Narrative:

Assigned 17000' by ZLA and was on a vector toward santa catalina VOR, with a cruise speed of 305 KIAS. Initially we were asked to slow to 280 KTS and descend to 1000'. At about 16000' we were cleared to 14000' and asked to slow to 250 KTS. At about 14000' we were cleared to 11000'. During the process of leveling at 11000', a clearance was received to descend to 5000', expediting through 6000', and to resume normal airspeed. The clearance was acknowledged xoe to descend to 5000', expediting through 6000'. One traffic advisory was given by ZLA when we were descending through about 10000', that they had a target at our 12 O'clock and 8 mi. As we were descending through about 8700', an aircraft was observed ahead at about the 12 O'clock position, a little low and approximately 1 mi ahead. Initially it appeared that the target was about 7500'. Out air carrier was leveled off at 8500' until we were clear. A comment was made to ZLA by the light twin that an aircraft had passed right over them. ZLA then inquired about xoe altitude and was told 8500' descending to 5000'. The center then replied that the clearance was for dgoe not xoe. Right after checking in on the effected sector frequency one call to dgoe was made, but neither the first officer nor I heard any additional calls to dgoa after that. The WX at the time of the incident was a solid cloud layer, with the tops somewhere around 4000'. We were flying into a very bright sun which was also reflecting off the clouds, and effectively reducing our forward visibility. One thing I do not understand is why ZLA did not notice our altitude deviation when we continued our descent from an assigned altitude of 11000 to 5000'. I did personally question why the center would tell us to return to normal speed when we would have to be at 250 KIAS below 10000', but because of the amount of radio traffic, I did not think it advisable to ask them. I also did not get a positive response from the center after acknowledging our clearance, but so often in a high density traffic area with a lot of radio xmissions, a clearance is given and an acknowledgement made west/O further communications. As it turned out my readback to ZLA was blocked by another transmission, and consequently the center never realized that both xoe and dgoe had read back the same clearance.

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Original NASA ASRS Text

Title: ACR-MLG ALT DEVIATION IN RESPONSE TO WRONG CALL SIGN AND CLOSE PROX WITH SAME DIR TRAFFIC A GA-SMT.

Narrative: ASSIGNED 17000' BY ZLA AND WAS ON A VECTOR TOWARD SANTA CATALINA VOR, WITH A CRUISE SPD OF 305 KIAS. INITIALLY WE WERE ASKED TO SLOW TO 280 KTS AND DSND TO 1000'. AT ABOUT 16000' WE WERE CLRED TO 14000' AND ASKED TO SLOW TO 250 KTS. AT ABOUT 14000' WE WERE CLRED TO 11000'. DURING THE PROCESS OF LEVELING AT 11000', A CLRNC WAS RECEIVED TO DSND TO 5000', EXPEDITING THROUGH 6000', AND TO RESUME NORMAL AIRSPD. THE CLRNC WAS ACKNOWLEDGED XOE TO DSND TO 5000', EXPEDITING THROUGH 6000'. ONE TFC ADVISORY WAS GIVEN BY ZLA WHEN WE WERE DSNDING THROUGH ABOUT 10000', THAT THEY HAD A TARGET AT OUR 12 O'CLOCK AND 8 MI. AS WE WERE DSNDING THROUGH ABOUT 8700', AN ACFT WAS OBSERVED AHEAD AT ABOUT THE 12 O'CLOCK POS, A LITTLE LOW AND APPROX 1 MI AHEAD. INITIALLY IT APPEARED THAT THE TARGET WAS ABOUT 7500'. OUT ACR WAS LEVELED OFF AT 8500' UNTIL WE WERE CLR. A COMMENT WAS MADE TO ZLA BY THE LIGHT TWIN THAT AN ACFT HAD PASSED RIGHT OVER THEM. ZLA THEN INQUIRED ABOUT XOE ALT AND WAS TOLD 8500' DSNDING TO 5000'. THE CENTER THEN REPLIED THAT THE CLRNC WAS FOR DGOE NOT XOE. RIGHT AFTER CHKING IN ON THE EFFECTED SECTOR FREQ ONE CALL TO DGOE WAS MADE, BUT NEITHER THE F/O NOR I HEARD ANY ADDITIONAL CALLS TO DGOA AFTER THAT. THE WX AT THE TIME OF THE INCIDENT WAS A SOLID CLOUD LAYER, WITH THE TOPS SOMEWHERE AROUND 4000'. WE WERE FLYING INTO A VERY BRIGHT SUN WHICH WAS ALSO REFLECTING OFF THE CLOUDS, AND EFFECTIVELY REDUCING OUR FORWARD VIS. ONE THING I DO NOT UNDERSTAND IS WHY ZLA DID NOT NOTICE OUR ALT DEVIATION WHEN WE CONTINUED OUR DSCNT FROM AN ASSIGNED ALT OF 11000 TO 5000'. I DID PERSONALLY QUESTION WHY THE CENTER WOULD TELL US TO RETURN TO NORMAL SPD WHEN WE WOULD HAVE TO BE AT 250 KIAS BELOW 10000', BUT BECAUSE OF THE AMOUNT OF RADIO TFC, I DID NOT THINK IT ADVISABLE TO ASK THEM. I ALSO DID NOT GET A POSITIVE RESPONSE FROM THE CENTER AFTER ACKNOWLEDGING OUR CLRNC, BUT SO OFTEN IN A HIGH DENSITY TFC AREA WITH A LOT OF RADIO XMISSIONS, A CLRNC IS GIVEN AND AN ACKNOWLEDGEMENT MADE W/O FURTHER COMS. AS IT TURNED OUT MY READBACK TO ZLA WAS BLOCKED BY ANOTHER XMISSION, AND CONSEQUENTLY THE CENTER NEVER REALIZED THAT BOTH XOE AND DGOE HAD READ BACK THE SAME CLRNC.

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.