Narrative:

Departed IFR from oso airport. Reported WX along route was from 55 scattered to 150 overcast and en route changing to 250 overcast with no reported icing forecast etc. At 5000 ft we were in and out of the layer with continuous light chop. I chose to climb to 7000. 10 mins later was in another layer of clouds with small accumulating light clear ice. Opted to go to 9000 to get on top. Solid clouds to 9000 and received very quick accumulation of moderate ice. No one else in area reporting any ice. Aircraft was not performing properly. Asked for lowest flight level and if this stuff was still solid below or scattered as forecast. Decided to get accumulation of ice off aircraft and check WX. Ask for vectors to nearest airport (not as an emergency). We vectored to mgw where aircraft lost most of the ice at 5000 ft but wanted to still check WX and aircraft instead of continuing. Mgw FSS reported pittsburg radar had a small 5-10 mi cell we must have hit and at the time no one else had ice problems. I followed forecast properly and took proper action to climb when I encountered thin clear ice in thin layer, however, unexpected and unforecasted solid layer ruined that safety action and when I realized it wasn't worth trying to get on top (clouds weren't supposed to be there) I had to go back down through ice to lower flight level. Which was forecast to be clear. This was not an ATC emergency but in my judgement was a precautionary diversion of flight to keep from having an emergency.

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Original NASA ASRS Text

Title: SMA HAS ICE BUILD UP, RECEIVES VECTORS TO DSND BELOW FREEZING LEVEL AND TO CLRER AREA.

Narrative: DEPARTED IFR FROM OSO ARPT. RPTED WX ALONG RTE WAS FROM 55 SCATTERED TO 150 OVCST AND ENRTE CHANGING TO 250 OVCST WITH NO RPTED ICING FORECAST ETC. AT 5000 FT WE WERE IN AND OUT OF THE LAYER WITH CONTINUOUS LIGHT CHOP. I CHOSE TO CLB TO 7000. 10 MINS LATER WAS IN ANOTHER LAYER OF CLOUDS WITH SMALL ACCUMULATING LIGHT CLR ICE. OPTED TO GO TO 9000 TO GET ON TOP. SOLID CLOUDS TO 9000 AND RECEIVED VERY QUICK ACCUMULATION OF MODERATE ICE. NO ONE ELSE IN AREA RPTING ANY ICE. ACFT WAS NOT PERFORMING PROPERLY. ASKED FOR LOWEST FLT LEVEL AND IF THIS STUFF WAS STILL SOLID BELOW OR SCATTERED AS FORECAST. DECIDED TO GET ACCUMULATION OF ICE OFF ACFT AND CHK WX. ASK FOR VECTORS TO NEAREST ARPT (NOT AS AN EMER). WE VECTORED TO MGW WHERE ACFT LOST MOST OF THE ICE AT 5000 FT BUT WANTED TO STILL CHK WX AND ACFT INSTEAD OF CONTINUING. MGW FSS RPTED PITTSBURG RADAR HAD A SMALL 5-10 MI CELL WE MUST HAVE HIT AND AT THE TIME NO ONE ELSE HAD ICE PROBLEMS. I FOLLOWED FORECAST PROPERLY AND TOOK PROPER ACTION TO CLB WHEN I ENCOUNTERED THIN CLR ICE IN THIN LAYER, HOWEVER, UNEXPECTED AND UNFORECASTED SOLID LAYER RUINED THAT SAFETY ACTION AND WHEN I REALIZED IT WASN'T WORTH TRYING TO GET ON TOP (CLOUDS WEREN'T SUPPOSED TO BE THERE) I HAD TO GO BACK DOWN THROUGH ICE TO LOWER FLT LEVEL. WHICH WAS FORECAST TO BE CLR. THIS WAS NOT AN ATC EMER BUT IN MY JUDGEMENT WAS A PRECAUTIONARY DIVERSION OF FLT TO KEEP FROM HAVING AN EMER.

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.