20170309001989I

Pilot reported loading 5 passengers and closing cabin door, starting #1 engine normally, obtaining clearance for IFR flight from hio to boi, and starting #2 engine after a longer than normal battery charge recovery period. Shortly after # 2 engine start a very high amp rate began to display on RH generator ammeter, followed by warning lights for "h stab ice fail" and battery feeder fail warning light. Pilot reported hearing circuit breakers popping and began engine shut down. Ground personnel on ramp reported seeing smoke coming out of aft fuselage and ran to notify pilot of fire concern not visible to crew. Aircraft evacuated with no injuries to crew or passengers and fire did not continue after shutdown of electrical system. Follow up investigation found the mosfet contactors above battery in aft fuselage had caught fire and melted. Damage found to wiring near mosfet component rack, and large amount of soot visible on metal above the 4 mosfets installed. 2 of the 4 mosfet contactors appeared very damaged with molten metal found by one unit. RH elevator heater horn also severely damaged by overheating.

Pilots reported aircraft had flown the night before and that the engine nacelle heat had been turned on due to icing concerns on approach. Both crewmembers said they did not turn on the h stab deice heat system switch the night before this event, OR engage it during startup of engines the day of the fire.

However, a conflicting narrative of these events was provided to FAA by a person present at this fire event. The reporter said that the PIC told him immediately after the fire that the h stab deice system was turned on in flight the night before and turned off again after a fail warning light came on for the h stab deice heat system.

Additional narrative amendment 1:

Cockpit voice recorder download obtained and reviewed by FSDO on 4/21/17. CVR recording of conversations between pilot and copilot from flight to hio on March 8 and on morning of departure preparation on March 9 show clearly that h stab deice system was used on March 8 and failed in flight. Narrative shows that both pilots discussed the failure warning light for the h stab deice system that came on the night before, and did not realize that the electrical system faults displaying on March 9 prior to taxi were caused by h stab deice system failure.

During several minutes of troubleshooting discussion CVR recording shows battery and generator issues were suspected of causing failure warning lights and other anomalies, until the entire electrical system went off line due to massive electrical short in mosfet units.

Discussion with pilots after event found that neither of them knew about or had followed the AFM emergency checklist for h stab ice fail light indication, and that the circuit breaker for the h stab deice system control was not pulled before shutting off the switch as required by the AFM checklist. Pilot action of shutting off the switch without first pulling the h stab deice control c/b after a warning fail light came on set the conditions which could lead to a mosfet fire.

The electrical fire on March 9 appears to have occurred as a result of a failed system which was not shut down by the crew per the AFM emergency checklist procedures after a failure warning light illuminated in-flight the night before the fire.

Attempting to dispatch the aircraft the next day with inoperative h stab deice system created conditions leading to mosfet fire in aft fuselage due to known defects in the mosfet units of this part number series.