Narrative:

Altimeter needle was bouncing rapidly; I thought. Turned out to be the needles were winding down rapidly. I had a brief period of vertigo even in VFR conditions. After the needles settled down I thought that I was at 12500 ft. At sometime after passing over ZZZ1 I realized that I had been at 13500 ft; and at that point; I looked and the 10000 ft needle was at just above sea level. It was at that point that I realized that the altimeter had failed. Again after the needle stopped bouncing; I thought it was just a drop of water in the static system; and I wanted to climb to 14500 ft. After about 10 mins I was still at the same indicated altitude but the airplane was not acting normal. Low manifold pressure; higher cabin altitude and the clouds in front of me; that I expected to be above me; were below me. I then looked at the transponder which has pressure altitude readout was saying 18300 ft. I wasn't sure which instrument was correct; but I descended immediately to below 18000 ft indication. I could have landed at ZZZ1 but I wasn't sure which instrument was correct and felt that I would prefer to land at ZZZ2; if I didn't have altitude indications. I estimate that when I was on the 350 degree radial from abc runway 32 DME I was out of the class a airspace and was there for 3-8 mins.

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Original NASA ASRS Text

Title: A P210 ACFT ALTIMETER FAILED. THE PILOT CLBED ABOVE WX TO 14500 AND REALIZED HE WAS AT FL183 AFTER READING HIS TRANSPONDER ALTIMETER.

Narrative: ALTIMETER NEEDLE WAS BOUNCING RAPIDLY; I THOUGHT. TURNED OUT TO BE THE NEEDLES WERE WINDING DOWN RAPIDLY. I HAD A BRIEF PERIOD OF VERTIGO EVEN IN VFR CONDITIONS. AFTER THE NEEDLES SETTLED DOWN I THOUGHT THAT I WAS AT 12500 FT. AT SOMETIME AFTER PASSING OVER ZZZ1 I REALIZED THAT I HAD BEEN AT 13500 FT; AND AT THAT POINT; I LOOKED AND THE 10000 FT NEEDLE WAS AT JUST ABOVE SEA LEVEL. IT WAS AT THAT POINT THAT I REALIZED THAT THE ALTIMETER HAD FAILED. AGAIN AFTER THE NEEDLE STOPPED BOUNCING; I THOUGHT IT WAS JUST A DROP OF WATER IN THE STATIC SYS; AND I WANTED TO CLB TO 14500 FT. AFTER ABOUT 10 MINS I WAS STILL AT THE SAME INDICATED ALT BUT THE AIRPLANE WAS NOT ACTING NORMAL. LOW MANIFOLD PRESSURE; HIGHER CABIN ALT AND THE CLOUDS IN FRONT OF ME; THAT I EXPECTED TO BE ABOVE ME; WERE BELOW ME. I THEN LOOKED AT THE XPONDER WHICH HAS PRESSURE ALT READOUT WAS SAYING 18300 FT. I WASN'T SURE WHICH INST WAS CORRECT; BUT I DSNDED IMMEDIATELY TO BELOW 18000 FT INDICATION. I COULD HAVE LANDED AT ZZZ1 BUT I WASN'T SURE WHICH INST WAS CORRECT AND FELT THAT I WOULD PREFER TO LAND AT ZZZ2; IF I DIDN'T HAVE ALT INDICATIONS. I ESTIMATE THAT WHEN I WAS ON THE 350 DEG RADIAL FROM ABC RWY 32 DME I WAS OUT OF THE CLASS A AIRSPACE AND WAS THERE FOR 3-8 MINS.

Data retrieved from NASA's ASRS site as of January 2009 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.