Pilot Debrief

CESSNA 208 near Fairbanks, AK — 2021-08-16

Final reportANC21LA073
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Date
2021-08-16
Location
Fairbanks, AK, USA
Aircraft
CESSNA 208
Registration
N97HA
Category
Airplane
Highest injury
None
Fatalities
0
Phase of flight
Climb

Probable cause

The pilot’s unexpected encounter with supercooled liquid droplets (SLD), which resulted in a loss of control due to ice accumulation. Contributing to the accident were: (1) the pilot’s failure to maintain the minimum airspeed for flight in icing conditions, (2) the pilot’s failure to disconnect the autopilot every 10 minutes to check for ice buildup, (3) the overweight airplane, (4) the lack of an SLD forecast product for the state of Alaska, and (5) the Federal Aviation Administration's failure to require weight and balance documentation for each flight in Title 14 Code of Federal Regulations Part 135 single-engine operations.

Contributing factors

Contributing to the accident were: (1) the pilot’s failure to maintain the minimum airspeed for flight in icing conditions, (2) the pilot’s failure to disconnect the autopilot every 10 minutes to check for ice buildup, (3) the overweight airplane, (4) the lack of an SLD forecast product for the state of Alaska, and (5) the Federal Aviation Administration's failure to require weight and balance documentation for each flight in Title 14 Code of Federal Regulations Part 135 single-engine operations.

NTSB narrative

After departure, the pilot climbed the airplane with the autopilot engaged to her assigned altitude of 10,000 ft mean sea level (msl), where she encountered instrument meteorological conditions and “light rime” icing conditions. To exit the icing conditions, she climbed the airplane to 10,500 ft msl. However, she was unable to exit the icing conditions and began a gradual descent back to 10,000 ft msl. She noted that while en route, the airplane’s deicing boots, and heated propeller were operating normally by removing ice accumulations. While in a gradual descent from 10,500 ft msl to 10,000 ft msl, the autopilot disengaged without warning, and the airplane entered an abrupt uncommand right bank followed by a steep, nose-down, spiraling descent. The pilot stated that during the upset, it felt as if the aileron controls were jammed as she tried to regain control of the airplane. After regaining control of the airplane, she declared an emergency and made an emergency landing back at the departure airport. The airplane sustained substantial damage to the wings and the right aileron during the upset. Postaccident examination revealed no evidence of preaccident mechanical malfunctions or anomalies with the airplane that would have precluded normal operation. Flight control continuity was confirmed from the cockpit to all control surfaces. Additionally, testing of the autopilot and deice systems did not reveal any anomalies. Weather soundings, forecasts, and experimental weather products indicated a 30% to 60% chance of light to moderate icing in the accident region with supercooled liquid water in the clouds from 7,000 ft to 13,000 ft and favorable conditions for supercooled large droplets (SLD). It is likely the airplane encountered SLD conditions that degraded the airplane’s performance, which resulted in the upset. Graphical aviation forecast information that showed areas of icing (trace to severe/heavy) was available in Alaska. However, that information did not depict areas with potential SLD icing. If a product that included SLD icing information for Alaska had been available, the accident may have been avoided. As a result, the National Transportation Safety Board (NTSB) issued recommendations to the Federal Aviation Administration (FAA) and the National Weather Service (NWS) to collaborate in the development of a graphical forecast showing the potential for SLD icing conditions in Alaska and to make this information available to pilots (Safety Recommendations A-22-21 and -22). Although the airplane was equipped for flight into known icing conditions, the pilot’s operating handbook (POH) and applicable supplements contained information that flight in freezing rain, freezing drizzle, mixed conditions, or conditions defined as severe were prohibited. In addition, the minimum speed for flight in icing conditions with the flaps up was 130 knots indicated airspeed with a maximum weight of 8,550 lbs. The POH further required that the autopilot be disconnected once every 10 minutes in icing conditions to check for any out-of-trim conditions caused by ice buildup. Data obtained from the autopilot revealed that it was not disconnected every 10 minutes through the course of the flight as required. The indicated air speed was consistently below 130 knots, and it was below 100 knots in the seconds preceding the upset. Furthermore, the low airspeed alert activated at 107 kts about 16 minutes, 5 minutes and 2 1/2 minutes before the upset, respectively. Additionally, the estimated gross weight of the airplane at the time of the accident was about 293.5 lbs over its maximum approved gross weight and 805.5 lbs over its approved maximum gross weight for flight in known icing conditions. For each flight in multiengine operations, Title 14 Code of Federal Regulations (CFR) 135.63(c) requires the preparation of a load manifest that includes, among other items the number of passengers, total weight of the loaded aircraft, the maximum allowable takeoff weight, and the center of gravity location of the loaded aircraft; one copy of the load manifest should be carried in the airplane, and the operator is required to keep the records for at least 30 days. Single-engine operations, such as the accident flight, are excluded from this requirement. Had the pilot been required to compute and record a weight and balance calculation, she may have been more aware of the airplane’s limitations for flight in known icing conditions and its overweight condition. The NTSB previously attempted to address this exclusion with the issuance of Safety Recommendations A-89-135, A-99-61, and A-15-29, which asked the FAA to amend the record-keeping requirements of 14 CFR 135.63(c) to apply to single-engine as well as multiengine aircraft. The FAA did not take the recommended action, and the NTSB classified Safety Recommendations A-89-135, A-99-61, and A-15-29 "Closed—Unacceptable Action" in 1990, 2014, and 2021, respectively.

Analysis

Primary failure mode
Controlled flight into terrain
First missed decision gate
Pilot should have disconnected autopilot every 10 minutes in icing conditions as required.

NTSB coding

Evidence available

  • ATC audio
  • ADS-B / radar
  • 29 docket documents
View NTSB final reportView NTSB docket

Docket documents29

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