Narrative:

I was PIC of a flight from mdt to avp. We were given a step climb up to 11000'. Passing 10000' I gave the 'out of 10000 for 11000' call. Out of 10900' I again gave the '100' to go' call. The first officer started to level off when ATC called and asked us to verify altitude. I looked at my altimeter and called 11000'. ATC advised he showed 11700' and climbing. I checked my altimeter and x-chked with first officer and both were showing 11000'. I told ATC that we would try another transponder and switched to the #2 position. We were given a descent to 9000'. After some consideration I advised the first officer that this aircraft may not be equipped with 2 xponders, so I again switched to the #1 transponder. ATC advised he showed us out of 10000' and that matched with what our altimeters showed. We had no other problems with altitude mode C. Supplemental information from acn 155908: upon arrival at avp the captain wrote up the discrepancy. An avionics technician was flown to avp to replace/recertify the system before its next scheduled flight. This aircraft indeed had only 1 transponder installed (the selector has 2 positions). Had there been a second transponder, center could have had an immediate verification of our actual altitude, thus leaving no room for doubt.

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Original NASA ASRS Text

Title: MALFUNCTIONING MODE C TRANSPONDER ELICITS PRECAUTIONARY DESCENT CLRNC ON PART OF ARTCC RADAR CTLR.

Narrative: I WAS PIC OF A FLT FROM MDT TO AVP. WE WERE GIVEN A STEP CLB UP TO 11000'. PASSING 10000' I GAVE THE 'OUT OF 10000 FOR 11000' CALL. OUT OF 10900' I AGAIN GAVE THE '100' TO GO' CALL. THE F/O STARTED TO LEVEL OFF WHEN ATC CALLED AND ASKED US TO VERIFY ALT. I LOOKED AT MY ALTIMETER AND CALLED 11000'. ATC ADVISED HE SHOWED 11700' AND CLBING. I CHKED MY ALTIMETER AND X-CHKED WITH F/O AND BOTH WERE SHOWING 11000'. I TOLD ATC THAT WE WOULD TRY ANOTHER XPONDER AND SWITCHED TO THE #2 POS. WE WERE GIVEN A DSNT TO 9000'. AFTER SOME CONSIDERATION I ADVISED THE F/O THAT THIS ACFT MAY NOT BE EQUIPPED WITH 2 XPONDERS, SO I AGAIN SWITCHED TO THE #1 XPONDER. ATC ADVISED HE SHOWED US OUT OF 10000' AND THAT MATCHED WITH WHAT OUR ALTIMETERS SHOWED. WE HAD NO OTHER PROBS WITH ALT MODE C. SUPPLEMENTAL INFO FROM ACN 155908: UPON ARR AT AVP THE CAPT WROTE UP THE DISCREPANCY. AN AVIONICS TECHNICIAN WAS FLOWN TO AVP TO REPLACE/RECERTIFY THE SYS BEFORE ITS NEXT SCHEDULED FLT. THIS ACFT INDEED HAD ONLY 1 XPONDER INSTALLED (THE SELECTOR HAS 2 POSITIONS). HAD THERE BEEN A SECOND XPONDER, CENTER COULD HAVE HAD AN IMMEDIATE VERIFICATION OF OUR ACTUAL ALT, THUS LEAVING NO ROOM FOR DOUBT.

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.