Narrative:

While en route from bna to pfn on an IFR flight plan in an small transport at 9000', I encountered moderate to heavy mixed icing. I made a request to huntsville approach control to climb to 11000', which looked like it would put me on top of the clouds with clear conditions above. The controller replied by telling me that he had my request and a short while later I heard him say, 'climb and maintain 11000', but I'll have to get clearance if you want any higher.' I immediately started a climb at vy to 11000'. Somewhere around 10400' the controller asked me to verify altitude. I replied, and by then was at 10800'. The controller then stated that he cleared me to 10000' and told me that he would have to get clearance for me to go to 11000'. He instructed me to descend to 10000'. I did so and shortly thereafter was cleared to 11000'. In my mind, I heard the clearance to 11000'. Cockpit noise levels were greatly increased due to ice shedding of the propellers as well as from the forming of the ice. The workload in the cockpit was also at a peak due to the constant monitoring of the ic build-up and cycling of the de-icer boots which was required every 1-1 1/2 mins. Perhaps it was in the controller's explanation that I heard 11000' and that may be when I set my altitude pointer (mounted on altimeter) to 11000'. I'm sure the urgency of the situation had something to do with it, but I was sure that I was cleared to 11000'.

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Original NASA ASRS Text

Title: CORP PLT CLIMBS ABOVE ASSIGNED ALT. MISINTERPRETATION OF CLRNC.

Narrative: WHILE ENRTE FROM BNA TO PFN ON AN IFR FLT PLAN IN AN SMT AT 9000', I ENCOUNTERED MODERATE TO HEAVY MIXED ICING. I MADE A REQUEST TO HUNTSVILLE APCH CTL TO CLB TO 11000', WHICH LOOKED LIKE IT WOULD PUT ME ON TOP OF THE CLOUDS WITH CLR CONDITIONS ABOVE. THE CTLR REPLIED BY TELLING ME THAT HE HAD MY REQUEST AND A SHORT WHILE LATER I HEARD HIM SAY, 'CLB AND MAINTAIN 11000', BUT I'LL HAVE TO GET CLRNC IF YOU WANT ANY HIGHER.' I IMMEDIATELY STARTED A CLB AT VY TO 11000'. SOMEWHERE AROUND 10400' THE CTLR ASKED ME TO VERIFY ALT. I REPLIED, AND BY THEN WAS AT 10800'. THE CTLR THEN STATED THAT HE CLRED ME TO 10000' AND TOLD ME THAT HE WOULD HAVE TO GET CLRNC FOR ME TO GO TO 11000'. HE INSTRUCTED ME TO DSND TO 10000'. I DID SO AND SHORTLY THEREAFTER WAS CLRED TO 11000'. IN MY MIND, I HEARD THE CLRNC TO 11000'. COCKPIT NOISE LEVELS WERE GREATLY INCREASED DUE TO ICE SHEDDING OF THE PROPS AS WELL AS FROM THE FORMING OF THE ICE. THE WORKLOAD IN THE COCKPIT WAS ALSO AT A PEAK DUE TO THE CONSTANT MONITORING OF THE IC BUILD-UP AND CYCLING OF THE DE-ICER BOOTS WHICH WAS REQUIRED EVERY 1-1 1/2 MINS. PERHAPS IT WAS IN THE CTLR'S EXPLANATION THAT I HEARD 11000' AND THAT MAY BE WHEN I SET MY ALT POINTER (MOUNTED ON ALTIMETER) TO 11000'. I'M SURE THE URGENCY OF THE SITUATION HAD SOMETHING TO DO WITH IT, BUT I WAS SURE THAT I WAS CLRED TO 11000'.

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.