Narrative:

We were cruising at 11000 ft MSL in and out of the clouds on V-458. Although there was no airmet for icing over the route, light rime icing had been reported by a pilot at this altitude and location 2 hours previously, so I was watching the OAT closely. We had experienced very slight rime and had requested climb to 13000 ft to clear the cloud layer. ATC was unable to climb us and, as we entered a dark cloud, we experienced forecast freezing rain, rapid ice buildup and continuous moderate turbulence. I requested and was granted a 180 degree turn. We also requested and were cleared to descend to 9000 ft MSL. As we completed the turn, ATC advised that if we wished to continue left we would need to declare an emergency. As I was still above 10000 ft and VMC was available on my left while the same clouds we had just encountered were on my right, I elected to declare an emergency to enable the controller to grant my heading request. We remained VMC while descending and selecting another airway to our destination, then requested vectors to the airway and continued at 9000 ft, experiencing rain without icing. I learned from this to take PIREPS seriously, and plan an alternate route when icing is reported or forecast, so as to remain below the freezing level. I know any emergency dramatically increases controller workload and impacts other aircraft who have not declared, and will try to avoid sits, which could lead to having to declare an emergency in the future. I appreciated the excellent help given by the controllers as I recovered from this misadventure.

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Original NASA ASRS Text

Title: M20 PLT ENCOUNTERS RAPID ICE BUILDUP AND DECLARES AN EMER IN ORDER TO FACILITATE CONTINUED DEV OUT OF ICING CONDITIONS.

Narrative: WE WERE CRUISING AT 11000 FT MSL IN AND OUT OF THE CLOUDS ON V-458. ALTHOUGH THERE WAS NO AIRMET FOR ICING OVER THE RTE, LIGHT RIME ICING HAD BEEN RPTED BY A PLT AT THIS ALT AND LOCATION 2 HRS PREVIOUSLY, SO I WAS WATCHING THE OAT CLOSELY. WE HAD EXPERIENCED VERY SLIGHT RIME AND HAD REQUESTED CLB TO 13000 FT TO CLR THE CLOUD LAYER. ATC WAS UNABLE TO CLB US AND, AS WE ENTERED A DARK CLOUD, WE EXPERIENCED FORECAST FREEZING RAIN, RAPID ICE BUILDUP AND CONTINUOUS MODERATE TURB. I REQUESTED AND WAS GRANTED A 180 DEG TURN. WE ALSO REQUESTED AND WERE CLRED TO DSND TO 9000 FT MSL. AS WE COMPLETED THE TURN, ATC ADVISED THAT IF WE WISHED TO CONTINUE L WE WOULD NEED TO DECLARE AN EMER. AS I WAS STILL ABOVE 10000 FT AND VMC WAS AVAILABLE ON MY L WHILE THE SAME CLOUDS WE HAD JUST ENCOUNTERED WERE ON MY R, I ELECTED TO DECLARE AN EMER TO ENABLE THE CTLR TO GRANT MY HDG REQUEST. WE REMAINED VMC WHILE DSNDING AND SELECTING ANOTHER AIRWAY TO OUR DEST, THEN REQUESTED VECTORS TO THE AIRWAY AND CONTINUED AT 9000 FT, EXPERIENCING RAIN WITHOUT ICING. I LEARNED FROM THIS TO TAKE PIREPS SERIOUSLY, AND PLAN AN ALTERNATE RTE WHEN ICING IS RPTED OR FORECAST, SO AS TO REMAIN BELOW THE FREEZING LEVEL. I KNOW ANY EMER DRAMATICALLY INCREASES CTLR WORKLOAD AND IMPACTS OTHER ACFT WHO HAVE NOT DECLARED, AND WILL TRY TO AVOID SITS, WHICH COULD LEAD TO HAVING TO DECLARE AN EMER IN THE FUTURE. I APPRECIATED THE EXCELLENT HELP GIVEN BY THE CTLRS AS I RECOVERED FROM THIS MISADVENTURE.

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.