Narrative:

We were flying level at FL310 when we checked in with lax center (female controller). Right after she 'rogered' our check-in, we received what we thought was clearance to FL240. Our flight number was XX00. We were unaware that an xxii, same company, was also on frequency. Passing FL300 center issued a traffic alert to us about opposite direction traffic at FL290 and said we were not cleared lower and clearance was for another aircraft. Other aircraft did not acknowledge, center did not catch that we acknowledged. All 3 crew members on our aircraft thought clearance was for us. This is a clear case of similar call signs being confused. Also controller's voice not clear and too soft. All 3 pilots were wearing headsets plus using speakers. Supplemental information from acn 118831: we were given a clearance to descend to FL240 and to slow to 280 KTS for spacing. The captain read back the clearance. I began D descent out of FL310. After passing through FL300 I saw an medium large transport to the left os us eastbound. Center called air carrier XX00 traffic alert. The controller who issued the traffic alert was a different controller than the one who issued the descent clearance. The controller said the descent clearance was for air carrier xxii, not air carrier XX00. The captain talked to lax center on the phone and was told that the tapes revealed the clearance was issued to air carrier xxii, but they were going to treat the incident as a controller error. I'm concerned how all 3 of us (air carrier crew members) listening to the clearance were 100% sure the clearance was for air carrier XX00. The answer I've come up with is fatigue. At the time of the incident I knew I was tired, but did not feel I was operating much below normal level until the incident happened on the second to last leg of a 4 day trip. During the trip we flew 24 hours 11 min. The last 2 days of the trip were preceded by an XXXX duty break followed by an XXXX duty day (xx flight time) followed by an FAA minimum duty break with a wakeup at XXXX EDT (XXXX body clock (we're lax based) for the last day of the trip.

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Original NASA ASRS Text

Title: PLT RESPONDED TO WRONG CALL SIGN AND DESCENDED FROM ASSIGNED ALT. PLT DEVIATION.

Narrative: WE WERE FLYING LEVEL AT FL310 WHEN WE CHECKED IN WITH LAX CENTER (FEMALE CTLR). RIGHT AFTER SHE 'ROGERED' OUR CHECK-IN, WE RECEIVED WHAT WE THOUGHT WAS CLRNC TO FL240. OUR FLT NUMBER WAS XX00. WE WERE UNAWARE THAT AN XXII, SAME COMPANY, WAS ALSO ON FREQ. PASSING FL300 CENTER ISSUED A TFC ALERT TO US ABOUT OPPOSITE DIRECTION TFC AT FL290 AND SAID WE WERE NOT CLRED LOWER AND CLRNC WAS FOR ANOTHER ACFT. OTHER ACFT DID NOT ACKNOWLEDGE, CENTER DID NOT CATCH THAT WE ACKNOWLEDGED. ALL 3 CREW MEMBERS ON OUR ACFT THOUGHT CLRNC WAS FOR US. THIS IS A CLEAR CASE OF SIMILAR CALL SIGNS BEING CONFUSED. ALSO CTLR'S VOICE NOT CLEAR AND TOO SOFT. ALL 3 PLTS WERE WEARING HEADSETS PLUS USING SPEAKERS. SUPPLEMENTAL INFORMATION FROM ACN 118831: WE WERE GIVEN A CLRNC TO DSND TO FL240 AND TO SLOW TO 280 KTS FOR SPACING. THE CAPT READ BACK THE CLRNC. I BEGAN D DSCNT OUT OF FL310. AFTER PASSING THROUGH FL300 I SAW AN MLG TO THE LEFT OS US EBND. CENTER CALLED ACR XX00 TFC ALERT. THE CTLR WHO ISSUED THE TFC ALERT WAS A DIFFERENT CTLR THAN THE ONE WHO ISSUED THE DSCNT CLRNC. THE CTLR SAID THE DSCNT CLRNC WAS FOR ACR XXII, NOT ACR XX00. THE CAPT TALKED TO LAX CENTER ON THE PHONE AND WAS TOLD THAT THE TAPES REVEALED THE CLRNC WAS ISSUED TO ACR XXII, BUT THEY WERE GOING TO TREAT THE INCIDENT AS A CTLR ERROR. I'M CONCERNED HOW ALL 3 OF US (ACR CREW MEMBERS) LISTENING TO THE CLRNC WERE 100% SURE THE CLRNC WAS FOR ACR XX00. THE ANSWER I'VE COME UP WITH IS FATIGUE. AT THE TIME OF THE INCIDENT I KNEW I WAS TIRED, BUT DID NOT FEEL I WAS OPERATING MUCH BELOW NORMAL LEVEL UNTIL THE INCIDENT HAPPENED ON THE SECOND TO LAST LEG OF A 4 DAY TRIP. DURING THE TRIP WE FLEW 24 HRS 11 MIN. THE LAST 2 DAYS OF THE TRIP WERE PRECEDED BY AN XXXX DUTY BREAK FOLLOWED BY AN XXXX DUTY DAY (XX FLT TIME) FOLLOWED BY AN FAA MINIMUM DUTY BREAK WITH A WAKEUP AT XXXX EDT (XXXX BODY CLOCK (WE'RE LAX BASED) FOR THE LAST DAY OF THE TRIP.

Data retrieved from NASA's ASRS site as of August 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.