Narrative:

I was inbound on the localizer 15 to mtn, and was past the FAF, I believe 12.4 DME at 2500 ft. Crossing this fix I had a step down altitude of 2000 ft until 6.6 DME. I descended to 2000 ft and was told by approach to contact tower. At 10 DME I contacted tower, who told me to climb to 2500 ft and contact approach. In doing so approach told me they showed me 500 ft low on the altimeter. I was also told that there was a tower out there that they needed to keep me away from. I advised them I was at 2000 ft and climbing. Looking at the copilot's side I noticed a difference in altitudes. I tapped on my altimeter and the needle suddenly jumped up. Unknown how/when or why this occurred. I thought I had iced up the pitot tube, but this was not the case. I reset a circuit breaker while being vectored for another approach, but I later learned it was not associated with the pitot. The second approach was uneventful. I could not reproduce the problem, and it did not show up using alternate static source. However, on my last leg going home, I did get some needle wavering. I advised our avionics maintenance people and the pilot's side altimeter was changed and the lines were drained.

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Original NASA ASRS Text

Title: PLT OF AN ATX CARGO ACFT FAILED TO MAINTAIN APCH PROC ALT DUE TO ERROR IN INTERRUPTING THE PROC AND A POSSIBLE FAULTY ALTIMETER.

Narrative: I WAS INBOUND ON THE LOC 15 TO MTN, AND WAS PAST THE FAF, I BELIEVE 12.4 DME AT 2500 FT. XING THIS FIX I HAD A STEP DOWN ALT OF 2000 FT UNTIL 6.6 DME. I DSNDED TO 2000 FT AND WAS TOLD BY APCH TO CONTACT TWR. AT 10 DME I CONTACTED TWR, WHO TOLD ME TO CLB TO 2500 FT AND CONTACT APCH. IN DOING SO APCH TOLD ME THEY SHOWED ME 500 FT LOW ON THE ALTIMETER. I WAS ALSO TOLD THAT THERE WAS A TWR OUT THERE THAT THEY NEEDED TO KEEP ME AWAY FROM. I ADVISED THEM I WAS AT 2000 FT AND CLBING. LOOKING AT THE COPLT'S SIDE I NOTICED A DIFFERENCE IN ALTS. I TAPPED ON MY ALTIMETER AND THE NEEDLE SUDDENLY JUMPED UP. UNKNOWN HOW/WHEN OR WHY THIS OCCURRED. I THOUGHT I HAD ICED UP THE PITOT TUBE, BUT THIS WAS NOT THE CASE. I RESET A CIRCUIT BREAKER WHILE BEING VECTORED FOR ANOTHER APCH, BUT I LATER LEARNED IT WAS NOT ASSOCIATED WITH THE PITOT. THE SECOND APCH WAS UNEVENTFUL. I COULD NOT REPRODUCE THE PROB, AND IT DID NOT SHOW UP USING ALTERNATE STATIC SOURCE. HOWEVER, ON MY LAST LEG GOING HOME, I DID GET SOME NEEDLE WAVERING. I ADVISED OUR AVIONICS MAINT PEOPLE AND THE PLT'S SIDE ALTIMETER WAS CHANGED AND THE LINES WERE DRAINED.

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.