Bonanza 35
Flight Crew Instrument; Flight Crew Private
Flight Crew Last 90 Days 20; Flight Crew Total 1400; Flight Crew Type 500
Other / Unknown; Situational Awareness; Workload
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Flight Crew Became Reoriented; Flight Crew Overcame Equipment Problem
Aircraft; Human Factors; Weather
Aircraft was assigned 6;000 FT when on IFR flight plan. Aircraft encountered trace ice (which was not in forecast as per flight briefing) in IMC conditions and pilot turned on pitot heat. Shortly thereafter; pilot lost primary guidance system (attitude indicator; HSI; airspeed indicator; electronic altimeter and VSI) and autopilot malfunctioned. Pilot reported loss of instruments to Center. Plane began descent when instruments lost and autopilot malfunctioned. Pilot then began navigation using secondary instruments. Pilot lost about 600 FT while transitioning to secondary instruments. Pilot then climbed back to assigned altitude Center very helpful during incident.
The pilot stated that he simply forgot to turn the pitot heat on until after he was IMC. By then the moisture had accumulated in the pitot tube and had become frozen enough that it was apparently solid and so would take time to thaw and exit the aft pitot tube. In the mean time he was flying on his ASPEN PFD system which should have warned him about the pitot tube status but he does not remember seeing it and so the ASPEN lost input also. His standby altitude did indicate accurately but he did not know if he could trust it until after verifying with ATC that in fact he was at 5;400 FT which coincided with that instrument. The ASPEN did indicate a right descending turn but again the Reporter was not certain it should be trusted. The HSI shut down when it sensed a discrepancy between inputs develop.
A BE35 lost primary guidance systems (attitude; HSI; airspeed; altimeter; VSI) in flight during unforecast icing conditions on and IFR flight plan. His backup ASPEN PFD did provide partial information.