Narrative:

After takeoff, it was noted that the barometric altimeter was not indicating an increasing altitude at a smooth rate of climb but rather the needle moved in short jerks as if it were hanging up. Aircraft was leveled at 2000' MSL as per clearance and I questioned departure control to 'confirm altitude' as I was not comfortable with the performance of the barometric altimeter. The controller replied '2000' assigned, I show you at 2500', altimeter setting is xx.' this setting was within a few hundredths of an inch of the setting in the instrument. The indicated altitude was fluctuating somewhat due to the light to moderate turbulence the aircraft was encountering but at no time did the altimeter indicate more than 2150'. I replied that 'I was showing about 2100' MSL but I will bring it down for you.' I leveled the aircraft at precisely 2000' indicated and asked 'what are you showing us at now?' controller was busy with another aircraft and did not answer. I was shortly handed off to another controller and was instructed to climb to 3000' and then was cleared to 7000' and asked to give them my best rate of climb to 6000'. The barometric altimeter again indicated the increasing altitude in the climb in short jerky movements of the needle. At this point I selected pitot-static system heat (even though the outside temperature was about 42 degrees fahrenheit) and in about 20-30 seconds the altimeter began to show a smooth steady rate of altitude increase. I believe from the above facts that the static system and/or static ports were contaminated with condensation and this was causing an inaccurate mode C indication. It is possible of course that the mode C and/or the barometric altimeter reading was also inaccurate.

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Original NASA ASRS Text

Title: ACFT ALT AND TRANSPONDER ALT DID NOT AGREE. PLT SUSPECTS PITOT ICING ON SMT TWIN ACFT.

Narrative: AFTER TKOF, IT WAS NOTED THAT THE BAROMETRIC ALTIMETER WAS NOT INDICATING AN INCREASING ALT AT A SMOOTH RATE OF CLB BUT RATHER THE NEEDLE MOVED IN SHORT JERKS AS IF IT WERE HANGING UP. ACFT WAS LEVELED AT 2000' MSL AS PER CLRNC AND I QUESTIONED DEP CTL TO 'CONFIRM ALT' AS I WAS NOT COMFORTABLE WITH THE PERFORMANCE OF THE BAROMETRIC ALTIMETER. THE CTLR REPLIED '2000' ASSIGNED, I SHOW YOU AT 2500', ALTIMETER SETTING IS XX.' THIS SETTING WAS WITHIN A FEW HUNDREDTHS OF AN INCH OF THE SETTING IN THE INSTRUMENT. THE INDICATED ALT WAS FLUCTUATING SOMEWHAT DUE TO THE LIGHT TO MODERATE TURB THE ACFT WAS ENCOUNTERING BUT AT NO TIME DID THE ALTIMETER INDICATE MORE THAN 2150'. I REPLIED THAT 'I WAS SHOWING ABOUT 2100' MSL BUT I WILL BRING IT DOWN FOR YOU.' I LEVELED THE ACFT AT PRECISELY 2000' INDICATED AND ASKED 'WHAT ARE YOU SHOWING US AT NOW?' CTLR WAS BUSY WITH ANOTHER ACFT AND DID NOT ANSWER. I WAS SHORTLY HANDED OFF TO ANOTHER CTLR AND WAS INSTRUCTED TO CLB TO 3000' AND THEN WAS CLRED TO 7000' AND ASKED TO GIVE THEM MY BEST RATE OF CLB TO 6000'. THE BAROMETRIC ALTIMETER AGAIN INDICATED THE INCREASING ALT IN THE CLB IN SHORT JERKY MOVEMENTS OF THE NEEDLE. AT THIS POINT I SELECTED PITOT-STATIC SYS HEAT (EVEN THOUGH THE OUTSIDE TEMP WAS ABOUT 42 DEGS FAHRENHEIT) AND IN ABOUT 20-30 SECS THE ALTIMETER BEGAN TO SHOW A SMOOTH STEADY RATE OF ALT INCREASE. I BELIEVE FROM THE ABOVE FACTS THAT THE STATIC SYS AND/OR STATIC PORTS WERE CONTAMINATED WITH CONDENSATION AND THIS WAS CAUSING AN INACCURATE MODE C INDICATION. IT IS POSSIBLE OF COURSE THAT THE MODE C AND/OR THE BAROMETRIC ALTIMETER READING WAS ALSO INACCURATE.

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.