Narrative:

I was working local control 1 position from XA17 to XC36. Controller training was in progress at the local control 1 position. (Trainee) had instructed small aircraft X to make a straight-in approach to runway 25L -- to report a 4 mi final. X was advised of numerous aircraft inbound from the east. Due to radio frequency congestion and overlapping radio xmissions -- I assumed local control position responsibility. At approximately XC12Z I heard small aircraft Y call 'entering downwind from the hills.' this was the first radio call received from Y. Due to the poor quality of the radio, I asked Y to 'say again and to give his position from the airport.' Y replied 'west of the airport.' I instructed Y to 'enter right traffic, runway 25R, report on the 45 to downwind.' within 5-8 seconds Y reported on the 45 to downwind. I did not observe Y on a 45 to right traffic nor in the left hand pattern. I made a radio transmission to Y advising 'not in sight, report mid-field on downwind.' time passed, Y was not observed on right downwind. My scan immediately shifted to the left pattern, Y was not in sight. I asked Y if he had passed mid-field on downwind? Y stated in a very garbled manner what sounded like 'affirmative' -- the rest of his radio call was not readable. Y was not in sight. X reported the 4 mi final for runway 25L. Y called 'base turning final.' no aircraft was observed on right base turning final. A quick scan to the left pattern produced sight of a twin-engine aircraft Y turning from left downwind to left base. Y was banked to the left and within 2 seconds had passed the inbound cessna X. No time for a TA so I asked Y if he was turning left base. Y answered 'affirmative.' I could not define if Y passed in front of, or behind X. I advised Y, you were issued right traffic, cleared to land, runway 25R, full stop. Radio xmissions from Y were difficult to interpret in the sterile isolated recorder-play back room and were even more difficult to understand in the control tower cabin attendant.

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Original NASA ASRS Text

Title: SMA Y HAD NMAC WITH SMA X ON FINAL APCH. SEE AND AVOID CONCEPT.

Narrative: I WAS WORKING LCL CTL 1 POS FROM XA17 TO XC36. CTLR TRAINING WAS IN PROGRESS AT THE LCL CTL 1 POS. (TRAINEE) HAD INSTRUCTED SMA X TO MAKE A STRAIGHT-IN APCH TO RWY 25L -- TO RPT A 4 MI FINAL. X WAS ADVISED OF NUMEROUS ACFT INBOUND FROM THE E. DUE TO RADIO FREQ CONGESTION AND OVERLAPPING RADIO XMISSIONS -- I ASSUMED LCL CTL POS RESPONSIBILITY. AT APPROX XC12Z I HEARD SMA Y CALL 'ENTERING DOWNWIND FROM THE HILLS.' THIS WAS THE FIRST RADIO CALL RECEIVED FROM Y. DUE TO THE POOR QUALITY OF THE RADIO, I ASKED Y TO 'SAY AGAIN AND TO GIVE HIS POS FROM THE ARPT.' Y REPLIED 'W OF THE ARPT.' I INSTRUCTED Y TO 'ENTER R TFC, RWY 25R, RPT ON THE 45 TO DOWNWIND.' WITHIN 5-8 SECONDS Y RPTED ON THE 45 TO DOWNWIND. I DID NOT OBSERVE Y ON A 45 TO R TFC NOR IN THE L HAND PATTERN. I MADE A RADIO XMISSION TO Y ADVISING 'NOT IN SIGHT, RPT MID-FIELD ON DOWNWIND.' TIME PASSED, Y WAS NOT OBSERVED ON R DOWNWIND. MY SCAN IMMEDIATELY SHIFTED TO THE L PATTERN, Y WAS NOT IN SIGHT. I ASKED Y IF HE HAD PASSED MID-FIELD ON DOWNWIND? Y STATED IN A VERY GARBLED MANNER WHAT SOUNDED LIKE 'AFFIRMATIVE' -- THE REST OF HIS RADIO CALL WAS NOT READABLE. Y WAS NOT IN SIGHT. X RPTED THE 4 MI FINAL FOR RWY 25L. Y CALLED 'BASE TURNING FINAL.' NO ACFT WAS OBSERVED ON R BASE TURNING FINAL. A QUICK SCAN TO THE L PATTERN PRODUCED SIGHT OF A TWIN-ENG ACFT Y TURNING FROM L DOWNWIND TO L BASE. Y WAS BANKED TO THE L AND WITHIN 2 SECONDS HAD PASSED THE INBOUND CESSNA X. NO TIME FOR A TA SO I ASKED Y IF HE WAS TURNING L BASE. Y ANSWERED 'AFFIRMATIVE.' I COULD NOT DEFINE IF Y PASSED IN FRONT OF, OR BEHIND X. I ADVISED Y, YOU WERE ISSUED R TFC, CLRED TO LAND, RWY 25R, FULL STOP. RADIO XMISSIONS FROM Y WERE DIFFICULT TO INTERPRET IN THE STERILE ISOLATED RECORDER-PLAY BACK ROOM AND WERE EVEN MORE DIFFICULT TO UNDERSTAND IN THE CTL TWR CAB.

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.