RPTR EXPERIENCES VIBRATION AND UNCOMMANDED ROLL IN ICING CONDITIONS.

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SCHEDULED ACR FLT FROM LAX TO LAS. ON DSCNT INTO LAS; LEAVING 17000 FT; DSNDING TO 10000 FT; ENTERED A STRATUS LAYER AT APPROX 16500 FT. AT 15000 FT (APPROX) STARTING ACCUMULATING MODERATE MIXED ICING. ALL DEICE AND ANTI-ICE SYS WERE ACTIVATED AND FUNCTIONING NORMALLY. DSNDING THROUGH APPROX 14500 FT; STARTED GETTING A VIBRATION IN THE CTL WHEEL; FELT AS IF THE VIBRATIONS WERE COMING FROM THE AILERONS. AS DSCNT CONTINUED; A SMALLER VIBRATION COULD ALSO BE FELT IN THE RUDDER PEDALS. THE ACFT WAS CARRYING A SUBSTANTIAL AMOUNT OF ICE. AT APPROX 12000- 13000 FT; FELT WHAT APPEARED TO BE AN UNCOMMANDED ROLL OF THE ACFT. THE ROLL WAS CORRECTED. ALL COMPANY AND ACFT PROCS WERE FOLLOWED IN FLYING IN ICE. ICE WAS RPTED TO ATC; A LOWER ALT WAS REQUESTED. LOWER ALT NOT AVAILABLE DUE TO HIGH MEA'S OVER THE SOUTHERN SIERRA NEVADA'S. EVENTUALLY DSNDING INTO WARMER AIR AND CLR OF ALL CLOUDS; CTL VIBRATIONS CEASED. AN UNEVENTFUL LNDG WAS MADE IN LAS. NO EMER WAS DECLARED; NO PRIORITY HANDLING WAS GIVEN. THE EVENT WAS COMMUNICATED TO COMPANY DISPATCH UPON ARR IN LAS. I BELIEVE A SUBSTANTIAL AMOUNT OF ICE FORMED ON OUR ACFT; IN PARTICULAR; ICE WAS BUILDING JUST AFT OF OUR LEADING EDGE DEICE BOOTS; THUS CAUSING A VIBRATION IN THE FLT CTLS. POSSIBLE CORRECTIONS TO THIS PROB ARE CURRENTLY BEING STUDIED AND EVALUATED AT THIS PRESENT TIME. A LARGER DEICE BOOT IS BEING TEST FLOWN. THIS MAY CORRECT THIS PROB. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR ACFT IS AN ATR 42 THAT REQUIRED ABOUT 50 PERCENT AILERON INPUT TO OFFSET ASYMMETRIC ICE ON THE WINGS. THE INDICATOR ON THE ACFT SHOWED 3/4 INCH ICE BUILDUP (RIME). THE BOOTS ARE USED AS ANTI-ICE. THE WINGS ARE COMPOSITE SO HEATED WINGS CANNOT BE USED. THE CAPT GOT ON THE CTLS TO VERIFY THE ASYMMETRY AND THE VIBRATION THAT THE RPTR WAS EXPERIENCING. THE FLT DATA RECORDER WAS PULLED AND THE ROLL WAS 5 DEGS BEFORE AILERON WAS APPLIED TO COUNTER THE ROLL. THE ACFT WAS NOT ON THE AUTOPLT. THERE WAS NO ICE ON THE WINDSHIELD. NEXT TIME THE RPTR WILL TRY TO GET OUT OF THE ICE SOONER AND IF ASYMMETRY GETS WORSE USE ASYMMETRIC PWR. THE RUDDER HAS YAW DAMPERS AND RUDDER WAS NOT USED IN THIS INCIDENT BUT THERE WAS A VIBRATION IN THE RUDDER. NONE OF THE VIBRATIONS WERE INTENSE ENOUGH THAT PAX WOULD NOTICE. THERE WAS NO FREEZING RAIN OR DRIZZLE IN THIS EVENT; ONLY RIME ICE. A PROB WITH THIS ACFT IS THE MISSION IN THAT IT NEVER GETS ABOVE THE WX BUT IS ALWAYS IN IT WHENEVER THERE IS ANY. GOV GROUP HAS BEEN NOTIFIED AND STUDYING THIS EVENT WITH SPECIAL INTEREST BECAUSE FREEZING RAIN AND DRIZZLE WAS NOT INVOLVED. SUPPLEMENTAL INFO FROM ACN 298971: THE CTLS FELT STIFF AND THERE WAS A CONTINUING BUZZING VIBRATION IN THE YOKE AND RUDDER PEDALS. WE WERE ABLE TO LOSE THE ICE AFTER WE REACHED 10000 FT AND WENT ON AND MADE AN UNEVENTFUL LNDG. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THE ACFT WAS AN ATR-42 AND WAS NOT MODIFIED WITH THE EXTENDED LEADING EDGE 'BOOT.' THE EMER AIRWORTHINESS DIRECTIVE PREVENTS THEM FROM GOING INTO FREEZING RAIN OR DRIZZLE ONLY. ALSO; THIS ACFT DOES NOT HAVE THE NEW STYLE PROPS INSTALLED ON THE NEW ATR-72 MODELS; ONE OF WHICH WAS INVOLVED IN THE FATAL CRASH OUTSIDE OF CHICAGO. HE FURTHER STATED THAT THE ACFT WAS BEING HAND FLOWN BY THE FO WHO WAS AN EXPERIENCED ATR AND CORP JET PLT. BOTH PLTS COULD READILY OBSERVE ICE ON THE LEADING EDGE OF THE WING WHICH WAS NOT COMING OFF WITH THE ANTI-ICE BOOTS ON. HE BELIEVES THAT THE DESIGN OF THE BOOTS IS 1 REASON FOR THE LACK OF BREAKING OFF THE ICE. UNLIKE MOST DEICING BOOTS THAT HAVE HORIZ INFLATION POCKETS THAT DEFLATE ALTERNATELY AFTER ICE FORMS; THIS BOOT HAS VERT RIB POCKETS RUNNING CORD WISE AND DO INFLATE ALTERNATELY; BUT ARE CONSIDERED ANTI-ICE RATHER THAN DEICE. HE STATED THAT THE ICE THAT THEY ENTERED WHEN DSNDING INTO CLOUD SAT 17000 FT WAS NOT FORECASTED AND WAS NOT PRESENT ON THE PREVIOUS FLT ACROSS THE SAME RTE EARLIER; AND THAT THERE WAS NO PIREP EITHER. RPTR FURTHER STATED THAT THERE HAS BEEN AN ICE EVIDENCE PROB PUT OUTSIDE THE CAPT'S WINDSHIELD FOR CONSTANT MONITOR WHICH ALSO SHOWED ICE.