What happened
An IFR flight operated by a Mitsubishi MU-2 departed from Salt Lake City, Utah, bound for Pocatello, Idaho. While cruising at 16,000 feet MS, the aircraft entered an area of structural icing. Radar tracking indicated that the aircraft's speed dropped from a cruise of approximately 190 knots to roughly 100 knots, accompanied by minor fluctuations in altitude and heading. During this period, the flight crew issued an unspecified emergency declaration before all radio communication ceased. The aircraft subsequently performed a right turn and entered a steep descent, resulting in a crash.
A pilot flying a Beech 1900, trailing the MU-2 by approximately 12 minutes, reported experiencing moderate rime icing at the same altitude. To mitigate the effects, the Beech pilot utilized deice boots and descended to 12,000 feet to escape the icing conditions.
Findings
Investigations into the accident revealed that the Mitsubishi MU-2 flight manual contains specific warnings regarding icing operations. The manual notes that stall warning devices may become unreliable in icing conditions and advises pilots to maintain a minimum cruise speed of 180 knots or exit icing conditions to prevent ice buildup.
Findings indicated that the captain was aware of maintenance deficiencies, specifically regarding the timer for the aircraft's deice boots. Additionally, while icing was predicted for the destination in Idaho, no icing had been forecasted for the route of flight. The flight was being conducted under 14 CFR 91 as part of a contractual arrangement for executive transportation.