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Analysis

Summary

The investigation was unable to determine with any certainty at what time during the flight the fire began. Indications are that the fire was probably not an in-flight fire. The damage to the surrounding structure of the wheel well appeared to indicate plastic deformation and some melting of the aluminium structure, and a probable maximum fire temperature of around 700 o C. The crew had reported good illumination from the landing lights for landing. This indicated that all lights were still operating, and that at that time the mechanical indicating fuse was intact. The pilot's actions in not selecting the standby boost pump to on, following the R FUEL PRESS LOW indication, may have inadvertently been a mitigating factor in the fire. The pump could have supplied the fire with extra fuel under pressure at a critical time.

Several years prior to the incident the right wing de-ice boots had been removed and the wing repaired as a result of hail damage. This repair entailed some disconnection and disturbance of the pneumatic lines and electrical wiring running through the forward area of the wing bay. It is possible that the missing landing light electrical wiring cable tie stand-off had been removed and not replaced at this time. The fact that the cable tie impressions on the pneumatic line fire sleeving were covered with soot from the fire, also suggests that the tie was not in place. The area immediately surrounding the landing light cable tie stand-off was also less severely heat affected than other areas in the zone where the remains of cable ties still existed. For example, in an adjacent area there was still considerable evidence of the cable tie that secured and positioned the standby pump electrical wiring. Had the landing light cable tie been in place at the time of the incident, some remains of it should still have been evident.

As there was no visible evidence of chafing between the landing light wiring and the fuel lines, it is possible that the electric arcing may have been the result of the plastic deformation and subsequent breakdown of the wire's ETFE insulation. This could have occurred while the wiring was in contact with the fuel lines, following the generation of excessive heat in the landing light wiring due to the excessive 'contact bounce' in the K11 relay.

The fuel ignition source may have initiated from one of several sources. For example: the electrical arcing between the electrical wiring and the fuel tubing, arcing of a fuel drenched electrical aircraft component, the main gear up position indicator switch, and/or possible static electricity generated by the fuel escaping from the damaged fuel lines. The fuel 'washing' marks against the upper panel suggests that the fire was not burning in that area. Further, the area immediately surrounding the motive flow line leak only exhibited evidence of heat damage and sooting.

The suitability of the ampere rating of the enclosed link, current limiting fuse was also discussed with the aircraft's manufacturer, due to the fact that the fuse delay had allowed the wiring to arc through the fuel lines. Following a review of the wiring system and the current limiter's rating, the aircraft's manufacturer decided that it was appropriate for the task.

The fire may have started following the holing of the engine supply line in the rear of the wing zone, and spread from there. There was evidence of a well-established fire in this area, with some of the ethylene-tetrafluoroethylene copolymer (ETFE) wiring insulation completely burnt away. It is also possible that the holed fuel lines allowed the fuel to run down onto the landing light gear up position indicator switch. This switch initiating the fire when it was momentarily powered during the landing gear extension cycle.

It is probable that the electrical arcing and fire occurred either immediately prior to, or just following landing. Had this fire been burning in flight it is likely that a more serious outcome would have resulted. The inability of the flightcrew to isolate the fuel leaks, together with the extreme heat of an inflight fire, could have resulted in the wing spar losing structural integrity, and a possible in-flight loss of the right wing.

 
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