Narrative:

I was flying IFR from plb to pne. Conditions were IFR the entire time. We climbed to 8000 ft, picked up light rime and continued up to 11000 ft. During this period we had intermittent carburetor icing problems and decided to divert to albany because of this. As we descended through 7000 ft, we found temperatures above freezing and moderate to heavy rain. While being vectored for an ILS runway 1 approach, at 4000 ft, we began to have more severe carburetor icing -- the engine fluctuated from full power, to no power at all. It continued to surge. Applying carburetor heat only seemed to exacerbate the problem. At that point, we advised the tower of our urgent need to land. The controller was magnificent. When we requested SVFR, the controller issued it and assisted us with vectors to the airport. The controller also declared an emergency on our behalf. I contacted our mechanic while on the ground and we spoke about the problem we had. He told us that the only way to resolve this issue was to lean the mixture, and that we might need to lean it almost completely out while at the same time applying carburetor heat. A contributing factor was certainly the conditions I was flying in -- just above freezing temperatures and plenty of moisture. The need to lean the mixture upon applying carburetor heat in certain conditions will never be forgotten after this experience.

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Original NASA ASRS Text

Title: C170 PLT ENCOUNTERS CARB ICING AND DIVERTS TO AN ALTERNATE FIELD.

Narrative: I WAS FLYING IFR FROM PLB TO PNE. CONDITIONS WERE IFR THE ENTIRE TIME. WE CLBED TO 8000 FT, PICKED UP LIGHT RIME AND CONTINUED UP TO 11000 FT. DURING THIS PERIOD WE HAD INTERMITTENT CARB ICING PROBS AND DECIDED TO DIVERT TO ALBANY BECAUSE OF THIS. AS WE DSNDED THROUGH 7000 FT, WE FOUND TEMPS ABOVE FREEZING AND MODERATE TO HVY RAIN. WHILE BEING VECTORED FOR AN ILS RWY 1 APCH, AT 4000 FT, WE BEGAN TO HAVE MORE SEVERE CARB ICING -- THE ENG FLUCTUATED FROM FULL PWR, TO NO PWR AT ALL. IT CONTINUED TO SURGE. APPLYING CARB HEAT ONLY SEEMED TO EXACERBATE THE PROB. AT THAT POINT, WE ADVISED THE TWR OF OUR URGENT NEED TO LAND. THE CTLR WAS MAGNIFICENT. WHEN WE REQUESTED SVFR, THE CTLR ISSUED IT AND ASSISTED US WITH VECTORS TO THE ARPT. THE CTLR ALSO DECLARED AN EMER ON OUR BEHALF. I CONTACTED OUR MECH WHILE ON THE GND AND WE SPOKE ABOUT THE PROB WE HAD. HE TOLD US THAT THE ONLY WAY TO RESOLVE THIS ISSUE WAS TO LEAN THE MIXTURE, AND THAT WE MIGHT NEED TO LEAN IT ALMOST COMPLETELY OUT WHILE AT THE SAME TIME APPLYING CARB HEAT. A CONTRIBUTING FACTOR WAS CERTAINLY THE CONDITIONS I WAS FLYING IN -- JUST ABOVE FREEZING TEMPS AND PLENTY OF MOISTURE. THE NEED TO LEAN THE MIXTURE UPON APPLYING CARB HEAT IN CERTAIN CONDITIONS WILL NEVER BE FORGOTTEN AFTER THIS EXPERIENCE.

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.