Beech F90
Flight Crew Instrument; Flight Crew Commercial; Flight Crew Flight Instructor; Flight Crew Multiengine
Flight Crew Last 90 Days 50; Flight Crew Total 1650; Flight Crew Type 170
Training / Qualification; Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Track / Heading All Types; Inflight Event / Encounter Weather / Turbulence
Flight Crew Took Evasive Action
Aircraft; Human Factors; Weather
I was flying under an IFR flight plan and requested deviation 30 degrees left/right for weather.  I proceeded and chose the best route according to XM and the onboard aircraft radar.  I ended up flying through clouds that appeared safer than the rest.  I proceeded to go through the clouds but inadvertently ended up losing altitude and heading due to what I learned a few moments later was a developing storm cell.  After I recovered from the altitude loss and proceeded back on course; the XM painted the area I just flew through as a developed cell.  My corrective action for the future would be understanding XM and onboard radar as tools that may be delayed by a few moments and not give me actual time relevant weather info.  I have also learned how to decipher my onboard radar with more accuracy to avoid problems in the future. Flying through the clouds; my autopilot kicked off and I had to manually take the controls to maintain the aircraft and I did not expect the autopilot to disengage as early as it did.  In the future I will be more diligent about the autopilot disengage as to not lose as much altitude and/or heading.  This was a learning experience that I will use in the future as to refrain from deviated altitude and heading.
King Air pilot reports using XM Nexrad and onboard Radar to avoid heavy weather but still encountering turbulence that causes the autopilot to disengage with resulting altitude and heading deviations.  Both systems show the area as a developed cell shortly after passing through.