Narrative:

On V26 sebound at 17000 ft indicated altitude, ZMP controller reported my altitude encoder indicated 16000 ft on the readout. I had departed rcx VFR and picked up my IFR clearance at about 4000 ft climbing southeast to auw. I had set the barometric pressure as provided by center when clearance was provided. I was approaching a cold front which was lying north/south over lake michigan. The controller asked if I had a backup encoder (as I was indicating 17000 ft and he saw 16000 ft on his readout). I said no, and asked for an altimeter setting. The setting provided was 1 inch lower than the previous provided setting (about 100 NM earlier). I turned the altitude squawk off and reset my altimeter. (I asked if I should stop altitude squawk). After the reset my altitude now indicated 16000 ft. I climbed up to 17000 ft indicated and turned on the altitude squawk. The controller then confirmed his readout was also 17000 ft. The problem evidently was a very steep pressure gradient behind and approaching the front from the west. The controller should have been more attentive to the altitude deviation his scope read out as I followed my indicated altitude, holding it at 17000 ft. (I have an autoplt with excellent altitude hold capability). There was no other traffic in the vicinity near the 17000 ft altitude and the controller was busy with lower en route, arrival and departure aircraft.

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Original NASA ASRS Text

Title: ACFT LOST 1000 FT AS IF FLEW FROM HIGH BAROMETRIC PRESSURE TO AN AREA WHERE THE PRESSURE WAS 1 INCH LOWER.

Narrative: ON V26 SEBOUND AT 17000 FT INDICATED ALT, ZMP CTLR RPTED MY ALT ENCODER INDICATED 16000 FT ON THE READOUT. I HAD DEPARTED RCX VFR AND PICKED UP MY IFR CLRNC AT ABOUT 4000 FT CLBING SE TO AUW. I HAD SET THE BAROMETRIC PRESSURE AS PROVIDED BY CENTER WHEN CLRNC WAS PROVIDED. I WAS APCHING A COLD FRONT WHICH WAS LYING N/S OVER LAKE MICHIGAN. THE CTLR ASKED IF I HAD A BACKUP ENCODER (AS I WAS INDICATING 17000 FT AND HE SAW 16000 FT ON HIS READOUT). I SAID NO, AND ASKED FOR AN ALTIMETER SETTING. THE SETTING PROVIDED WAS 1 INCH LOWER THAN THE PREVIOUS PROVIDED SETTING (ABOUT 100 NM EARLIER). I TURNED THE ALT SQUAWK OFF AND RESET MY ALTIMETER. (I ASKED IF I SHOULD STOP ALT SQUAWK). AFTER THE RESET MY ALT NOW INDICATED 16000 FT. I CLBED UP TO 17000 FT INDICATED AND TURNED ON THE ALT SQUAWK. THE CTLR THEN CONFIRMED HIS READOUT WAS ALSO 17000 FT. THE PROBLEM EVIDENTLY WAS A VERY STEEP PRESSURE GRADIENT BEHIND AND APCHING THE FRONT FROM THE W. THE CTLR SHOULD HAVE BEEN MORE ATTENTIVE TO THE ALT DEV HIS SCOPE READ OUT AS I FOLLOWED MY INDICATED ALT, HOLDING IT AT 17000 FT. (I HAVE AN AUTOPLT WITH EXCELLENT ALT HOLD CAPABILITY). THERE WAS NO OTHER TFC IN THE VICINITY NEAR THE 17000 FT ALT AND THE CTLR WAS BUSY WITH LOWER ENRTE, ARR AND DEP ACFT.

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.