Flight and Icing Encounter
A Mitsubishi MU-2 departed Salt Lake City, Utah, on an IFR flight to Pocatello, Idaho. The aircraft climbed to 16,000 feet MSL. During cruise flight, the MU-2 encountered structural icing conditions. Radar data showed that the aircraft began slowing from a cruise speed of about 190 knots, with slight deviations from its assigned heading and altitude. The airspeed decreased to approximately 100 knots.
Emergency and Descent
The flight crew declared an unspecified emergency. Shortly thereafter, radio contact was lost. The MU-2 began a right turn and then entered a steep descent, ultimately crashing.
A Beech 1900 pilot, trailing the MU-2 by about 12 minutes, reported encountering moderate rime icing at 16,000 feet. The Beech pilot activated his deice boots three times and descended to 12,000 feet to exit the icing conditions.
Maintenance and Pilot Information
The MU-2 flight manual included warnings that during flight in icing conditions, stall warning devices may not be accurate and should not be relied upon. It advised maintaining a minimum cruise speed of 180 knots or exiting icing conditions to minimize ice accumulation.
An investigation determined that the captain of the MU-2 was aware of deficiencies in the timer for the deice boots, as well as other maintenance deficiencies. The captain held a medical certificate dated 11/17/94. He was providing executive transportation for compensation under an agreement for "contractual flights" under 14 CFR 91.
Weather forecasts indicated icing conditions in the destination area, but no icing was forecast for the en route portion of the flight.
Probable Cause
The official probable cause findings state: Continued flight by the flightcrew into icing conditions with known faulty deice equipment; structural (airframe) ice; and failure of the flight crew to maintain adequate airspeed, which resulted in the loss of aircraft control and collision with terrain. A factor relating to the accident was: the en route weather (icing) condition, which was not forecast (inaccurate forecast).