Narrative:

On this flight we were cruising at 15000 ft MSL, mostly IMC, when we approached an area of a cluster of buildups just west of fwa. We observed our radar for a possible path through the area and requested/received permission to deviate from ZAU. Very rapidly we found ourselves in a literal mine field of buildups that were either not registering on our WX radar or were masked by other buildups or attenuated by the heavy precipitation we encountered. We started to experience altdevs plus or minus 300 ft. ZAU offered us a block altitude if we wished. We requested 14000- 16000 ft and it was immediately given. Our deviations regained constantly changing headings from approximately 020 degrees to 180 degrees as we progressed eastbound to reestablish on the dtw mizar 1 arrival via the fwa transition. ZAU conferred with fwa approach radar and advised that if we proceeded northeast for 10 mi we would exit the area of thunderstorms. We advised we were 'working our way' in that direction. We finally cleared the area of thunderstorms and advised we could now maintain 15000 ft and take a heading to rejoin the mizar 1 arrival. We were cleared to maintain 15000 ft and given a heading to intercept the fwa radial. I believe we complied with ATC clrncs but we were not aware that this WX area, that appeared as many we have routinely transitted, was so severe. Hindsight might ask why we didn't turn back to ind, but we had just come through very similar conditions to that point, and had no reason to suspect the extreme change -- especially since the radar didn't depict any more than it did.

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Original NASA ASRS Text

Title: AN ACR SMT GOT CAUGHT IN HVY TSTM WX REQUIRING MANY DEVS AND A BLOCK ALT.

Narrative: ON THIS FLT WE WERE CRUISING AT 15000 FT MSL, MOSTLY IMC, WHEN WE APCHED AN AREA OF A CLUSTER OF BUILDUPS JUST W OF FWA. WE OBSERVED OUR RADAR FOR A POSSIBLE PATH THROUGH THE AREA AND REQUESTED/RECEIVED PERMISSION TO DEVIATE FROM ZAU. VERY RAPIDLY WE FOUND OURSELVES IN A LITERAL MINE FIELD OF BUILDUPS THAT WERE EITHER NOT REGISTERING ON OUR WX RADAR OR WERE MASKED BY OTHER BUILDUPS OR ATTENUATED BY THE HVY PRECIPITATION WE ENCOUNTERED. WE STARTED TO EXPERIENCE ALTDEVS PLUS OR MINUS 300 FT. ZAU OFFERED US A BLOCK ALT IF WE WISHED. WE REQUESTED 14000- 16000 FT AND IT WAS IMMEDIATELY GIVEN. OUR DEVS REGAINED CONSTANTLY CHANGING HDGS FROM APPROX 020 DEGS TO 180 DEGS AS WE PROGRESSED EBOUND TO REESTABLISH ON THE DTW MIZAR 1 ARR VIA THE FWA TRANSITION. ZAU CONFERRED WITH FWA APCH RADAR AND ADVISED THAT IF WE PROCEEDED NE FOR 10 MI WE WOULD EXIT THE AREA OF TSTMS. WE ADVISED WE WERE 'WORKING OUR WAY' IN THAT DIRECTION. WE FINALLY CLRED THE AREA OF TSTMS AND ADVISED WE COULD NOW MAINTAIN 15000 FT AND TAKE A HDG TO REJOIN THE MIZAR 1 ARR. WE WERE CLRED TO MAINTAIN 15000 FT AND GIVEN A HDG TO INTERCEPT THE FWA RADIAL. I BELIEVE WE COMPLIED WITH ATC CLRNCS BUT WE WERE NOT AWARE THAT THIS WX AREA, THAT APPEARED AS MANY WE HAVE ROUTINELY TRANSITTED, WAS SO SEVERE. HINDSIGHT MIGHT ASK WHY WE DIDN'T TURN BACK TO IND, BUT WE HAD JUST COME THROUGH VERY SIMILAR CONDITIONS TO THAT POINT, AND HAD NO REASON TO SUSPECT THE EXTREME CHANGE -- ESPECIALLY SINCE THE RADAR DIDN'T DEPICT ANY MORE THAN IT DID.

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.