Transponder repeatedly sends ATC an altitude readout more than 2000' below actual altitude.

Attributes
ACN	143394
Time
Date	199004
Day	Thu
Local Time Of Day	1201 To 1800
Place
Locale Reference	atc facility : ord airport : mdw
State Reference	IL
Altitude	msl bound lower : 4600 msl bound upper : 6600
Environment
Flight Conditions	Mixed
Light	Daylight
Aircraft 1
Controlling Facilities	tracon : ord
Operator	common carrier : air carrier
Make Model Name	Light Transport, High Wing, 2 Turboprop Eng
Flight Phase	climbout : intermediate altitude
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
Experience	flight time last 90 days : 300 flight time total : 6000 flight time type : 300
ASRS Report	143394
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument
Events
Anomaly	aircraft equipment problem : less severe
Independent Detector	other controllera other flight crewa
Resolutory Action	none taken : unable
Consequence	Other
Supplementary
Primary Problem	Aircraft
Air Traffic Incident	other

Narrative:

As the crew was climbing out ord departure control got an altitude readout 2000' or less than what we were actually at. He remarked about it because the erroneous altitude readout put us in conflict with another airlines jet. We told him our real altitude. A few moments later, the transponder read correctly. At the time, the controller thought it was his equipment. His comment was, 'I got a bad sweep.' trip continued to destination cmi with no further problems. Upon departing cmi back to ord, cmi departure commented that our mode C altitude was again 2000', lower than our actual altitude. We switched over to our #1 encoding transponder and flight was uneventful back to ord. At ord we wrote up #2 transponder as inoperative. I also called ord tower and explained what happened. Maintenance worked on the transponder and claimed they 'swapped out' some parts and the transponder was now working correctly. We departed ord again, and as luck would have it, were being controled by the same controller as in the previous flight out of ord. Again we experienced a mode C readout 2000' or more lower than actual. We switched back to #1 transponder and continued to our destination, bmi. At bmi we again wrote up #2 transponder.

Original NASA ASRS Text

Title: TRANSPONDER REPEATEDLY SENDS ATC AN ALT READOUT MORE THAN 2000' BELOW ACTUAL ALT.

Narrative: AS THE CREW WAS CLBING OUT ORD DEP CTL GOT AN ALT READOUT 2000' OR LESS THAN WHAT WE WERE ACTUALLY AT. HE REMARKED ABOUT IT BECAUSE THE ERRONEOUS ALT READOUT PUT US IN CONFLICT WITH ANOTHER AIRLINES JET. WE TOLD HIM OUR REAL ALT. A FEW MOMENTS LATER, THE XPONDER READ CORRECTLY. AT THE TIME, THE CTLR THOUGHT IT WAS HIS EQUIP. HIS COMMENT WAS, 'I GOT A BAD SWEEP.' TRIP CONTINUED TO DEST CMI WITH NO FURTHER PROBS. UPON DEPARTING CMI BACK TO ORD, CMI DEP COMMENTED THAT OUR MODE C ALT WAS AGAIN 2000', LOWER THAN OUR ACTUAL ALT. WE SWITCHED OVER TO OUR #1 ENCODING XPONDER AND FLT WAS UNEVENTFUL BACK TO ORD. AT ORD WE WROTE UP #2 XPONDER AS INOP. I ALSO CALLED ORD TWR AND EXPLAINED WHAT HAPPENED. MAINT WORKED ON THE XPONDER AND CLAIMED THEY 'SWAPPED OUT' SOME PARTS AND THE XPONDER WAS NOW WORKING CORRECTLY. WE DEPARTED ORD AGAIN, AND AS LUCK WOULD HAVE IT, WERE BEING CTLED BY THE SAME CTLR AS IN THE PREVIOUS FLT OUT OF ORD. AGAIN WE EXPERIENCED A MODE C READOUT 2000' OR MORE LOWER THAN ACTUAL. WE SWITCHED BACK TO #1 XPONDER AND CONTINUED TO OUR DEST, BMI. AT BMI WE AGAIN WROTE UP #2 XPONDER.

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.