Narrative:

While deviating north off course in cloud to avoid thunderstorms on descent from ict to rdu we encountered a sudden, unexpected, isolated short burst of turbulence, which rendered the autoplt and us incapable of maintaining our assigned altitude. This turbulence lasted approximately 5 seconds in duration and was also accompanied by a short burst of frozen precipitation. An altitude excursion of approximately 300 ft lower occurred. We immediately corrected by climbing back to the assigned altitude. In spite of the suggestion by the PF to announce our situation to the controller, because of very dense radio traffic I believed it was best to guard radio silence and did so. We had climbed back to our assigned altitude in several seconds when the controller asked us if we were at our assigned altitude to which I replied 'yes we are.' by that time we were again at our assigned altitude, so nothing more was said. Due to the isolated, sudden and violent nature of the burst of turbulence, it is difficult to identify measures that may be taken to prevent a recurrence. Distance from convective activity shown on radar seemed to us to have been appropriate. In retrospect, perhaps a higher degree of anticipation for unexpected turbulence may have prepared us to recover sooner from the effects of the turbulence. An immediate announcement to ATC of our situation regardless of radio traffic density would have been appropriate and I should have done so to alert other traffic in the area of the potential for problems resulting from unexpected turbulence and icing potential.

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Original NASA ASRS Text

Title: FLT CREW OF HAWKER 400XP ENCOUNTER BRIEF BURST OF TURB CAUSING THEM TO LOSE 300 FT FROM ASSIGNED ALT.

Narrative: WHILE DEVIATING N OFF COURSE IN CLOUD TO AVOID TSTMS ON DSCNT FROM ICT TO RDU WE ENCOUNTERED A SUDDEN, UNEXPECTED, ISOLATED SHORT BURST OF TURB, WHICH RENDERED THE AUTOPLT AND US INCAPABLE OF MAINTAINING OUR ASSIGNED ALT. THIS TURB LASTED APPROX 5 SECONDS IN DURATION AND WAS ALSO ACCOMPANIED BY A SHORT BURST OF FROZEN PRECIPITATION. AN ALT EXCURSION OF APPROX 300 FT LOWER OCCURRED. WE IMMEDIATELY CORRECTED BY CLBING BACK TO THE ASSIGNED ALT. IN SPITE OF THE SUGGESTION BY THE PF TO ANNOUNCE OUR SIT TO THE CTLR, BECAUSE OF VERY DENSE RADIO TFC I BELIEVED IT WAS BEST TO GUARD RADIO SILENCE AND DID SO. WE HAD CLBED BACK TO OUR ASSIGNED ALT IN SEVERAL SECONDS WHEN THE CTLR ASKED US IF WE WERE AT OUR ASSIGNED ALT TO WHICH I REPLIED 'YES WE ARE.' BY THAT TIME WE WERE AGAIN AT OUR ASSIGNED ALT, SO NOTHING MORE WAS SAID. DUE TO THE ISOLATED, SUDDEN AND VIOLENT NATURE OF THE BURST OF TURB, IT IS DIFFICULT TO IDENT MEASURES THAT MAY BE TAKEN TO PREVENT A RECURRENCE. DISTANCE FROM CONVECTIVE ACTIVITY SHOWN ON RADAR SEEMED TO US TO HAVE BEEN APPROPRIATE. IN RETROSPECT, PERHAPS A HIGHER DEGREE OF ANTICIPATION FOR UNEXPECTED TURB MAY HAVE PREPARED US TO RECOVER SOONER FROM THE EFFECTS OF THE TURB. AN IMMEDIATE ANNOUNCEMENT TO ATC OF OUR SIT REGARDLESS OF RADIO TFC DENSITY WOULD HAVE BEEN APPROPRIATE AND I SHOULD HAVE DONE SO TO ALERT OTHER TFC IN THE AREA OF THE POTENTIAL FOR PROBS RESULTING FROM UNEXPECTED TURB AND ICING POTENTIAL.

Data retrieved from NASA's ASRS site as of July 2007 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.