Small aircraft has mode C readout different from indicated altitude with proper altimeter setting.

Attributes
ACN	203447
Time
Date	199203
Day	Sun
Local Time Of Day	0601 To 1200
Place
Locale Reference	airport : teb
State Reference	NJ
Altitude	msl bound lower : 1400 msl bound upper : 2000
Environment
Flight Conditions	VMC
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : n90 tower : teb
Operator	general aviation : personal
Make Model Name	Small Aircraft, High Wing, 1 Eng, Fixed Gear
Flight Phase	other
Flight Plan	None
Person 1
Affiliation	Other
Function	flight crew : single pilot
Qualification	pilot : commercial pilot : instrument
Experience	flight time last 90 days : 25 flight time total : 405 flight time type : 11
ASRS Report	203447
Person 2
Affiliation	government : faa
Function	controller : local
Qualification	controller : non radar
Events
Anomaly	non adherence : far other anomaly other
Independent Detector	other controllera
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	Pilot Deviation

Narrative:

On a left downwind departure from runway 24 teb we were level at 1400 MSL. The tower was calling traffic to an arrival and reported us at 2000 (which would have put us into the TCA) and our altimeter was set correctly. We asked the tower to verify his mode C readout, and he said it was 2000 so we shut our mode C off (after telling the tower and again reporting level at 1400). Later we checked with approach and our mode C was equal to our indicated altitude. On our return we checked again with teb tower and the mode C readout was equal to the indicated altitude. The only explanation is that the tower was tracking another aircraft, but try to tell the FAA that if they're looking for a violation.

Original NASA ASRS Text

Title: SMA HAS MODE C READOUT DIFFERENT FROM INDICATED ALT WITH PROPER ALTIMETER SETTING.

Narrative: ON A L DOWNWIND DEP FROM RWY 24 TEB WE WERE LEVEL AT 1400 MSL. THE TWR WAS CALLING TFC TO AN ARR AND RPTED US AT 2000 (WHICH WOULD HAVE PUT US INTO THE TCA) AND OUR ALTIMETER WAS SET CORRECTLY. WE ASKED THE TWR TO VERIFY HIS MODE C READOUT, AND HE SAID IT WAS 2000 SO WE SHUT OUR MODE C OFF (AFTER TELLING THE TWR AND AGAIN RPTING LEVEL AT 1400). LATER WE CHKED WITH APCH AND OUR MODE C WAS EQUAL TO OUR INDICATED ALT. ON OUR RETURN WE CHKED AGAIN WITH TEB TWR AND THE MODE C READOUT WAS EQUAL TO THE INDICATED ALT. THE ONLY EXPLANATION IS THAT THE TWR WAS TRACKING ANOTHER ACFT, BUT TRY TO TELL THE FAA THAT IF THEY'RE LOOKING FOR A VIOLATION.

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.