Narrative:

We were cleared from FL30 to 6000', and on descent we entered clouds at approximately 230 KTS indicated. As we entered clouds, we immediately got icing detection warnings and a moderate to severe vibration. As our anti-icing was only just warming up, we concluded the vibration was due to icing. We changed propeller RPM and tried different speeds and descent rates. Nothing deterred the vibration until gear and flaps were selected down. At this time no ice remained on aircraft, yet the vibration continued. We did a very thorough post-flight inspection and found nothing abnormal. After talking with maintenance, they decided there had to be some ice on the aircraft and to go ahead and fly the return trip to den. On climb out from rap, everything remained normal until level at FL210. As we accelerated through 180 KTS, the vibration returned. At this time there was no way that we had ice on the aircraft. If we stayed below 180 KTS, we had a smooth ride. We continued to den at a slower than normal speed and grounded the aircraft pending further maintenance. Supplemental information from acn 180073: the F/a's informed us that there was a loud, grinding noise in the aircraft (loud enough to scare the passenger) each time the aircraft exhibited the unusual behavior. Throughout the return to den, we were in contact with maintenance and the chief pilot, but no one had any ideas. Maintenance in den found several defects in the vertical and horizontal stabilizers and fixed them. In retrospect, I believe I should have been more forceful with the captain and refused to depart rapid city until main had inspected the aircraft.

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Original NASA ASRS Text

Title: COMMUTER FLT CREW ENCOUNTERS SEVERE VIBRATION AND LOUD NOISE ON DESCENT INTO RAP.

Narrative: WE WERE CLRED FROM FL30 TO 6000', AND ON DSNT WE ENTERED CLOUDS AT APPROX 230 KTS INDICATED. AS WE ENTERED CLOUDS, WE IMMEDIATELY GOT ICING DETECTION WARNINGS AND A MODERATE TO SEVERE VIBRATION. AS OUR ANTI-ICING WAS ONLY JUST WARMING UP, WE CONCLUDED THE VIBRATION WAS DUE TO ICING. WE CHANGED PROP RPM AND TRIED DIFFERENT SPDS AND DSNT RATES. NOTHING DETERRED THE VIBRATION UNTIL GEAR AND FLAPS WERE SELECTED DOWN. AT THIS TIME NO ICE REMAINED ON ACFT, YET THE VIBRATION CONTINUED. WE DID A VERY THOROUGH POST-FLT INSPECTION AND FOUND NOTHING ABNORMAL. AFTER TALKING WITH MAINT, THEY DECIDED THERE HAD TO BE SOME ICE ON THE ACFT AND TO GO AHEAD AND FLY THE RETURN TRIP TO DEN. ON CLBOUT FROM RAP, EVERYTHING REMAINED NORMAL UNTIL LEVEL AT FL210. AS WE ACCELERATED THROUGH 180 KTS, THE VIBRATION RETURNED. AT THIS TIME THERE WAS NO WAY THAT WE HAD ICE ON THE ACFT. IF WE STAYED BELOW 180 KTS, WE HAD A SMOOTH RIDE. WE CONTINUED TO DEN AT A SLOWER THAN NORMAL SPD AND GNDED THE ACFT PENDING FURTHER MAINT. SUPPLEMENTAL INFO FROM ACN 180073: THE F/A'S INFORMED US THAT THERE WAS A LOUD, GRINDING NOISE IN THE ACFT (LOUD ENOUGH TO SCARE THE PAX) EACH TIME THE ACFT EXHIBITED THE UNUSUAL BEHAVIOR. THROUGHOUT THE RETURN TO DEN, WE WERE IN CONTACT WITH MAINT AND THE CHIEF PLT, BUT NO ONE HAD ANY IDEAS. MAINT IN DEN FOUND SEVERAL DEFECTS IN THE VERT AND HORIZ STABILIZERS AND FIXED THEM. IN RETROSPECT, I BELIEVE I SHOULD HAVE BEEN MORE FORCEFUL WITH THE CAPT AND REFUSED TO DEPART RAPID CITY UNTIL MAIN HAD INSPECTED THE ACFT.

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.