Narrative:

I was on an IFR flight plan and climbed to 16;000 ft uneventfully. During climb vectored left to 240 toward precipitation per satellite report but none seen on weather radar. During climb 10-20 seconds of light precipitation encountered. Trace rime ice on windshield wipers. Windshield defrost engaged. Climb rate 800 to 1000 FPM. No ice on wings or engine inlet. Tops at 14;500 to 15;000 ft clear above. Leveled at 16;000 ft. Airspeed about 180 KTS. Autopilot engaged. Fuel flow and RPM reduced for cruise. Airspeed noted to decrease. Still no ice on wings or engine inlet. Pitot heat; prop heat and engine heat engaged. Fuel flow increased. Autopilot disengaged. Airspeed 120 KTS. Sudden violent roll/yaw to the left several times and each time corrected with rudder and spoiler input. At some point the nose pitched down with several more violent rolls to left with correction. Then a left spin developed. Airspeed remained between 120 to 150 KTS. Control was restored at about 6200 ft. Engine gages indicated normal readings except for brief elevation of exhaust temperature on the right engine. The trace ice on the wiper blades had melted. Airspeed stabilized at about 180 KTS. I notified center after level flight and control was restored. My initial impression was that I encountered severe turbulence with a forceful down draft as I was near the backside of a passing front. I discussed the turbulence with center. I did not declare an emergency after leveling off as all engine gages; airspeed appeared stable and fight control functioned normally. Upon further consideration other factors may have been at play. The violent roll/yaw to the left may have been due to improper fuel scheduling to the engine due to icing of the P2/T2 sensor. Fuel icing may have also been a factor. The downward pitch may have been due to a decrease in engine power but more likely due to a tail stall. The spin may have been the result of the stall and the power surge of the right engine. As far as what could have been done to prevent this event a precautionary cycle of the deicing boots when airspeed deteriorated despite no ice being seen on the wings may have helped. More vigilance of the outside air temperature with earlier application of deicing equipment when outside air temperature is at or below 0 C. The spin was arrested by decreasing the power in the right engine (condition lever to flight idle).

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Original NASA ASRS Text

Title: MU2 pilot leveling at 16;000 FT reported inability to maintain cruise speed with normal power setting. As airspeed decreased controllability issues developed and aircraft entered a left spin. Recovery was initiated by reducing power on the right engine and aircraft leveled at 6200 FT.

Narrative: I was on an IFR flight plan and climbed to 16;000 FT uneventfully. During climb vectored left to 240 toward precipitation per satellite report but none seen on weather radar. During climb 10-20 seconds of light precipitation encountered. Trace rime ice on windshield wipers. Windshield defrost engaged. Climb rate 800 to 1000 FPM. No ice on wings or engine inlet. Tops at 14;500 to 15;000 FT clear above. Leveled at 16;000 FT. Airspeed about 180 KTS. Autopilot engaged. Fuel flow and RPM reduced for cruise. Airspeed noted to decrease. Still no ice on wings or engine inlet. Pitot heat; prop heat and engine heat engaged. Fuel flow increased. Autopilot disengaged. Airspeed 120 KTS. Sudden violent roll/yaw to the left several times and each time corrected with rudder and spoiler input. At some point the nose pitched down with several more violent rolls to left with correction. Then a left spin developed. Airspeed remained between 120 to 150 KTS. Control was restored at about 6200 FT. Engine gages indicated normal readings except for brief elevation of exhaust temperature on the right engine. The trace ice on the wiper blades had melted. Airspeed stabilized at about 180 KTS. I notified Center after level flight and control was restored. My initial impression was that I encountered severe turbulence with a forceful down draft as I was near the backside of a passing front. I discussed the turbulence with Center. I did not declare an emergency after leveling off as all engine gages; airspeed appeared stable and fight control functioned normally. Upon further consideration other factors may have been at play. The violent roll/yaw to the left may have been due to improper fuel scheduling to the engine due to icing of the P2/T2 sensor. Fuel icing may have also been a factor. The downward pitch may have been due to a decrease in engine power but more likely due to a tail stall. The spin may have been the result of the stall and the power surge of the right engine. As far as what could have been done to prevent this event a precautionary cycle of the deicing boots when airspeed deteriorated despite no ice being seen on the wings may have helped. More vigilance of the outside air temperature with earlier application of deicing equipment when outside air temperature is at or below 0 C. The spin was arrested by decreasing the power in the right engine (condition lever to flight idle).

Data retrieved from NASA's ASRS site as of April 2012 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.