N9794MCessna 207A · 1981

On November 20, 2021, about 1755 Alaska standard time, a Cessna 207 airplane, N9794M, sustained substantial damage when it was involved in an accident near Bethel, Alaska. The pilot and five passengers were not injured. The airplane was operated by Yute Commuter Service as a Title 14 Code of Federal Regulations Part 135 scheduled passenger flight.

The purpose of the flight was to transport five passengers and cargo to Kwethluk, Alaska, which is located about 12 miles east of Bethel. The pilot reported that shortly after departure, he noticed that the red emergency locator transmitter (ELT) light on the instrument-panel-mounted remote switch had illuminated. The pilot then asked the Bethel tower operator if they were hearing an ELT signal, and the tower operator responded that no signal was being received. Moments later, the pilot began to smell an electrical burn smell and elected to return to Bethel.

The pilot said that about 1 minute later, the electrical burn smell intensified, which was followed by visible smoke in the cockpit, and he then declared an emergency to the Bethel tower. The pilot then turned off the airplane’s master electrical switch, and subsequently opened his side window for ventilation and smoke removal. He said he briefly turned the master switch back on to again declare an emergency with Bethel tower, and to inform the tower operator that he was planning to land on runway 1L.

The pilot said that during the landing roll, he realized that the nosewheel steering system and brake system were both inoperative. After the airplane rolled to a stop on the left side of runway 1L, he ordered all the passengers to evacuate the airplane.

The pilot reported that after all the passengers had safely departed the airplane, heavy smoke filled the cockpit and passenger compartment, and he saw a candle-like flame behind the pilot and co-pilot seats, just beneath the floorboards of the airplane. Moments after all the passengers and pilot had exited the airplane, it was immediately engulfed in flames. See figure 1.

Figure 1 Photo of accident airplane while still on Runway 1L.

Examination of the maintenance records showed that on January 27, 2011, the Capstone system, phase 1, was installed on the accident airplane under Federal Aviation Administration (FAA) Supplemental Type Certificate (STC) number STC2154AK. The Capstone Program is a voluntary safety demonstration involving the installation, at FAA expense, of avionics suites in approximately 200 aircraft operated in western Alaska. The avionics suites, manufactured by UPS Aviation Technologies, consist of an MX-20 multifunction display (MFD), a GX-60 GPS nav/com, a Universal Access Transceiver (UAT), and an installation kit with all necessary cables, antennas, and mounting hardware, and an instruction book.

A National Transportation Safety Board (NTSB) postaccident examination of the airplane wreckage revealed the origin of the fire to be centered behind the pilot’s row of seats, where a wire harness, which was discovered to be part of the Capstone Program equipment, was found improperly installed on top of the aft fuel line from the left fuel tank. The fuel line exhibited chafing and thermal damage. Following removal of the fuel line, a small pinhole was detected in the fuel line utilizing a flashlight. NTSB Materials Laboratory examination of the wire harness found a range of thermal and electrical damage, from thermal discoloration and sooting to beading and welding in areas on individual conductors. The damage was consistent with the wire being energized at the time the damage occurred.

During the on-scene NTSB examination of the accident airplane wreckage, a second Cessna 207, which was purchased at the same time as the accident airplane, was examined in the operator’s hangar. The NTSB IIC, along with the entire investigative team, discovered that the same Capstone wire harness condition found in the accident airplane existed in the second Cessna 207 in the same location.

As a result of the NTSB investigation, the FAA issued a notification letter to alert registered aircraft owners and operators of the need to inspect the area inside inspection panels from the rear door frame forward under the pilots and passengers’ floorboards at the next maintenance function, 100 hour, or Annual inspections. FAA database records indicate there is over 200 aircraft that had the CAPSTONE phase 1 equipment installation under STC2154AK and therefore need to be inspected.