B737 FLT CREW RECEIVES GPWS TERRAIN WARNING JUST OUTSIDE WASON AT 5500 FEET AS THEY PREPARE FOR THE VISUAL TO RWY 11L AT TUS. AT 800 AGL ON THE SAME APPROACH THEY RECEIVE A TCAS RA AND GO AROUND.

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ON THE DINGO 5 ARR TO TUS; ATC CLRED US DIRECT TO WASON; DSND TO 7000 FT. WE WERE AWARE OF THE TERRAIN IN THE AREA AND HAD BRIEFED IT. PRIOR TO WASON; WE CALLED 'FIELD IN SIGHT' AND WERE CLRED FOR VISUAL APCH TO RWY 11L. I DSNDED TO REACH 5500 FT 3-5 MI PRIOR TO WASON TO LEVEL AND SLOW PRIOR TO CAPTURING GS. THE HIGHEST TERRAIN IN THE AREA WAS 4000 FT AND THE HIGHEST OBSTACLE WAS 4687 FT. JUST AFTER LEVELING AT 5500 FT; ATC ISSUED A LOW ALT ALERT STATING MVA WAS 5700 FT. BEFORE WE COULD INITIATE THE 200 FT CLB; WE RECEIVED A TERRAIN PULL-UP WARNING AND IMMEDIATELY COMPLIED. THE TERRAIN DISPLAY WAS SELECTED 'ON' PRIOR TO THE EVENT AND ALL TERRAIN TARGETS APPEARED GREEN. NO CAUTIONS WERE RECEIVED PRIOR TO THE 'PULL UP WARNING.' WE STOPPED CLB AT 6500 FT AS WE JOINED THE LOC (NOW 1000 FT ABOVE PUBLISHED ALT); BUT WERE ABLE TO FULLY CONFIGURE AND ESTABLISH STABLE APCH PRIOR TO 1000 FT AGL. A FEW SECONDS LATER; ATC CALLED OUT HELI TFC WHICH WE SPOTTED AND CALLED IN SIGHT AS WE PASSED APPROX 800 FT AGL; BUT A TCAS RA RESULTED AND WE HAD TO EXECUTE A GAR TO COMPLY WITH THE RA. ATC ISSUED R TFC AND A SECOND VISUAL APCH WHICH REQUIRED SOME NONSTANDARD PLANNING DUE TO THE SAME TERRAIN NEAR WASON. THE SECOND APCH WAS COMPLETED TO A NORMAL LNDG. I AM CONCERNED THAT WE RECEIVED A 'PULL UP' WARNING EVEN THOUGH WE WERE IN LEVEL FLT; WELL ABOVE TERRAIN AND OBSTACLES AND HAVING RECEIVED NO TERRAIN CAUTIONS. I AM ALSO CONCERNED THAT WE WERE ABLE TO FALL INTO THIS TRAP DESPITE HAVING BRIEFED AND PLANNED FOR THE TERRAIN. IF THIS TERRAIN WARNING WAS NOT IN ERROR; PERHAPS IT WOULD BE APPROPRIATE TO SECURE AN ATC AGREEMENT TO ISSUE ALL VISUALS FROM THIS DIRECTION AS 'MAINTAIN 6000 FT TILL ESTABLISHED ON FINAL...'