Narrative:

Aircraft X was northwest of tulsa southeast bound to lit; was level at FL270. Air carrier Y northeast of tulsa southwest bound pilot's discretion descent to FL280. Issued traffic to both aircraft. After acknowledging the traffic; aircraft X asked what the altitude I was showing on aircraft Y because it passed underneath him. I advised aircraft X that I showed the aircraft Y at FL290. Air carrier Y; when asked; advised that he passed under aircraft X and that he was at F280. All instruments showed the aircraft at the altitudes that were displayed on my radar and in their cockpits. There was no conflict alert and no TCASII warnings. But one of these aircraft was 2000-2500 ft off. Both aircraft verified 29.92 altimeter settings.

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Original NASA ASRS Text

Title: ZKC CTLR DESCRIBED NMAC AT FL270 WHEN ATC AND ACFT EQUIP INDICATED MORE THAN REQUIRED SEPARATION.

Narrative: ACFT X WAS NW OF TULSA SE BOUND TO LIT; WAS LEVEL AT FL270. ACR Y NE OF TULSA SW BOUND PLT'S DISCRETION DSCNT TO FL280. ISSUED TFC TO BOTH ACFT. AFTER ACKNOWLEDGING THE TFC; ACFT X ASKED WHAT THE ALT I WAS SHOWING ON ACFT Y BECAUSE IT PASSED UNDERNEATH HIM. I ADVISED ACFT X THAT I SHOWED THE ACFT Y AT FL290. ACR Y; WHEN ASKED; ADVISED THAT HE PASSED UNDER ACFT X AND THAT HE WAS AT F280. ALL INSTS SHOWED THE ACFT AT THE ALTS THAT WERE DISPLAYED ON MY RADAR AND IN THEIR COCKPITS. THERE WAS NO CONFLICT ALERT AND NO TCASII WARNINGS. BUT ONE OF THESE ACFT WAS 2000-2500 FT OFF. BOTH ACFT VERIFIED 29.92 ALTIMETER SETTINGS.

Data retrieved from NASA's ASRS site as of January 2009 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.