Date: 201303
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Weather elements / visibility: Snow; 10
Light: Daylight
Aircraft: Bonanza 36
Operator: Personal
Operating under: Part 91
Flight plan: IFR
Mission: Personal
Flight phase: Initial Approach
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Reporter organization: Personal
Reporter function: Single Pilot
Reporter qualification: Flight Crew Instrument; Flight Crew Private
Human factors: Confusion; Distraction; Situational Awareness; Troubleshooting
Anomaly: Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Flight Crew
When detected: In-flight
Contributing factors: Human Factors; Weather
Primary problem: Weather

Narrative: Prior to departure I signed into DUAT and received a weather briefing. I also consulted aviationweather.gov. I concluded that my planned flight could be made safely and filed IFR; expecting visual conditions with only intermittent IMC. I noted no pilot reports of icing.On V1 at 7;000 approaching [Class C airport]; I passed through a light snowstorm but saw no accumulation on the wings and so continued. A few minutes later I noted IAS falling to 130 KTS; below the expected 160 KTS or higher. I also noted vibration in the aircraft and there was no increase in IAS with full throttle/mixture/prop.I concluded that aircraft might be accumulating ice on the prop (though induction air blockage also is a possibility) and requested descent to 3;000 MSL and a diversion to the nearest suitable field. ATC approved descent and provided vectors for an RNAV 9 approach.While trying to get the airport weather and requesting RNAV 27 (as it was more suitable than the RNAV to Runway 9) and tending to the avionics; I gradually and inadvertently descended to 2;000 FT; 1;000 below my assigned altitude. I spent a few minutes in gradual descent before noting the discrepancy and correcting immediately to 3;000. VMC existed at 3;000 and below.Contributing factors to my failure to maintain assigned altitude: Failure to note the effect of snow on possible induction blockage; causing pilot anxiety to locate a suitable field and get on the ground. Failure to manage the process of locating a suitable field; obtaining proper frequencies; obtaining latest weather at the field; while flying the airplane as required. Failure to properly include altimeter in scan [and] failure to set autopilot correctly to maintain altitude.Contributing factor: existence of VMC at and below 3;000 led to my lax behavior concerning altitude (I could see where everything was) which led to insufficient vigilance concerning altitude.To prevent recurrence; I need more practice with the 'sudden diversion' scenario with my instructor; with key elements including proper use of autopilot and proper use of manual trim (if autopilot inoperative or inadvisable) while obtaining field information and communicating with ATC. Also review of Emergency Procedures in the POH which would have led me to consider the Alt Air handle (if induction icing was in fact the issue).




Synopsis: After an IFR BE36 pilot suffered reduced power and loss of airspeed after passing through a light snow shower he received a clearance to the nearest suitable airport. While doing so he inadvertently descended 1;000 FT below his cleared altitude of 3;000 MSL.