Accident Summary
On August 16, 2021, at approximately 1452 Alaska daylight time, a Cessna 208B, registration N97HA, operated by Wright Air Service as flight 440, sustained substantial damage during an in-flight upset near Fairbanks, Alaska. The pilot and eight passengers were not injured. The flight was conducted under Title 14 CFR Part 135 as a scheduled commuter flight.
Flight History
The flight departed Fairbanks International Airport (FAI) at about 1424, destined for Huslia, Alaska (HLA). After departure, the pilot climbed to an assigned altitude of 10,000 ft msl and encountered light rime icing. She requested and received a block altitude of 10,000 to 12,000 ft msl and climbed to 10,500 ft msl but could not exit icing conditions. She began a gradual descent back to 10,000 ft msl. The autopilot was engaged, and deicing boots and heated propeller operated normally.
During the descent, the autopilot disengaged without warning, and the airplane entered an abrupt uncommanded right bank followed by a steep, nose-down, spiraling descent. The pilot reported that aileron controls felt jammed. After regaining control, she declared an emergency and landed safely at FAI.
Weather Conditions
Flight track data showed the flight proceeded west-northwest for about 47 nautical miles at 10,000 ft msl before a climb to about 10,700 ft msl, then a descent to 10,300 ft msl preceding the upset. The Fairbanks upper air soundings indicated cloud cover from 3,000 ft msl through 18,000 ft msl with moderate icing above 9,500 ft msl and a freezing level around 6,440 ft msl. Radar showed light to moderate precipitation. Experimental satellite data indicated supercooled liquid water from 7,000 to 13,000 ft msl. Icing products showed a 30% to 60% chance of trace to moderate icing; the Current Icing Product indicated an unknown probability of supercooled large droplets (SLD) between 8,000 and 11,000 ft msl. Passengers reported ice accumulation on the windshield and strut, and chunks of ice flying past during the descent.
Aircraft Examination
Postflight examination revealed substantial damage to the wings and right aileron. No evidence of preaccident mechanical malfunctions or anomalies was found. Flight control continuity was confirmed, and testing of the autopilot and deice systems revealed no anomalies.
Pilot Operating Handbook Compliance
The Cessna 208B POH prohibited flight in freezing rain, freezing drizzle, mixed conditions, or severe icing. Minimum speed for icing conditions with flaps up was 130 knots indicated airspeed (KIAS) at a maximum weight of 8,550 lbs. The POH required disconnecting the autopilot every 10 minutes in icing conditions to check for out-of-trim conditions. Flight data from the Garmin GFC 600 autopilot showed it was not disconnected every 10 minutes. Indicated airspeed was consistently below 130 KIAS and dropped below 100 knots before the upset. Low airspeed alerts activated at 107 KIAS at 16, 5, and 2.5 minutes before the upset.
Weight and Balance
The airplane's basic empty weight was 5,202.5 lbs with a center of gravity (CG) of 188.2 inches. Maximum takeoff gross weight (MTOW) was 9,062 lbs; MTOW for flight into known icing was 8,550 lbs. Estimated weight at the time of the accident was about 9,420 lbs, exceeding the icing MTOW, with a CG of 199.8 inches (within range for MTOW but not for icing limit). The weight estimate included pilot (115 lbs), passengers and baggage (1,606 lbs and 832 lbs), and fuel (about 1,700 lbs).
NTSB Recommendations
The NTSB determined the airplane encountered SLD. The pilot reviewed weather from the NWS Alaska Aviation Weather Unit website, but no SLD forecast product was available for Alaska. The NTSB issued Safety Recommendation A-22-21 to the FAA and A-22-22 to the NWS to develop and make available a graphical SLD forecast for Alaska. Additionally, the NTSB noted that 14 CFR 135.63 does not require weight and balance documentation for single-engine aircraft, and issued recommendations A-89-135, A-99-61, and A-15-029 to amend the rule; these were closed as unacceptable action.