Eclipse 500
Flight Crew Flight Instructor; Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 23; Flight Crew Total 15300; Flight Crew Type 975
Situational Awareness
Aircraft Equipment Problem Critical; Deviation - Altitude Excursion From Assigned Altitude; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Aircraft Equipment Problem Dissipated; Flight Crew Diverted; Flight Crew Landed As Precaution; Flight Crew Took Evasive Action; General Maintenance Action
Aircraft; Weather
I was attempting to overfly convective activity as previous aircraft were going over at FL380 so requested FL380.  I started to experience moderate turbulence and IMC conditions about the same time the AIRSPEED DISAGREE Crew Alert System annunciated.  The autopilot disconnected and would not reconnect.  Then the stall warning activated; which is a continuous voice 'stall' every second.  At the same time the the LH PFD was showing erratic airspeed and a stall indication.  I attempted to silence the stall warning by alternately selecting the ADC source which is the QRH procedure for erroneous 'Stall' Indications or Airspeed Erroneous.  I was still manually flying at FL380.  I told Center that I would need to descend to FL280.  He noticed my altitude fluctuations and asked if I was still RVSM compliant.  I said I was not.  He then asked me to maintain FL380 for traffic which I did to the best of my ability.  I knew I was over a large cell of weather and would be descending into it so I turned further north to avoid it.  I was still getting 'stall' warnings while trying to maintain altitude and airspeed on the standby A/I.  I was able to eventually silence the stall warning and get stable airspeed and altitude.  The Center then cleared me to FL260 for traffic.  I continued flying looking for a divert airport.  Going through approximately 17;000 the erroneous airspeed condition cleared which confirmed my suspicions.  I used improper phraseology to convey situation to ATC.  I requested a lower altitude (FL280) to exit RVSM instead of stating that I was 'unable RVSM.'  After reviewing RVSM procedures; I should have stated 'unable RVSM' due to equipment failure and turbulence.In the past; the erroneous airspeed and stall indication has been caused by moisture inadvertently getting into the static system and freezing at altitude.  Eclipse installed spring loaded drain valves in the pitot/static system (which I did accomplish on the preflight).  Most operators also use static covers to prevent moisture accumulation.  This has proven to be the most effective deterrent.  This airplane normally has the covers but for some reason they were not on the airplane.  The weather forecast did call for scattered showers the night before but I determined it was not enough of a chance to hangar the airplane.  Another issue Maintenance will look at is the pitot system not getting to proper temperature because the occurrence did happen after I had entered IMC.  After fueling; I attempted to drain any residual moisture using the drain valves.  The problem with the Eclipse design; in my estimation; is there is no way of knowing if you have drained any water as it is drained in an inaccessible forward fuselage area.  Some operators will blow gently on the pitot tubes while someone is holding the drain valves open. Again I didn't think I had accumulated enough moisture to try this.  Another issue Maintenance will look at is the pitot system not getting to proper temperature.  The occurrence did happen after I had entered IMC.  In the future I will insure that the static covers are on the airplane and/or hangar it if any chance of precipitation.
EA50 pilot experiences erroneous airspeed indications and stall warnings at FL380 while IMC above a thunderstorm.  A descent and diversion is executed with ATC approval and the pitot static system returns to normal passing 17;000 feet.