A LANCAIR IVP PLT ENCOUNTERED ICING AT FL250 AND CLBED 500 FT; PROMPTING ATC TO QUESTION ALT.

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ACFT IS A LANCAIR IVP WITH A PT6-20 TURBINE EXPERIMENTAL. I FUELED IN ZZZ1; CHKED THE WX WITH FSS BY PHONE AND FILED A FLT PLAN. THE WX IN ZZZ2 WAS CLEAR AND WITH A CHANCE OF SHOWERS; CEILINGS OF 3000 FT. I USED ZZZ3 FOR AN ALTERNATE; THE CURRENT AND FORECAST WAS CLEAR. FREEZING LEVEL WAS RPTED AS 18000 FT. SEVERAL TURB WARNINGS WERE ALSO ISSUED. I DID CHK THE WSI WX AT FBO IN ZZZ1 AND WAS AWARE OF THE SHOWERS IN THE NORTHERN PORTION OF THE AREA MOVING TO THE S. I FIGURED THAT I WOULD ARRIVE IN ZZZ2 45 MINS BEFORE THE SHOWERS WOULD HIT. THERE WERE NO ECHOES ALONG THE RTE. ALL THE WX TO THE S OF MY DEST WAS GOOD VFR. THE AREA FORECAST SHOWED TOPS AT 25000 FT. BASED ON MY FLT FROM ZZZ4 AND LNDG IN ZZZ1 40 MINS EARLIER; I THOUGHT THAT FL210 WOULD BE THE BEST ALT FOR BEING BETWEEN LAYERS AND A SMOOTH RIDE. I DEPARTED RWY 28 WITH A R TURN TO A HDG OF 65 DEGS AND 12000 FT. SHORTLY AFTER TURNING TO THE HDG; I WAS CLRED DIRECT ZZZ2 AND FL210. I PICKED UP A TRACE OF ICE AT APPROX 13000 FT. BY 15500 FT I WAS BETWEEN LAYERS AND SLOWED THE CLB TO LET THE ICE BURN OFF; WHICH IT DID BEFORE I HIT THE NEXT LAYER AT FL200. WHEN I ENTERED THIS LAYER THERE WAS MODERATE MIXED ICING AND BUILDING QUICKLY. WHILE CLBING; I HAD REQUESTED FL250 AND WAS CLRED TO FL250 BEFORE REACHING FL210. APPROX 23000 FT; I BROKE OUT OF A LAYER AND WAS CLEAR OF THE CLOUDS AT THE TIME. I WAS APCHING ANOTHER LARGE BUILDUP WHEN I REQUESTED A BLOCK FROM FL250 TO FL270. MY CLRNC WAS STILL FOR FL250 I WAS AT FL255; AND WHEN ASKED BY CTR RPTED SUCH. I SAID THAT I COULD MAKE A TURN BACK TO THE W OR TURN TO THE S. THE CTLR CAME BACK WITH A BLOCK FL250 TO FL260. IF I READ THAT BACK; IN MY MIND I WAS STILL GOING TO FL270 TO AVOID GOING INTO THE TOPS AHEAD. THINGS THAT CHANGED FROM THE BRIEFING TO THE FLT: THE FREEZING LEVEL WAS LOWER THAN FORECAST; IE; ICING STARTED AT 12000 FT. AS I WAS CLBING; THE INFLT RADAR UPDATED SHOWING US CLBING UP THE W SIDE OF A CELL THAT WAS APPROX 20 MILES IN DIAMETER. IT UPDATED WHEN I WAS IN THE CLB ABOVE 15000 FT. WHEN I SAW THAT; I KNEW THAT DSNDING BACK DOWN ON THAT HDG WOULD NOT WORK BASED ON THE LEVEL OF INTENSITY AND TEMPS. HAVING A VISUAL OF THE TOPS OF THE CELLS CONFIRM WHAT THE INFLT WX WAS SHOWING. I FELT I STILL HAD A COUPLE OF OPTIONS; BUT THE RISING CLOUD DECK WAS APCHING FASTER THAN I COULD GET THE CLRNCES NEEDED. 1) CIRCLE TO THE W AND REMAIN VISUALLY SEPARATED FROM THE CLOUDS AND WAIT FOR THE BLOCK CLRNC TO GO OVER THE TOP. 2) HEAD TO THE S AT THE EXISTING ASSIGNED ALT OF FL250. THIS WOULD HAVE BEEN BACK IN TO IMC AND FORECAST BETTER WX. 3) STAYED ON COURSE INTO THE BUILDUP. HOW IT WAS DISCOVERED: WHEN THE CTR CALLED ON MY ALT I KNEW I WAS HIGH FL255 VERSUS FL250 ASSIGNED. THE CTLR KEPT TRYING TO KEEP ME LEGAL BY ASSIGNING A BLOCK FL250 TO FL260. I SAY THAT NOT HAVING THE RECORDER TO REPLAY. I MAY HAVE READBACK THE CORRECT BLOCK BUT CONTINUED TO CLB TO FL270. THE DIFFERENCE BETWEEN WHAT I REQUESTED AND RECEIVED? CHAIN OF EVENTS: THE BUILDUP CAUGHT ME OFF GUARD AS I WAS EXPECTING JUST STRATUS LAYERS UNTIL I WAS FARTHER E THEN DSNDING TO BELOW THE FREEZING LEVEL FOR THE ARR INTO BISMARCK. THE INTENSITY OF THE CELL ON THE INFLT WX; THE RATE AT WHICH THE ICE BUILT IN THE LAST LAYER; THE FACT THAT I WAS CLBING INTO THE SIDE OF THE CELL; THE VISUAL LOOK THAT I HAD WHILE CLEAR OF CLOUDS. ALL PLAYED IN MY DECISION. I SHOULD OF ENGAGED THE AUTOPLT WHILE I WAS WORKING ON MY OPTIONS. THIS WOULD OF KEPT ME ON ALT AT FL250. AS FAR AS THE READBACK; I HAVE NO ANSWER IF I READ BACK THE CORRECT ALT. IT WAS MINDSET TO GO TO FL270. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT HIS DEV FROM FL250 TO FL255 WAS INTENTIONAL. HE WAS VERY CONCERNED ABOUT ICING ON HIS ACFT; WHICH HAS NO DE-ICE/ANTI-ICE CAPABILITY. OF PARTICULAR CONCERN WAS THE TURBINE INLET AIR PASSAGE ACCUMULATING ICE AND SUBSEQUENTLY STARVING THE ENG OF AIR. HE SAID IN HIS ACFT AT THE TIME OF HIS ALT TRANSGRESSION; THE WORKLOAD WAS VERY HIGH BECAUSE THEPDF DISPLAY DOES NOT INTEGRATE CURRENT WX DATA. THEREFORE; THE PLT MUST MULTITASK; SWITCHING BETWEEN THE WX DISPLAY AND THE NAV DISPLAY. THE NEWEST VERSION OF THE LANCAIR; NAMED COLUMBIA; IS ESSENTIALLY THE SAME ACFT BUT WITH INTEGRATED FLT DATA PRESENTATION. THE NEWER VERSION IS MUCH LESS DEMANDING ON THE PLT BECAUSE THE NECESSARY INFO IS PRESENTED MORE CLEARLY.