ACR X TCASII RA FROM SMA Y EVASIVE ACTION TURN DSCNT BELOW ASSIGNED ALT. PLTDEV.

Narrative

ACR X ON THE CRESO ARR FOR VISUAL APCH TO RWY 25L AT LAS. AS USUAL; WE WERE BACKING THIS UP WITH PF ON ILS TO RWY 25L AND PNF ON VOR LAS FOR DME. IT APPEARED TO US THAT APCH CTL HAD KEPT US HIGH. WE WERE GIVEN A SERIES OF L TURNS AND DSCNTS; TURNING US TO FINAL; PF'S ILS SHOWED US ABOVE GS. THE LAST TURN WAS TO 310 DEGS; AND DSCNT TO 6000 FT. PF WAS SLOWING AND DSNDING AT ABOUT 1500 FPM TO GET NEAR GS. AT ABOUT 6500 FT; TCASII SOUNDED WITH A RED SQUARE AND EITHER 'MONITOR VERT SPD' OR 'REDUCE DSCNT.' THE TARGET WAS OFF THE L WING. I LOOKED ACROSS CAPT AND OUT HIS SLIDING SIDE WINDOW AND SAW WHAT LOOKED LIKE AN SMA Y. THE LIGHT ACFT WAS HDG EASTERLY OR NORTHEASTERLY; AND WE CROSSED HIS NOSE FROM R TO L ABOVE HIM ABOUT 500 FT; BUT WITH OUR L WING POINTED RIGHT AT HIM DUE TO THE TURN. PF; HAVING HAD NO CLUE FROM APCH CTL ABOUT THIS ACFT KEPT HIS EYES ON HIM AND DID EVASIVE ACTION UNTIL HE WAS SURE THE LIGHT ACFT WAS BEHIND US. PNF (MYSELF) QUERIED APCH CTL AND MENTIONED OUR TCASII ADVISORY. PF NOTICED WE HAD LEVELED AT 5800 FT AFTER EVASIVE ACTION AND CLBED BACK TO 6000 FT. AFTER SOME HESITATION; APCH CTL SAID THE LIGHT ACFT WAS ON 'ANOTHER SECTOR FREQ' AND THAT WE HAD STANDARD VFR SEPARATION. THEN THE CTLR ASKED OUR ALT AND I (PNF) RESPONDED CLBING BACK TO 6000 FT. THE CTLR THEN CLRED US FOR VISUAL; SENT US TO TWR; THEN SAID THE PROBABLE CAUSE OF THE TCASII WAS THAT WE WERE AT 5700 FT. HOWEVER; TCASII HAD BEEN GOING OFF WELL BEFORE THE LEVEL-OFF ALT -- THAT'S HOW I SAW THE LIGHT ACFT IN THE FIRST PLACE. KEY POINTS: IF APCH CTL HAD ADVISED US OF THE TFC OR EXPLAINED WHY HE WAS KEEPING US HIGH; WE COULD HAVE NOT REACTED TO THE RA ON TCASII. I DON'T THINK WE WERE EVER CLOSER THAN 1 NM AND 300 FT TO THE LIGHT ACFT; AND THAT WAS AFTER WE CROSSED HIS NOSE. WE WERE ABOUT 1 1/2 MI AND 500 FT AS WE CROSSED HIM; HOWEVER; WE PROBABLY LOOKED CLOSER TO HIM DUE TO OUR SIZE AND THE FACT THE L WING WAS POINTED AT HIM. WITH THE RA ON TCASII AND THE TARGET ACQUIRED VISUALLY; THE PF'S MAIN WAY TO AVOID THE TFC WAS EVASIVE ACTION. THIS DID RESULT IN US DSNDING 200 FT (OUR ALTIMETERS) BELOW ASSIGNED ALT. APCH CTL KEPT TALKING AFTER CLRING US FOR VISUAL AND SENDING US TO TWR. THIS WAS A SERIOUS DISTR AND COMPLICATED THE PROBLEM OF LINING UP ON FINAL; SLOWING; DSNDING TO GS; AND COMPLETING THE CHKLISTS.