Accident Details
Approximately 13 minutes after departure for a medical transport flight, while climbing through about 14,900 feet mean sea level (msl), the pilot informed air traffic control (ATC) of a smoke smell in the cockpit and stated an intention to return to the originating airport. ATC cleared the flight to return with a descent at pilot's discretion to 9,000 feet msl. The pilot acknowledged and mentioned that he expected to lose some power shortly. Subsequently, the pilot declared an emergency, reported smoke in the cockpit, and requested ATC contact the fire department. The last radio transmission, made about 1 minute and 15 seconds after the initial smoke report, indicated three people on board.
Wreckage and Examination
The wreckage was located approximately 9 hours later in an area of brush and heavily forested terrain. Burned and fragmented wreckage was scattered along a debris path about 2,400 feet in length, consistent with an inflight breakup. The center fuselage and cockpit areas were largely intact and showed no evidence of fire; however, thermal damage was found on the forward fuselage consistent with an inflight fire. This damage was primarily limited to the floor area between the two forward seats near the main bus tie circuit breaker panel, extending to the forward edge of the wing spar. All exposed surfaces were heavily sooted. Localized melting and thermal-related tearing of aluminum structure were present. The primer paint on floor panels under the right aft corner of the pilot seat and the left aft corner of the co-pilot seat was discolored dark brown. An aluminum stringer in this location exhibited broomstrawing, indicating heating near its melting point prior to impact.
Wiring and Components
A single wire in the area showed notching consistent with mechanical rubbing. The main bus tie circuit breakers were partially missing; remaining breakers were heavily sooted on their aft ends, with one breaker thermally discolored. Areas of charring were on the backside of the panel. Examination of wiring in this area revealed electrical arcing damage. Four hydraulic lines servicing the landing gear system were located in this area; all exhibited signs of thermal exposure, with melting and missing sections of material.
Examination of Exemplar Aircraft
Six exemplar airplanes of the same make and model as the accident airplane were examined. On all six, instances of unsafe conditions were found where electrical lines and hydraulic lines in the area of the main bus tie circuit breaker panel were in direct contact. Some wires in the exemplar airplanes showed chafing between hydraulic lines and electrical wires, which, if left uncorrected, could have led to electrical arcing and subsequent fire.
Conclusion
Based on the unsafe conditions found during examination of the exemplar airplanes and the thermal damage to the area near the main bus tie circuit breaker panel on the accident airplane—including broomstrawing of aluminum structure, electrical arcing damage to wiring, and melting of hydraulic lines—it is likely that an electrical wire near the tie bus circuit breakers chafed on a hydraulic line and/or airplane structure, resulting in arcing and a subsequent in-flight fire that was fed by hydraulic fluid.