Narrative:

IFR flight to ZZZ; VFR conditions most of way with numerous contingency airfields en route. Passed over ZZZ1 noting VFR conditions there. I had received current WX for ZZZ2 from FSS about 25 mins prior; WX above minimums with lower clouds and higher overcast. Flight continued northeast of ZZZ1 between layers; overcast above; broken clouds about 2000 ft below; with no precipitation encountered. Unable to hear the ZZZ AWOS-3 WX report. Unfortunately this AWOS shares frequency with that of ZZZ3 and is overpowered/unreadable until close to ZZZ. We first received the WX from ZZZ2; well above minimums; and continued towards ZZZ originally at 15000 ft MSL (filed altitude) we stepped down to 14000 ft MSL then 13000 ft MSL per ATC clearance. About the time ZZZ AWOS became readable our flight entered solid overcast IMC conditions with severe icing. All de-ice/anti-ice equipment including boots/hot propellers/windscreen protection were functioning normally but it was apparent that the conditions encountered were rapidly overwhelming the aircraft capability. The altered flight characteristics; burble; airframe vibration and general control deterioration from ice buildup required immediate acton. I turned off/disconnected the autoplt and turned 180 degrees while descending to 12000 ft MSL the MVA in search of warmer air and to fly out of the IMC/icing conditions we had entered. While turning/descending I declared an emergency with center and apprised the controller of our change in course/altitude. The aircraft flew out of IMC conditions at 12000 ft and we terminated our emergency status with center. ATC then handed us off to ZZZ1 approach control and we landed at ZZZZ1 uneventfully. While inbound approach control took down the icing information from our experience/passed it along/resulting in an icing SIGMET issuance. How the problem arose: I am experienced in flying in mountain WX having lived in ZZZ for 10 yrs. The WX conditions at ZZZ are often different than ZZZ2 even though only 20 NM apart. We continued our flight towards ZZZ to hear the AWOS-3 aware that diversion to ZZZ1 was always possible if ZZZ was below minimums or unavailable due to WX conditions. Contributing factors: ZZZ3 AWOS operates on the same frequency with either higher power or much better antenna location than ZZZ. One has to fly close to ZZZ to get current WX. Human performance considerations: it is important to realize and recognize that the de-ice/anti-ice capability of propeller driven GA aircraft is inadequate to handle severe icing sits. I am convinced our immediate change in direction to reverse course was the correct response to WX encountered. Always have an 'out' or plan 'B' if WX conditions are different from forecast. Recommendations: often far better WX conditions exist west of the continental divide in colorado. It would benefit all aviators to be able to hear current AWOS/ASOS WX as broadcast from the many airfields with this capability before entering the mountainous terrain. If necessary use repeaters located up high to ensure current field conditions are receivable to assist pilots in early decision making. This early information will allow for delay (holding) or diversion in WX outside mountainous terrain.

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Original NASA ASRS Text

Title: C340 PILOT REPORTS ENCOUNTERING SEVERE ICING CONDITIONS AT 13000 FT OVER THE ROCKIES. AN IMMEDIATE COURSE REVERSAL AND DESCENT IS INITIATED. AN EMERGENCY IS DECLARED AND ATC IS ADVISED OF CONDITIONS AND NEW DESTINATION.

Narrative: IFR FLT TO ZZZ; VFR CONDITIONS MOST OF WAY WITH NUMEROUS CONTINGENCY AIRFIELDS ENRTE. PASSED OVER ZZZ1 NOTING VFR CONDITIONS THERE. I HAD RECEIVED CURRENT WX FOR ZZZ2 FROM FSS ABOUT 25 MINS PRIOR; WX ABOVE MINIMUMS WITH LOWER CLOUDS AND HIGHER OVCST. FLT CONTINUED NE OF ZZZ1 BTWN LAYERS; OVCST ABOVE; BROKEN CLOUDS ABOUT 2000 FT BELOW; WITH NO PRECIP ENCOUNTERED. UNABLE TO HEAR THE ZZZ AWOS-3 WX RPT. UNFORTUNATELY THIS AWOS SHARES FREQ WITH THAT OF ZZZ3 AND IS OVERPOWERED/UNREADABLE UNTIL CLOSE TO ZZZ. WE FIRST RECEIVED THE WX FROM ZZZ2; WELL ABOVE MINIMUMS; AND CONTINUED TOWARDS ZZZ ORIGINALLY AT 15000 FT MSL (FILED ALT) WE STEPPED DOWN TO 14000 FT MSL THEN 13000 FT MSL PER ATC CLRNC. ABOUT THE TIME ZZZ AWOS BECAME READABLE OUR FLT ENTERED SOLID OVCST IMC CONDITIONS WITH SEVERE ICING. ALL DE-ICE/ANTI-ICE EQUIP INCLUDING BOOTS/HOT PROPS/WINDSCREEN PROTECTION WERE FUNCTIONING NORMALLY BUT IT WAS APPARENT THAT THE CONDITIONS ENCOUNTERED WERE RAPIDLY OVERWHELMING THE ACFT CAPABILITY. THE ALTERED FLT CHARACTERISTICS; BURBLE; AIRFRAME VIBRATION AND GENERAL CTL DETERIORATION FROM ICE BUILDUP REQUIRED IMMEDIATE ACTON. I TURNED OFF/DISCONNECTED THE AUTOPLT AND TURNED 180 DEGS WHILE DSNDING TO 12000 FT MSL THE MVA IN SEARCH OF WARMER AIR AND TO FLY OUT OF THE IMC/ICING CONDITIONS WE HAD ENTERED. WHILE TURNING/DSNDING I DECLARED AN EMER WITH CTR AND APPRISED THE CTLR OF OUR CHANGE IN COURSE/ALT. THE ACFT FLEW OUT OF IMC CONDITIONS AT 12000 FT AND WE TERMINATED OUR EMER STATUS WITH CTR. ATC THEN HANDED US OFF TO ZZZ1 APCH CTL AND WE LANDED AT ZZZZ1 UNEVENTFULLY. WHILE INBOUND APCH CTL TOOK DOWN THE ICING INFO FROM OUR EXPERIENCE/PASSED IT ALONG/RESULTING IN AN ICING SIGMET ISSUANCE. HOW THE PROB AROSE: I AM EXPERIENCED IN FLYING IN MOUNTAIN WX HAVING LIVED IN ZZZ FOR 10 YRS. THE WX CONDITIONS AT ZZZ ARE OFTEN DIFFERENT THAN ZZZ2 EVEN THOUGH ONLY 20 NM APART. WE CONTINUED OUR FLT TOWARDS ZZZ TO HEAR THE AWOS-3 AWARE THAT DIVERSION TO ZZZ1 WAS ALWAYS POSSIBLE IF ZZZ WAS BELOW MINIMUMS OR UNAVAILABLE DUE TO WX CONDITIONS. CONTRIBUTING FACTORS: ZZZ3 AWOS OPERATES ON THE SAME FREQ WITH EITHER HIGHER PWR OR MUCH BETTER ANTENNA LOCATION THAN ZZZ. ONE HAS TO FLY CLOSE TO ZZZ TO GET CURRENT WX. HUMAN PERFORMANCE CONSIDERATIONS: IT IS IMPORTANT TO REALIZE AND RECOGNIZE THAT THE DE-ICE/ANTI-ICE CAPABILITY OF PROP DRIVEN GA ACFT IS INADEQUATE TO HANDLE SEVERE ICING SITS. I AM CONVINCED OUR IMMEDIATE CHANGE IN DIRECTION TO REVERSE COURSE WAS THE CORRECT RESPONSE TO WX ENCOUNTERED. ALWAYS HAVE AN 'OUT' OR PLAN 'B' IF WX CONDITIONS ARE DIFFERENT FROM FORECAST. RECOMMENDATIONS: OFTEN FAR BETTER WX CONDITIONS EXIST W OF THE CONTINENTAL DIVIDE IN COLORADO. IT WOULD BENEFIT ALL AVIATORS TO BE ABLE TO HEAR CURRENT AWOS/ASOS WX AS BROADCAST FROM THE MANY AIRFIELDS WITH THIS CAPABILITY BEFORE ENTERING THE MOUNTAINOUS TERRAIN. IF NECESSARY USE REPEATERS LOCATED UP HIGH TO ENSURE CURRENT FIELD CONDITIONS ARE RECEIVABLE TO ASSIST PLTS IN EARLY DECISION MAKING. THIS EARLY INFO WILL ALLOW FOR DELAY (HOLDING) OR DIVERSION IN WX OUTSIDE MOUNTAINOUS TERRAIN.

Data retrieved from NASA's ASRS site as of May 2009 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.