Narrative:

Departed sgj on an IFR flight plan for fay, nc. Preflight WX brief indicated 30 scattered to broken 80 overcast for route and lr- until sav, GA. Soon after departure and near crg, entered IMC. Encountered light to moderate turbulence. Other aircraft in area reporting turbulence and 200-300 ft altdevs. After crg, I encountered moderate turbulence with inability to maintain altitude. VMC/IMC. ATC asked about flight conditions. Stated moderate turbulence with altdevs. I asked for heading deviations to avoid buildups. Moderate to heavy rain developed. Entered into IMC conditions. Upon exiting a cloud bank entered a small area (1-2 mi) of VMC. Noticed an unusually dark buildup with billowing sides and bases that possibly contained moderate to severe turbulence. This was directly in our path. Immediately asked ATC for 30-40 degree west deviations (we were heading nne). ATC approved. Encountered 300 ft altdevs due to updrafts/downdrafts. I initiated a 180 degree turn back to east without a clearance. In the turn told ATC what I was doing. I told him I was doing 'a 360' (should have said 180 degree). ATC said 'never to do that again, traffic may have been intrail.' a SIGMET was broadcast for moderate to severe turbulence from chs-jax-alma. I don't remember if that was before or after the event, but very near the same time. We were stormscope equipped which showed activity behind but never in our path. ATC was reporting level 3 activity. Cause of problem: surprised by WX different from forecast. No reports of turbulence, thunderstorms, ice. In my judgement, an immediate deviation was necessary but I forgot to ask first. ATC was correct. I did have 30 seconds to ask.

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Original NASA ASRS Text

Title: HDG TRACK DEV BY SMA PLT WHEN IN PROX OF TSTM ACTIVITY.

Narrative: DEPARTED SGJ ON AN IFR FLT PLAN FOR FAY, NC. PREFLT WX BRIEF INDICATED 30 SCATTERED TO BROKEN 80 OVCST FOR RTE AND LR- UNTIL SAV, GA. SOON AFTER DEP AND NEAR CRG, ENTERED IMC. ENCOUNTERED LIGHT TO MODERATE TURB. OTHER ACFT IN AREA RPTING TURB AND 200-300 FT ALTDEVS. AFTER CRG, I ENCOUNTERED MODERATE TURB WITH INABILITY TO MAINTAIN ALT. VMC/IMC. ATC ASKED ABOUT FLT CONDITIONS. STATED MODERATE TURB WITH ALTDEVS. I ASKED FOR HDG DEVS TO AVOID BUILDUPS. MODERATE TO HVY RAIN DEVELOPED. ENTERED INTO IMC CONDITIONS. UPON EXITING A CLOUD BANK ENTERED A SMALL AREA (1-2 MI) OF VMC. NOTICED AN UNUSUALLY DARK BUILDUP WITH BILLOWING SIDES AND BASES THAT POSSIBLY CONTAINED MODERATE TO SEVERE TURB. THIS WAS DIRECTLY IN OUR PATH. IMMEDIATELY ASKED ATC FOR 30-40 DEG W DEVS (WE WERE HDG NNE). ATC APPROVED. ENCOUNTERED 300 FT ALTDEVS DUE TO UPDRAFTS/DOWNDRAFTS. I INITIATED A 180 DEG TURN BACK TO E WITHOUT A CLRNC. IN THE TURN TOLD ATC WHAT I WAS DOING. I TOLD HIM I WAS DOING 'A 360' (SHOULD HAVE SAID 180 DEG). ATC SAID 'NEVER TO DO THAT AGAIN, TFC MAY HAVE BEEN INTRAIL.' A SIGMET WAS BROADCAST FOR MODERATE TO SEVERE TURB FROM CHS-JAX-ALMA. I DON'T REMEMBER IF THAT WAS BEFORE OR AFTER THE EVENT, BUT VERY NEAR THE SAME TIME. WE WERE STORMSCOPE EQUIPPED WHICH SHOWED ACTIVITY BEHIND BUT NEVER IN OUR PATH. ATC WAS RPTING LEVEL 3 ACTIVITY. CAUSE OF PROB: SURPRISED BY WX DIFFERENT FROM FORECAST. NO RPTS OF TURB, TSTMS, ICE. IN MY JUDGEMENT, AN IMMEDIATE DEV WAS NECESSARY BUT I FORGOT TO ASK FIRST. ATC WAS CORRECT. I DID HAVE 30 SECONDS TO ASK.

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.