Narrative:

Under the aforementioned conditions, my aircraft was traveling along the victor airway in level flight. The altimeter indicated level at 8000 ft MSL. Before takeoff, the altimeter was set to proper altimeter setting, at which time it indicated no measurable deviation from the known airport elevation. In flight an airline pilot reported having a 'visual' on my aircraft level at his altitude of 9000 ft MSL. Also, his TCASII reported me at 9000 ft. ATC advised me that my mode C was indicating 9000 ft. ATC asked me to verify my altimeter setting and altitude. The proper setting was already set in the altimeter, and the altimeter displayed 8000 ft MSL. I advised ATC of my altitude and also that I had no visual of the airline jet. The only conclusion I can draw is that, somehow, some moisture may have entered the static port the prior night. The airport was hit by severe thunderstorms the prior night. The old PA28 I was flying was not equipped with an alternate static air source (aside from smashing the vsi). The pitot-static system check is not due until jun/xx/95, so the aircraft system should have been functioning properly. No irregularities were found during the pre-flight inspection. Human factor played no part in the incident as I was well rested and fit for flight. Incidently, another person was on board the aircraft and verified my altimeter to display level at 8000 ft MSL. He too did not see the airline jet during the entire incident.

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Original NASA ASRS Text

Title: ALTDEV -- EQUIP FAILURE.

Narrative: UNDER THE AFOREMENTIONED CONDITIONS, MY ACFT WAS TRAVELING ALONG THE VICTOR AIRWAY IN LEVEL FLT. THE ALTIMETER INDICATED LEVEL AT 8000 FT MSL. BEFORE TKOF, THE ALTIMETER WAS SET TO PROPER ALTIMETER SETTING, AT WHICH TIME IT INDICATED NO MEASURABLE DEV FROM THE KNOWN ARPT ELEVATION. IN FLT AN AIRLINE PLT RPTED HAVING A 'VISUAL' ON MY ACFT LEVEL AT HIS ALT OF 9000 FT MSL. ALSO, HIS TCASII RPTED ME AT 9000 FT. ATC ADVISED ME THAT MY MODE C WAS INDICATING 9000 FT. ATC ASKED ME TO VERIFY MY ALTIMETER SETTING AND ALT. THE PROPER SETTING WAS ALREADY SET IN THE ALTIMETER, AND THE ALTIMETER DISPLAYED 8000 FT MSL. I ADVISED ATC OF MY ALT AND ALSO THAT I HAD NO VISUAL OF THE AIRLINE JET. THE ONLY CONCLUSION I CAN DRAW IS THAT, SOMEHOW, SOME MOISTURE MAY HAVE ENTERED THE STATIC PORT THE PRIOR NIGHT. THE ARPT WAS HIT BY SEVERE TSTMS THE PRIOR NIGHT. THE OLD PA28 I WAS FLYING WAS NOT EQUIPPED WITH AN ALTERNATE STATIC AIR SOURCE (ASIDE FROM SMASHING THE VSI). THE PITOT-STATIC SYS CHK IS NOT DUE UNTIL JUN/XX/95, SO THE ACFT SYS SHOULD HAVE BEEN FUNCTIONING PROPERLY. NO IRREGULARITIES WERE FOUND DURING THE PRE-FLT INSPECTION. HUMAN FACTOR PLAYED NO PART IN THE INCIDENT AS I WAS WELL RESTED AND FIT FOR FLT. INCIDENTLY, ANOTHER PERSON WAS ON BOARD THE ACFT AND VERIFIED MY ALTIMETER TO DISPLAY LEVEL AT 8000 FT MSL. HE TOO DID NOT SEE THE AIRLINE JET DURING THE ENTIRE INCIDENT.

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.