UNPLANNED LAST MIN TURN ON A VECTORED VISUAL APCH TO RWY 19L PLACES AN ARR B737-300 FLT OVER THE STRATOSPHERE TWR LOCATED ABOUT 5 NM N OF LAS; NV.

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WE OVERFLEW THE STRATOSPHERE TWR; TURNING FINAL FROM R BASE TO RWY 19L DURING LATE NIGHT VISUAL APCH AT LAS. OUR ALT WAS 3500 FT ASSIGNED BY APCH CTLR UNTIL CLR OF TWR; AUTOPLT ENGAGED; LEVEL; GEAR/F15. AT THE SAME TIME; TCASII TA (-03 'TFC') IN CLOSE PROX TO OUR ACFT. GPWS ACTIVATED/IN USE. NO WARNINGS ISSUED/HAZARDS DEPICTED. CAPT WAS PF. ON KEPEC ARR; INITIALLY TOLD TO EXPECT RWY 25L. VECTORED OFF ARR AFTER KEPEC AT 12000 FT FOR VECTORS TO RWY 19L. TOLD TO FIND STRATOSPHERE. IN SIGHT; DIRECTED TO MAINTAIN 3500 FT UNTIL PAST; CLRED VISUAL RWY 19L. WITH THE BUILDING AT APPROX 2 O'CLOCK POS; 1 MI; PF MADE ABRUPT 45 DEG R TURN. TA ACTIVATED SHORTLY AFTER. I WAS CAUGHT OFF GUARD BY THE SUDDEN TURN; THEN THE TA; AND A BRIEF DISCUSSION ENSUED AS TO PF INTENT. DARK ARPT FOOTPRINT VISIBLE; BUT RWY 19L/R NOT YET IN SIGHT FROM THE 90 DEG BASE LEG PRIOR TO THE 45 DEG TURN. PF THOUGHT WE WERE GOING TO OVERSHOOT FINAL AS REASON FOR TURN; ATTN WAS CHANNELED ON ACQUIRING THE RWY AND WAS SLIGHTLY CONFUSED AS TO ACFT AND RWY RELATIVE POS. WITH AUTOPLT ENGAGED/LEVEL AT ASSIGNED ALT; THERE WAS NO SENSE OF DANGER OF COLLISION WITH BUILDINGS; AND WITH TA ACTIVATION; ATTEMPTING TO ACQUIRE VISUAL ON RWY AND DETERMINE PF INTENT/ACTIONS; I DID NOT NOTICE OUR FLT PATH TAKING US DIRECTLY OVER THE STRATOSPHERE UNTIL TOO LATE TO ALTER COURSE. SOON AFTER ACQUIRED VISUAL ON RWY; LANDED WITHOUT FURTHER INCIDENT. FACTORS INCLUDE NIGHT LOOK-DOWN AGAINST BRIGHTLY-LIT; CLUTTERED BACKGND; TALL STRUCTURES IN CLOSE PROX TO RWY 19L R; SLIGHT FATIGUE (LATE HR BODY CLOCKS); UNFAMILIAR APCH ANGLE TO SELDOM USED RWY; NO INST APCH REF; POSSIBLY THE XCOCKPIT LOOK ANGLE FOR THE PF; AND DISTR OF TA ACTIVATION. CHANNELED ATTN AND NO VERBAL CUES FROM THE PF AS TO CONFUSION OR CONCERN OVER PERCEIVED POS RELATIVE TO OUR RWY CONTRIBUTED. ALTHOUGH THE BUILDINGS ARE DEPICTED WITH ELEVATIONS ON APCH PLATES; WE DID NOT DISCUSS THEM DUE TO SUDDEN CHANGE IN RWY ASSIGNMENT; LACK OF INST APCH TO ASSIGNED RWY; AND BUSY LET-DOWN TASK LOAD FROM STARTING POS OFF ARR (THE ARR CHART WAS ON TOP OF THE RWY 25L PLATE ON MY YOKE CLIP). IF THE TA WAS GENERATED BY A XPONDER ON THE STRATOSPHERE; IT IS NOT NOTED AS IS THE ONE ON SANDIA PEAK NEAR ABQ. THE TA SERVED TO PRECLUDE ANY DESIRE ON OUR PART TO DSND; HAD WE INTENDED TO DO SO. AS A TECHNIQUE; I ROLL THE RWY HDG INTO MY CDI ON APCHS TO NON INSTRUMENTED RWYS (IMPROVES SITUATIONAL AWARENESS). I REFED THIS DURING THE CONFUSED MOMENTS AFTER THE PF'S 45 DEG TURN; AND HELPED REGAIN ORIENTATION AND RELATIVE POS AWARENESS. I RECOMMEND THAT ACFT NOT BE GIVEN VISUAL APCHS AT NIGHT TO RWY 19L/R FROM WESTERLY ARRS; DUE TO THE EXTREMELY BUSY DSCNT TASK LOADING; THE OBSTACLE ISSUES AND DIFFICULT VISUAL ACQUISITION CHALLENGES MENTIONED EARLIER.