AN SMA PLT FAILED TO SET HIS ALTIMETER WHEN CLBING INTO CLASS A AIRSPACE RESULTING IN AN ALTDEV.

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CRUISING AT 16500 FT VFR DIRECT TO BEND; OR; WERE ISSUED AN IFR CLRNC TO MAINTAIN FL180. AFTER CLBING TO FL180; CTR CLRED US PRESENT POS DIRECT TO THE BAM 085 DEG 30 DME FIX. THIS TOOK US A CONSIDERABLE DISTANCE OFF COURSE TO THE E IN ORDER TO AVOID THE AUSTIN MOA. CTR ADVISED THAT FL250 WOULD BE THE MINIMUM ALT TO REMAIN ON OUR FILED RTE. AS WE WERE NEAR OUR MAX ALT; A TURN WAS INITIATED TOWARD THE CLRED FIX. WE WERE IMMEDIATELY CONFRONTED WITH TWRING CUMULUS WHICH WITH PREVAILING CONVECTIVE ACTION; AND AMBIENT TEMPS IN THE ICING RANGE; WOULD HAVE ALMOST CERTAINLY PRESENTED ICING RISK. THE TURN TO THE CLRED FIX WAS DELAYED A FEW MOMENTS TO AVOID ENTERING CLOUD. RADIO TFC BTWN CTR AND OTHER ACFT PREVENTED OUR ADVISING CTR OF OUR CONDITION. THE CTLR TOOK US TO TASK FOR NOT COMPLYING IN A MORE TIMELY MANNER. HE ALSO RPTED OUR ALT READOUT TO SHOW FL184. THIS WAS RESOLVED BY SETTING 29.92 IN THE KOLLSMAN AND AIR DATA COMPUTER. EXTENSIVE CONVECTIVE CLOUD COVERAGE AND INCREASING HEIGHTS IN OUR INTENDED DIRECTION OF FLT RESULTED IN MAKING THE DECISION TO CANCEL IFR AND DSND THROUGH THE BROKEN LAYER. UPON SIMULTANEOUSLY CANCELING IFR AS WE DSNDED FROM FL180; THE CTLR STATED THAT WE COULD NOT CANCEL IFR 'IN CLASS B AIRSPACE.' I BELIEVE HE MEANT CLASS A AIRSPACE; HOWEVER; I REPLIED THAT WE WERE OUT OF FL180 AT THE MOMENT OF CANCELING. I FEEL BADLY IF THE CTLR BECAME UPSET OVER THE INCIDENT. I WOULD HAVE FELT A LOT WORSE HAD WE SPUN OUT OF THE CLOUDS WITH A LOAD OF ICE. THE REMAINDER OF THE TRIP TO OUR DEST WAS UNEVENTFUL IN VMC BELOW THE CLOUDS. THE OBVIOUS LESSON HERE IS TO NOT GET INTO THE POS WHERE YOU ARE AT MAX PERFORMANCE ALT FOR YOUR ACFT AND WHERE MORE ALT MIGHT BE NEEDED TO STAY OUT OF TROUBLE.