Date: 201207
Local time of day: 1201-1800
Location: ZMP.ARTCC, MN
Flight conditions: Mixed
Weather elements / visibility: Rain; Turbulence; Windshear
Light: Daylight
Aircraft: Cessna Stationair/Turbo Stationair 6
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Cruise
Airspace: Class E ZMP
ATC / advisory: Center ZMP
Aircraft component: Altitude Hold/Capture
Component problem: Improperly Operated
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Commercial; Flight Crew Instrument; Flight Crew Multiengine
Human factors: Human-Machine Interface
Anomaly: Aircraft Equipment Problem Less Severe; Deviation - Altitude Overshoot; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control
When detected: In-flight
Result: Air Traffic Control Issued Advisory / Alert; Flight Crew Returned To Clearance
Contributing factors: Aircraft; Human Factors; Weather
Primary problem: Human Factors

Narrative: Direct IFR to MSN at 11;000 FT. The buildups were going higher than 11;000 FT. I requested and was assigned 13;000 due to cloud buildups. The buildups were getting bigger and more numerous as the flight progressed. The top of the cloud deck was also rising. Nexrad showed small areas of moderated precipitation about 40 NM ahead; Storm Scope showed no discharges nearby. I entered one relatively benign looking build up. The turbulence was more than expected. One of bumps upset the contents of the cabin. I requested descent to 7;000 to get out of the clouds. MSP Center assigned me 9;000 and I set 9;000 FT in the C206 autopilot and programmed the descent rate and slowed the plane down. During the descent; I asked for and was given deviations left and right of course to stay out of the darker more turbulent clouds. I was looking at the Nexrad and Storm Scope displays to avoid additional surprises. There were notable updrafts and down drafts. The autopilot made moderate changes to the pitch attitude to maintain the descent rate. I was transferred to Volk approach. Shortly after the transfer; Volk reiterated 9;000 was my assigned altitude. I looked at the altimeter and my altitude was 8;200 FT. The autopilot did not capture the 9;000 FT setting. I immediately disconnected the autopilot and initiated a climb back to 9;000 FT. I did not monitor the altitude adequately; due turbulence and preoccupation using the Nexrad and Storm Scope to avoid further upsets. The autopilot did have 9;000 FT set. I do not known why it did not capture 9;000 FT. I have many hours using the C206. Occasionally it will not arm when climbing to a pre-selected altitude and this is something I watch for. I had not ever had it fail to arm and capture a descent altitude. The autopilot seemed to function fine for the duration of the flight. Perhaps I double pressed or hit another button during the set due to turbulence? Switching frequencies happened during a high workload part of the flight; while I was assessing the clouds (did not want to be surprised again). Check; check and recheck even in high workload phases of flight.




Synopsis: C206 pilot reports an altitude deviation while attempting to descend to 9;000 FT and deviate around buildups using a Storm scope and Nexrad with limited success. The autopilot was set to capture 9;000 FT but did not do so and the reporter was alerted by ATC at 8;200 FT.