What happened
On November 20, 2021, a Cessna 207A operating as a scheduled passenger flight departed near Bethel, Alaska, bound for Kwethluk. Shortly after takeoff, the pilot noticed the red emergency locator transmitter (ELT) light illuminate on the instrument panel. Although the Bethel tower reported no ELT signal, the pilot soon detected an electrical burning odor and decided to return to the airport.
As the aircraft approached Bethel, the smell intensified and smoke became visible in the cockpit, prompting the pilot to declare an emergency. The pilot turned off the master electrical switch and opened a side window for ventilation. During the landing roll on runway 1L, the pilot discovered that both the brake system and the nosewheel steering were inoperative.
After the aircraft came to a stop, the pilot ordered the five passengers to evacuate. Once the passengers had exited, heavy smoke filled the cabin, and the pilot observed a flame beneath the floorboards behind the seats. Shortly after the pilot evacuated, the aircraft N9794M was completely engulfed in flames. All occupants were not injured.
The investigation
NTSB investigators determined the fire originated behind the pilot's row of seats. The investigation focused on a wire harness belonging to the Capstone Program avionics suite, which had been installed under a Supplemental Type Certificate. This harness was found improperly installed directly on top of the aft fuel line from the left fuel tank.
The fuel line showed evidence of chafing and thermal damage, and a small pinhole was discovered in the line. Examination of the wire harness revealed thermal discoloration, sooting, and electrical beading, indicating the wires were energized during the event.
During the investigation, the NTSB examined a second Cessna 207 located in the operator's hangar. Investigators found that the same improper wire harness installation existed in the same location on the second aircraft.