Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Weather; Human Factors
Altitude deviation; few miles south of LHS VOR on V459 at 10;000 feet. IMC with light icing condition. Airspeed indicator showed slow decrease from 135 KIAS. I checked that pitot heat switch was on position and wing boots clear the ice. I thought that ice was over and or under the wing. As the airspeed indicator approached 110; I requested 9;000 feet to ATC. ATC said 'stand by'. Airspeed indicator kept decreasing. Airspeed approached 100; I reported 'unable to maintain 10;000' and lowered the pitch [to] gain airspeed. But airspeed did not increase; then I tried to recycle pitot heat switch. Few second later airspeed showed normal; and I climbed back to 10;000. I deviated 800 feet.Cause:1. Pitot/Static heat switch did not work properly.  2. I did not check other side of instruments (lack of scan). 3. When I requested 9;000; I should say why (due to icing etc.) 4. When I started to descend; I should declare emergency.
C-208 pilot reported deviating from assigned altitude when he experienced abnormal airspeed indications while in icing conditions; pitot/static heat switch problem was suspected to be a factor.