Narrative:

Center asked altitude. I reported 16000 ft. Center responded with a 500 ft discrepancy at 16500 ft. Given a barometric setting of 30.55. Switched xponders to #2 system. Center reported 'in compliance.' no explanation of why system #1 reported 500 ft out. Cannot duplicate occurrence on system.

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Original NASA ASRS Text

Title: A CPR PLT'S MODE C READS 500 FT HIGH ACCORDING TO AN ARTCC RADAR CTLR. THE PLT SWITCHED TO THE #2 XPONDER WHICH GAVE A CORRECT READING.

Narrative: CTR ASKED ALT. I RPTED 16000 FT. CTR RESPONDED WITH A 500 FT DISCREPANCY AT 16500 FT. GIVEN A BAROMETRIC SETTING OF 30.55. SWITCHED XPONDERS TO #2 SYS. CTR RPTED 'IN COMPLIANCE.' NO EXPLANATION OF WHY SYS #1 RPTED 500 FT OUT. CANNOT DUPLICATE OCCURRENCE ON SYS.

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.