Friendship Flying Service Beech 58 accident

After takeoff, the flight proceeded toward the destination airport and air traffic control communications were transferred between several facilities. While flying at 9,000 feet, the pilot attempted to establish contact with Knoxville Approach Control, but the controller did not reply. About 28 seconds later, the pilot broadcast on the same frequency that he needed to declare an emergency because of a fire but did not specify the location or extent of the fire. There were no further recorded radio transmissions from the pilot, and his declaration of a fire was the first and only transmission describing any abnormal event. Primary radar returns for the airplane depicted a turn to the northeast then north after which the returns were no longer noted. The airplane crashed in a wooded area located about 61 degrees and 3,140 feet from the last primary radar return, using straight line distance. No determination could be made regarding the airplane’s flightpath between the last primary radar return and the accident site; however, witnesses described seeing the airplane turn sharply to the right then hearing an explosion. Structural components consisting of the outer portion of the left wing, right wing outboard of the engine nacelle, vertical stabilizer, rudder, and both horizontal stabilizers were separated from the main wreckage, the farthest of which was located about 400 feet away. Examination of the fracture surfaces of the separated structural pieces revealed no evidence of preimpact failure or malfunction. The cockpit, cabin, instrument panel, nose compartment, empennage, and inboard sections of both wings were nearly consumed by the postcrash fire. The right wing’s upper skin between stations 105 and 115 exhibited fire damage but the lower wing skin just inboard and aft of the structurally separated area did not exhibit any fire damage. This indicates that the fire damage to the upper skin occurred after the lower skin piece separated.

Examination of a recovered portion of the instrument panel glare shield revealed thermal damage concentrated around two openings on the right side of the glare shield’s surface. The bottom surface of the glare shield had a distinct area of thermal damage; the damage extended from the forward most area on the right side all the way aft. The severity of the thermal damage was more pronounced in the middle of this area.

Photographs of the combustion heater revealed a lack of extensive fire damage and insulation that still covered the conductor on several electrical wires. Further, the surrounding area of the airframe near the heater (for example, the nose landing gear door) did not exhibit extensive fire damage. Therefore, the combustion heater is an unlikely source of the pilot-reported in-flight fire, even though, according to the airplane’s maintenance records, the combustion heater was beyond the recommended overhaul interval specified by the heater manufacturer.

The airplane’s maintenance records indicated compliance with a 2008 airworthiness directive (AD) requiring replacement of the circuit breaker toggle switches with switches of an improved design. The AD resulted from reports of overheating of certain circuit breaker toggle switches used in various electrical systems throughout the affected airplanes. Although a portion of the circuit breaker panel from the accident airplane was severely damaged by fire, it did not reveal any signatures consistent with electrical arcing or any other electrical anomaly.

The time interval between the pilot’s declaration of an emergency and the last radar return from the airplane was a little less than 1 minute, suggesting that the fire grew quickly without much of an incipient stage. These characteristics are consistent with a fuel-fed fire. Based on localized fire damage to the lower surface of the glare shield, the fire most likely occurred beneath the right side of the instrumental panel, an area that is near the direct-read oil pressure gauges. The exact source of the fire could not be determined because extensive fire damage precluded distinguishing between damage incurred in flight and postcrash.

Verbatim NTSB analysis and probable cause from the NTSB dataset