A rockwell turbo commander 980 has an apparent conflict in altitude with pwm approach controller. Pilot's altimeter reading was 3800 ft versus 3000 ft in the TRACON. Flight was supposed to be at 4000 ft.

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Attributes
ACN	392750
Time
Date	199802
Day	Tue
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : pwm
State Reference	ME
Altitude	msl bound lower : 3000 msl bound upper : 4000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : pwm tracon : rdu
Operator	general aviation : corporate
Make Model Name	Turbo Commander 980
Operating Under FAR Part	Part 121
Navigation In Use	Other
Flight Phase	descent : approach descent other other
Flight Plan	IFR
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : instrument pilot : atp
Experience	flight time last 90 days : 55 flight time total : 3890 flight time type : 2917
ASRS Report	392750
Person 2
Affiliation	government : faa
Function	controller : approach
Qualification	controller : radar
Events
Anomaly	aircraft equipment problem : less severe altitude deviation : overshoot non adherence : clearance other anomaly other
Independent Detector	atc equipment other atc equipment : unspecified other controllera
Resolutory Action	none taken : anomaly accepted
Consequence	faa : reviewed incident with flight crew Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation other

Narrative:

We had been cleared down to 4000 ft when portland approach asked us about our altitude and we replied 4000 ft. But our actual indicated altitude was 3800 ft. Portland approach told us that he was showing 3000 ft for an altitude. At this point I was a little bit confused and recycled the transponder. In the meantime we had been cleared for a visual approach to runway 29 and also instructed to call the tower. On the ground I talked to our avionics shop and was told that it is possible for the encoding altimeter to act up like this. On the next flight everything was working normally again.

Original NASA ASRS Text

Title: A ROCKWELL TURBO COMMANDER 980 HAS AN APPARENT CONFLICT IN ALT WITH PWM APCH CTLR. PLT'S ALTIMETER READING WAS 3800 FT VERSUS 3000 FT IN THE TRACON. FLT WAS SUPPOSED TO BE AT 4000 FT.

Narrative: WE HAD BEEN CLRED DOWN TO 4000 FT WHEN PORTLAND APCH ASKED US ABOUT OUR ALT AND WE REPLIED 4000 FT. BUT OUR ACTUAL INDICATED ALT WAS 3800 FT. PORTLAND APCH TOLD US THAT HE WAS SHOWING 3000 FT FOR AN ALT. AT THIS POINT I WAS A LITTLE BIT CONFUSED AND RECYCLED THE XPONDER. IN THE MEANTIME WE HAD BEEN CLRED FOR A VISUAL APCH TO RWY 29 AND ALSO INSTRUCTED TO CALL THE TWR. ON THE GND I TALKED TO OUR AVIONICS SHOP AND WAS TOLD THAT IT IS POSSIBLE FOR THE ENCODING ALTIMETER TO ACT UP LIKE THIS. ON THE NEXT FLT EVERYTHING WAS WORKING NORMALLY AGAIN.

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.