Narrative:

Was cleared to descend from FL270 to FL230 and at same time handed off from one chicago center controller to the next when severe turbulence was encountered. This lasted approximately 5 minutes. The windscreen iced over immediately. The autopilot kicked off. During this time I hit my head hard on the cockpit ceiling twice. There were short periods of time that I could not read the flight instruments because of motion. I tried to maintain attitude within 30 degrees of bank and pitch within 10 degrees of level. At times the attitude indicator pitched up while the vsi showed a descent. By the time I was able to regain complete control the aircraft was 2;000 feet to 3;000 feet below my assigned altitude of FL230. I reported this to the next sector controller who issued a new clearance. The rest of the flight was unremarkable. The route of flight was south of and parallel to a line of thunderstorms. Just before the turbulence began the onboard radar suddenly showed an area of red immediately in front. This did not appear to be present until just as we entered the area. I feel that this occurrence could have been avoided if I would have deviated to the south further from the line of convective activity. The center controllers were very proactive in providing me weather radar updates during the entire flight. In the future I will stay further away from convective activity. It concerns me deeply that I was involved in an altitude deviation that could have endangered another aircraft. The aircraft was equipped with a stormscope and onboard radar. I did not have aboard access to NWS radar. Perhaps if it had been onboard I would have had a better understanding of the overall picture and would have moved my route further south.

Related reports:

Google
 

Original NASA ASRS Text

Title: A small transport pilot encountered severe turbulence and descended up to 3;000 feet from his assigned altitude while flying south of and parallel to a line of Midwestern thunderstorms.

Narrative: Was cleared to descend from FL270 to FL230 and at same time handed off from one Chicago Center Controller to the next when severe turbulence was encountered. This lasted approximately 5 minutes. The windscreen iced over immediately. The autopilot kicked off. During this time I hit my head hard on the cockpit ceiling twice. There were short periods of time that I could not read the flight instruments because of motion. I tried to maintain attitude within 30 degrees of bank and pitch within 10 degrees of level. At times the attitude indicator pitched up while the VSI showed a descent. By the time I was able to regain complete control the aircraft was 2;000 feet to 3;000 feet below my assigned altitude of FL230. I reported this to the next sector Controller who issued a new clearance. The rest of the flight was unremarkable. The route of flight was South of and parallel to a line of thunderstorms. Just before the turbulence began the onboard radar suddenly showed an area of red immediately in front. This did not appear to be present until just as we entered the area. I feel that this occurrence could have been avoided if I would have deviated to the south further from the line of convective activity. The Center controllers were very proactive in providing me weather radar updates during the entire flight. In the future I will stay further away from convective activity. It concerns me deeply that I was involved in an altitude deviation that could have endangered another aircraft. The aircraft was equipped with a stormscope and onboard radar. I did not have aboard access to NWS radar. Perhaps if it had been onboard I would have had a better understanding of the overall picture and would have moved my route further south.

Data retrieved from NASA's ASRS site as of July 2013 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.