Narrative:

Zxy from ewr to sfo. Xxy from pit to sfo. On more than one day, both aircraft have been on the same frequency, same routing, expecting the same clrncs. It has been reported as a similar call sign problem with no resolution because the scheduled 'block' times are not in conflict. (?, Flts show block times 2 mins apart). While working the low altitude center sector, I took a handoff on zxy, showed level at FL240. Xxy checked on and was issued (to zxy) a descent cross cedes at 110 and 250 KTS. Xxy had been issued FL280 for traffic from the high altitude sector. Zxy was at FL240, on my scope, but not on frequency. Xxy reported TCASII traffic. I advised zxy that I had no traffic. At that time zxy came on frequency requesting lower and advising they were too high to meet the expected restriction. I asked which aircraft had the TCASII alert. Xxy responded and I asked his altitude. Xxy was passing through FL250 and I stopped the aircraft at FL240. The high altitude sector realized what had happened and gave me the handoff on xxy, which I would have worked soon anyway. Xxy has apparently responded to the TCASII, leveled off in the descent, and avoided traffic at FL260 that he had been stopped for. Zxy did not change frequencys. Xxy took zxy's frequency change. Xxy acknowledged zxy's clearance to 110, I did not recognize call sign difference -- not aware that xxy was even on frequency because I had handoff and should have been talking to zxy. This was during a busy evening rush inbound to sfo. The controller at high altitude was a developmental with one radar certification. Low altitude was combined with a departure sector -- single controller. Zxy was a widebody transport and xxy was an medium large transport. If 'heavy' were still required, this may have helped. TCASII on xxy prevented a potential disaster. Xxy had taken zxy's frequency change at least once earlier in flight. They were advised of each other at least once. Solution: when controllers say there is a call sign problem -- believe it, and change them! Supplemental information from acn 214975. Checked in with oak center at FL310, full call sign of xxy was used in all xmissions. Clearance to cross modesto at 250 KTS, 11000 ft was issued. Both first officer and myself believed it to be for us. I acknowledged with full call sign and PF started descent. Passing through 26000 ft we received a TA on 9 O'clock traffic. I asked oak center about it, using full call sign and they responded that they didn't see any traffic. We were not told of zxy heavy being on frequency controller never called zxy heavy; only zxy. Controller obviously did not listen to our full call sign. It's possible that we subconsciously heard xxy instead of zxy, as we were expecting descent. Similar call signs are dangerous. Further inquiry showed that center has the call sign conflict every night and yet did not take precautions.

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Original NASA ASRS Text

Title: ACR XXY HAS TCASII TA ON ACR ZXY XING TFC IN DSCNT.

Narrative: ZXY FROM EWR TO SFO. XXY FROM PIT TO SFO. ON MORE THAN ONE DAY, BOTH ACFT HAVE BEEN ON THE SAME FREQ, SAME ROUTING, EXPECTING THE SAME CLRNCS. IT HAS BEEN RPTED AS A SIMILAR CALL SIGN PROBLEM WITH NO RESOLUTION BECAUSE THE SCHEDULED 'BLOCK' TIMES ARE NOT IN CONFLICT. (?, FLTS SHOW BLOCK TIMES 2 MINS APART). WHILE WORKING THE LOW ALT CTR SECTOR, I TOOK A HDOF ON ZXY, SHOWED LEVEL AT FL240. XXY CHKED ON AND WAS ISSUED (TO ZXY) A DSCNT CROSS CEDES AT 110 AND 250 KTS. XXY HAD BEEN ISSUED FL280 FOR TFC FROM THE HIGH ALT SECTOR. ZXY WAS AT FL240, ON MY SCOPE, BUT NOT ON FREQ. XXY RPTED TCASII TFC. I ADVISED ZXY THAT I HAD NO TFC. AT THAT TIME ZXY CAME ON FREQ REQUESTING LOWER AND ADVISING THEY WERE TOO HIGH TO MEET THE EXPECTED RESTRICTION. I ASKED WHICH ACFT HAD THE TCASII ALERT. XXY RESPONDED AND I ASKED HIS ALT. XXY WAS PASSING THROUGH FL250 AND I STOPPED THE ACFT AT FL240. THE HIGH ALT SECTOR REALIZED WHAT HAD HAPPENED AND GAVE ME THE HDOF ON XXY, WHICH I WOULD HAVE WORKED SOON ANYWAY. XXY HAS APPARENTLY RESPONDED TO THE TCASII, LEVELED OFF IN THE DSCNT, AND AVOIDED TFC AT FL260 THAT HE HAD BEEN STOPPED FOR. ZXY DID NOT CHANGE FREQS. XXY TOOK ZXY'S FREQ CHANGE. XXY ACKNOWLEDGED ZXY'S CLRNC TO 110, I DID NOT RECOGNIZE CALL SIGN DIFFERENCE -- NOT AWARE THAT XXY WAS EVEN ON FREQ BECAUSE I HAD HDOF AND SHOULD HAVE BEEN TALKING TO ZXY. THIS WAS DURING A BUSY EVENING RUSH INBOUND TO SFO. THE CTLR AT HIGH ALT WAS A DEVELOPMENTAL WITH ONE RADAR CERTIFICATION. LOW ALT WAS COMBINED WITH A DEP SECTOR -- SINGLE CTLR. ZXY WAS A WDB AND XXY WAS AN MLG. IF 'HVY' WERE STILL REQUIRED, THIS MAY HAVE HELPED. TCASII ON XXY PREVENTED A POTENTIAL DISASTER. XXY HAD TAKEN ZXY'S FREQ CHANGE AT LEAST ONCE EARLIER IN FLT. THEY WERE ADVISED OF EACH OTHER AT LEAST ONCE. SOLUTION: WHEN CTLRS SAY THERE IS A CALL SIGN PROBLEM -- BELIEVE IT, AND CHANGE THEM! SUPPLEMENTAL INFO FROM ACN 214975. CHKED IN WITH OAK CTR AT FL310, FULL CALL SIGN OF XXY WAS USED IN ALL XMISSIONS. CLRNC TO CROSS MODESTO AT 250 KTS, 11000 FT WAS ISSUED. BOTH FO AND MYSELF BELIEVED IT TO BE FOR US. I ACKNOWLEDGED WITH FULL CALL SIGN AND PF STARTED DSCNT. PASSING THROUGH 26000 FT WE RECEIVED A TA ON 9 O'CLOCK TFC. I ASKED OAK CTR ABOUT IT, USING FULL CALL SIGN AND THEY RESPONDED THAT THEY DIDN'T SEE ANY TFC. WE WERE NOT TOLD OF ZXY HVY BEING ON FREQ CTLR NEVER CALLED ZXY HVY; ONLY ZXY. CTLR OBVIOUSLY DID NOT LISTEN TO OUR FULL CALL SIGN. IT'S POSSIBLE THAT WE SUBCONSCIOUSLY HEARD XXY INSTEAD OF ZXY, AS WE WERE EXPECTING DSCNT. SIMILAR CALL SIGNS ARE DANGEROUS. FURTHER INQUIRY SHOWED THAT CTR HAS THE CALL SIGN CONFLICT EVERY NIGHT AND YET DID NOT TAKE PRECAUTIONS.

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.