PLT DEVIATED FROM CLRNC COURSE WHEN HE SPIRALLED IN DSCNT TO AVOID RAPIDLY BUILDING CUMULUS; WHILE ON AN IFR FLT PLAN.

Narrative

WHILE ON AN IFR FLT PLAN IN A SINGLE ENG; LOW WING 180 HP ACFT FROM SMITH MOUNTAIN LAKE ARPT AT W91; VA; TO MQI; NC; AT THE ASSIGNED ALT OF 7000 FT; I OBSERVED RATHER RAPIDLY BUILDING CUMULUS AHEAD. I REQUESTED 9000 FT AND RECEIVED APPROVAL. AT 8500 FT IT BECAME APPARENT THAT I WOULD NOT CLR THE TOPS AND REALLY DID NOT CARE TO FLY INTO THE BUILDING CLOUDS IF IT COULD BE AVOIDED; SO I REQUESTED BACK DOWN TO 7000 FT AND WAS APPROVED FOR A BLOCK BTWN 8500 FT AND 7000 FT. AT 7000 FT I REQUESTED 5000 FT AND WAS APPROVED TO DSND FROM 7000 FT TO 3000 FT. AS I WAS GETTING VERY CLOSE TO THE OMINOUS LOOKING CLOUDS I SPIRALED DOWN TO AVOID ENTERING THE CHURNING CLOUDS. AT 3500 FT; ATC QUERIED ME AS TO MY HDG -- SAID HE WAS GETTING A STRANGE INDICATION. (I WAS ON COURSE BUT APPARENTLY SHOWING NO FORWARD MOTION.) I ADVISED ATC THAT I WAS SPIRALING DOWN TO AVOID ENTERING THE BLACK CHURNING CLOUDS. ATC RESPONDED; 'THAT IS OK BUT 'I' SHOULD HAVE ADVISED THEM BEFORE DOING IT.' AT MY ASSIGNED DSCNT ALT OF 3000 FT I WAS BELOW THE CLOUDS AND ROLLED OUT ON COURSE HDG AND PROCEEDED ON THE FLT AT THE ASSIGNED ALT OF 3000 FT. REST OF THE TRIP WAS UNEVENTFUL AND LANDED AT MQI; NC.