CAPT OF AN ATR 42-500 OVERSHOT FINAL DUE TO BEING TOO HIGH AND TO OBTAIN MORE DISTANCE NEEDED TO DSND TO ASSIGNED APCH ALT. THE CAPT FLEW THROUGH LOC TOWARD THE OTHER PARALLEL RWY CAUSING THE CTLR TO INTERVENE AND ASK FOR A 180 DEG TURN BACK TO FINAL. THE CAPT WHILE MAKING THE TURN ALSO SLOWED THE ACFT AND SUBSEQUENTLY MADE A SUCCESSFUL LNDG.

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THE CAPT WAS FLYING (WITH THE AUTOPLT ENGAGED) AND WE WERE ON A L DOWNWIND FOR RWY 27 AT IAH AT 6000 FT WITH AN ASSIGNED AIRSPD OF 210 KTS. THE CTLR ASKED US IF WE HAD THE FIELD IN SIGHT AND WHEN THE CAPT NODDED I REPLIED WE DID. THE CTLR CLRED US FOR A VISUAL APCH AND INSTRUCTED US TO CROSS THE MARKER (6 DME) AT OR BELOW 2000 FT. (THIS MEANT THAT THERE WAS TFC DEPARTING RWY 14L THAT WOULD BE MAKING A L TURNOUT TO THE N.) HE ALSO INFORMED US THAT WE WERE NUMBER ONE FOR THE APCH WHICH TOLD ME THAT THERE WOULD BE OTHER TFC (PROBABLY MORE THAN ONE ACFT) ON THE LOC BEHIND US AT AN UNKNOWN DISTANCE. WHILE I WAS READING BACK THE APCH CLRNC THE CAPT TURNED THE HDG BUG (SET AT 27 DEGS OF BANK) L TO 360 DEGS AND SET A VERT SPD DOWN OF 2000 FPM. I NOTED THAT WE WERE ONLY 8.5 DME AND WHEN I FINISHED TALKING TO THE CTLR SAID 'THIS IS GOING TO BE REALLY TIGHT.' IT WAS OBVIOUS THAT WE COULD NOT MAKE THE XING RESTR WITH THIS DSCNT RATE BUT I ASSUMED THAT THE CAPT WOULD INCREASE THE DSCNT RATE TO COMPLY. WE WERE ALMOST AT 360 DEGS AND IT WAS TOO LATE TO TURN BACK R TO GIVE US MORE SPACE AS IT WOULD TURN US DIRECTLY OUTBOUND ON THE LOC INTO THE TFC THAT WAS SUPPOSED TO FOLLOW US IN (CONTINUING N IS NOT AN OPTION AS IT WOULD TAKE US INTO THE LOC FOR THE PARALLEL APCH RWY 26). A 3000 FPM DSCNT DOWN TO 2000 FT IS NOT SAFE IN MY OPINION BUT SINCE WE COULD NOT TURN BACK R OR CONTINUE N; IT WAS OUR ONLY OPTION TO AVOID BLOWING THE XING RESTR. THE CTLR MUST ALSO HAVE NOTICED THAT WE HAD BEGUN OUR INBOUND TURN TOO TIGHT AND SAID 'I NEED YOU AT 2000 FT AT THE MARKER.' (MEANING TO ME THAT THERE MUST ACTUALLY BE NBOUND TFC TURNING L FROM RWY 14L.) THE CAPT CALLED FOR THE VISUAL APCH CHKLIST AND I INITIATED IT. WHILE I WAS RUNNING THE CHKLIST I NOTICED THAT THE CAPT HAD NOT INCREASED OUR DSCNT RATE BUT INSTEAD HAD STOPPED TURNING L (AT 360 DEGS) IN ORDER TO GIVE US MORE TIME TO DSND. I STOPPED RUNNING THE CHKLIST TO WARN HIM THAT THE LOC WAS COMING IN FAST. AT THIS POINT THE CTLR SAID 'TFC IS A B737 AT 2 O'CLOCK ON THE PARALLEL AND I NEED YOU TO CROSS THE MARKER AT 2000 FT.' WE WERE STILL DOING OVER 200 KTS (BEING UNABLE TO SLOW DOWN IN THE DSCNT) AND I GUESS THE CAPT WAS CONCENTRATING ON THE XING RESTR AND NOT THE B737 BECAUSE HE STARTED TO TURN R INSTEAD OF L. I IMMEDIATELY SAID 'NO; TURN L; TURN L; HE'S RIGHT THERE!' AND AT THE SAME TIME WE GOT A TCASII ALERT. I PUT MY HAND ON THE YOKE AT THE SAME TIME I SAID THIS SO I COULD DISCONNECT THE AUTOPLT AND FLY THE PLANE MYSELF IF THE CAPT DID NOT IMMEDIATELY TURN L OR IN CASE WE NEEDED MORE THAN 27 DEGS OF BANK. HE DID TURN L IMMEDIATELY AND AS WE BEGAN OUR TURN I SAW THE B737 APPROX 1/2 TO 1 MI TO OUR R. WE HAD BEEN TURNING DIRECTLY INTO HIM AND PROBABLY DOING ABOUT 80 KTS FASTER. AS OUR HDG REACHED 270 DEGS AND WE WERE MOVING SAFELY AWAY FROM THE B737; THE CTLR SAID TWICE; 'TURN L 180 DEGS IMMEDIATELY!' THE CAPT STARTED A L TURN AND I SAID 'WE HAVE THE TFC AND RWY IN SIGHT.' THE CTLR CLRED US AGAIN FOR RWY 27. NOW WE WERE ON A HDG OF ABOUT 200 DEGS BELOW 1500 FT DOING 200 KTS. I SAID 'WE'RE NOT GOING TO MAKE RWY 27' BUT DIDN'T WANT TO CALL FOR A GAR ON THIS HDG BECAUSE WE WOULD CLB INTO THE TFC FROM RWY 14L. SO I DECIDED TO WAIT UNTIL WE WERE ALIGNED WITH THE RWY AND THEN CALL FOR THE GAR. DURING THE TURN THE CAPT BROUGHT THE NOSE UP AND STARTED CONFIGURING THE ACFT. HE DID GET IT CONFIGURED IN TIME SO I DIDN'T CALL FOR THE GAR AND WE LANDED.