Narrative:

X 123 was a scheduled passenger flight between atl and cle. At the time of the occurrence we were proceeding r-nav direct to vxu VOR and climbing to our assigned altitude of FL290. I rogered the clearance and noted my first officer (who was flying the aircraft) put FL290 in the altitude reminder window. Shortly thereafter, we were advised by ATC center that we had crossing traffic ahead, Y 123 at FL310 and we could expect higher when clear. I rogered that transmission, but failed to note that my first officer put FL310 in the altitude reminder. Continuing our climb, I called 'out of FL280 for FL290' as we passed FL280. When I looked at the altitude reminder I saw FL310 and my first officer said we were cleared to FL310. I incorrectly made the assumption I had missed the clearance. Passing FL293, both of us noted the traffic ahead on the TCASII and noted his altitude as 1600 ft above us. This sent up a 'red flag' and immediately queried ZTL about our clearance to FL310. The controller stated 'negative maintain FL290,' at about the same time that we acquired the traffic visually. I reached over, retarded the throttles and told my first officer to get back down to FL290. I believe our maximum altitude was FL296. In order to avoid a potential conflict we were directed to make a 90 degree turn to the right. Once clear of the traffic we were given a heading to join J-91 north of vxu. Had I not been preoccupied with my own problems (sinus headache and congestion from allergies) at the time I might have caught the incorrect insertion of FL310 in the altitude reminder. Regardless, I broke my own 'cardinal rule' by not taking questions over clrncs out of the cockpit to the source, instead of querying other crewmembers about a clearance. I also feel the similar call signs, X 123 and Y 123, contributed to this altitude excursion. Note: we did not get a TCASII advisory (traffic alert) or RA.

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Original NASA ASRS Text

Title: ACFT CLBED THROUGH ASSIGNED ALT THINKING THAT THE ATC HAD CLRED THEM TO A HIGHER ALT, BUT CLRNC WAS FOR OTHER ACFT WITH SIMILAR CALL SIGN.

Narrative: X 123 WAS A SCHEDULED PAX FLT BTWN ATL AND CLE. AT THE TIME OF THE OCCURRENCE WE WERE PROCEEDING R-NAV DIRECT TO VXU VOR AND CLBING TO OUR ASSIGNED ALT OF FL290. I ROGERED THE CLRNC AND NOTED MY FO (WHO WAS FLYING THE ACFT) PUT FL290 IN THE ALT REMINDER WINDOW. SHORTLY THEREAFTER, WE WERE ADVISED BY ATC CTR THAT WE HAD XING TFC AHEAD, Y 123 AT FL310 AND WE COULD EXPECT HIGHER WHEN CLR. I ROGERED THAT XMISSION, BUT FAILED TO NOTE THAT MY FO PUT FL310 IN THE ALT REMINDER. CONTINUING OUR CLB, I CALLED 'OUT OF FL280 FOR FL290' AS WE PASSED FL280. WHEN I LOOKED AT THE ALT REMINDER I SAW FL310 AND MY FO SAID WE WERE CLRED TO FL310. I INCORRECTLY MADE THE ASSUMPTION I HAD MISSED THE CLRNC. PASSING FL293, BOTH OF US NOTED THE TFC AHEAD ON THE TCASII AND NOTED HIS ALT AS 1600 FT ABOVE US. THIS SENT UP A 'RED FLAG' AND IMMEDIATELY QUERIED ZTL ABOUT OUR CLRNC TO FL310. THE CTLR STATED 'NEGATIVE MAINTAIN FL290,' AT ABOUT THE SAME TIME THAT WE ACQUIRED THE TFC VISUALLY. I REACHED OVER, RETARDED THE THROTTLES AND TOLD MY FO TO GET BACK DOWN TO FL290. I BELIEVE OUR MAX ALT WAS FL296. IN ORDER TO AVOID A POTENTIAL CONFLICT WE WERE DIRECTED TO MAKE A 90 DEG TURN TO THE R. ONCE CLR OF THE TFC WE WERE GIVEN A HDG TO JOIN J-91 N OF VXU. HAD I NOT BEEN PREOCCUPIED WITH MY OWN PROBS (SINUS HEADACHE AND CONGESTION FROM ALLERGIES) AT THE TIME I MIGHT HAVE CAUGHT THE INCORRECT INSERTION OF FL310 IN THE ALT REMINDER. REGARDLESS, I BROKE MY OWN 'CARDINAL RULE' BY NOT TAKING QUESTIONS OVER CLRNCS OUT OF THE COCKPIT TO THE SOURCE, INSTEAD OF QUERYING OTHER CREWMEMBERS ABOUT A CLRNC. I ALSO FEEL THE SIMILAR CALL SIGNS, X 123 AND Y 123, CONTRIBUTED TO THIS ALT EXCURSION. NOTE: WE DID NOT GET A TCASII ADVISORY (TFC ALERT) OR RA.

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.