One altimeter gives erroneous readings while at cruise.

Attributes
ACN	171319
Time
Date	199102
Day	Mon
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : crw
State Reference	WV
Altitude	msl bound lower : 12000 msl bound upper : 12500
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	artcc : zid
Operator	general aviation : corporate
Make Model Name	Small Transport, Low Wing, 2 Recip Eng
Flight Phase	cruise other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : cfi pilot : atp
Experience	flight time last 90 days : 100 flight time total : 3500 flight time type : 1000
ASRS Report	171319
Person 2
Affiliation	government : faa
Function	controller : radar
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : excursion from assigned altitude non adherence : clearance
Independent Detector	atc equipment other atc equipment : unspecified other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

While cruising at 12000' over charleston, wv, I noticed my altimeter (pilot side) showing a descending trend. A x-chk with the copilot altimeter revealed that 1 unit was displaying an incorrect altitude. I reported my problem to center and asked for an altitude readout. Center still showed me level at 12000'. Suddenly the pilot side altimeter increased to 12500' in about 2-3 seconds and the digitizer reported this to center. Center called and told me it was showing 12500'. Glancing over to the copilot side I noticed the copilot altimeter now reading 12500'. I quickly descended to 12000' using the copilot INS having determined the pilot altimeter to be malfunctioning. Both altimeters stabilized on 12000' and the rest of the flight proceeded west/O further incident. I had the static system checked at my destination. The mechanic thought water in the system might be the culprit. No water was found.

Original NASA ASRS Text

Title: ONE ALTIMETER GIVES ERRONEOUS READINGS WHILE AT CRUISE.

Narrative: WHILE CRUISING AT 12000' OVER CHARLESTON, WV, I NOTICED MY ALTIMETER (PLT SIDE) SHOWING A DSNDING TREND. A X-CHK WITH THE COPLT ALTIMETER REVEALED THAT 1 UNIT WAS DISPLAYING AN INCORRECT ALT. I RPTED MY PROB TO CENTER AND ASKED FOR AN ALT READOUT. CENTER STILL SHOWED ME LEVEL AT 12000'. SUDDENLY THE PLT SIDE ALTIMETER INCREASED TO 12500' IN ABOUT 2-3 SECS AND THE DIGITIZER RPTED THIS TO CENTER. CENTER CALLED AND TOLD ME IT WAS SHOWING 12500'. GLANCING OVER TO THE COPLT SIDE I NOTICED THE COPLT ALTIMETER NOW READING 12500'. I QUICKLY DSNDED TO 12000' USING THE COPLT INS HAVING DETERMINED THE PLT ALTIMETER TO BE MALFUNCTIONING. BOTH ALTIMETERS STABILIZED ON 12000' AND THE REST OF THE FLT PROCEEDED W/O FURTHER INCIDENT. I HAD THE STATIC SYS CHKED AT MY DEST. THE MECH THOUGHT WATER IN THE SYS MIGHT BE THE CULPRIT. NO WATER WAS FOUND.

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.