen displays to blank because of a design deficiency. The NTSB wants a separation of electrical power sources, protection from moisture and replacement of the Ultem2200 material. The seven NTSB recommendations, four of them urgent, want to supersede a December 2005 non-mandatory All Operators Message. That suggested a wiring modification that would avoid loss of all electrics, deployment of the air-driven generator 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. The NTSB sees urgent action as being critical to flight safety.

In one NTSB test the water quickly evaporated due to the short circuit and 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. In one case the fire lasted for more than 100 seconds before power was lost as the wiring burnt away. The insulating material previously used (G-9 melamine resin laminate) was discontinued because of manufacturing difficulties and because the release of fibers might pose an environmental threat. NTSB tests showed that the G-9 evinced no problems when the contactor terminals were bridged with water. Even though the aim is to ultimately replace the Ultem2200, the short term fix must be to stop water from reaching the contactor and shorting across it. This could be done by installing a deflector shield and applying a sealant to the terminals themselves. The NTSB is also concerned that the Ultem2200 manufacturer was unaware of its arc-tracking characteristics, despite its similarity to Kapton, and expressed concern that it may be being used in other aircraft.

Similar to the fire triangle of fuel-oxygen-flame, explosive arc-tracking electrical fires only require a short circuit and a conductive surface in order to flare and propagate. Water and other fluids are common initiators for that short. In a November 29, 2000 incident an AirTran DC-9 barely made it back to Atlanta after a chorus of continuing circuit-breaker trips and heavy cabin smoke. Investigators found that maintenance had failed to reinstall the drip shield that prevented lavatory rinse fluid from waste tank overflows from flowing down over the electrical disconnect panel located at fuselage station 237 (the fire's location). Boeing issued a service letter to operators to stress the importance of properly sealing floor panels.

Despite it being known that water can trigger such events, the fact remains that the CRJ200 had no history of such problems, despite entrance-door water ingress in inclement weather. The problems started only when the contactor's insulating material was changed. The lesson is obvious. Materials matter.