ALT BUST. FLC MISCOM RESULTED IN THE FO THINKING THE CAPT HAD RECEIVED CLRNC FOR A VISUAL APCH WHEN IN FACT THEY HAD NOT. FO DSNDED BELOW CLRED ALT WHILE ON THE GS. CAPT GOT A CLRNC WHEN HE REALIZED WHAT HAD HAPPENED. THE ACFT IS A B737-200.

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WE WERE ON A VECTOR TO INTERCEPT THE RWY 18L LOC AND CLRED TO DSND TO 4000 FT. THE COPLT WAS FLYING AND SAID HE HAD THE FIELD AND THE TFC ABOUT 3.5 MI AHEAD OF US IN SIGHT. I THINK HE THOUGHT I CALLED THE FIELD AND/OR TFC AND I KNOW HE THOUGHT WE WERE CLRED FOR A VISUAL APCH TO RWY 18L. WE WERE STILL PRETTY FAST FOR OUR DISTANCE TO THE RWY AND ONLY ABOUT 3.5 MI BEHIND THE TFC AHEAD. BECAUSE I WAS CONCERNED ABOUT OUR SEPARATION I DIDN'T WANT THE VISUAL APCH YET SO I DID NOT CALL TFC OR FIELD. NORMAL ALT CALLOUTS WERE MADE; BUT THINKING WE WERE CLRED FOR THE VISUAL; HE DSNDED BELOW 4000 FT DOWN TO APPROX 3600 FT BEFORE WE WERE ACTUALLY CLRED FOR THE VISUAL. HIS STRONG BELIEF THAT WE HAD ALREADY BEEN CLRED FOR THE VISUAL LED HIM TO QUESTION MY ALT WARNINGS WHILE CONTINUING THE DSCNT. HE EXPECTED ME TO CALL THE FIELD/TFC AND THEN MADE THE ASSUMPTION THAT I HAD AND ALONG WITH THAT EXPECTATION CAME THE BELIEF THAT WE HAD BEEN CLRED. I BELIEVE THIS ALTDEV COULD HAVE BEEN AVOIDED HAD I COMMUNICATED THAT I WASN'T READY FOR HIM TO ACCEPT A VISUAL APCH AT THE TIME HE STATED THAT HE HAD THE FIELD/TFC IN SIGHT. I THINK THIS IS A COMMON SCENARIO OF A LACK OF COM AND ASSUMPTIONS MADE BASED ON A VERY OFTEN ENCOUNTERED SIT REGARDING VISUAL APCHS AND THE RPTING OF FIELD/TFC AND THEN RECEIVING VISUAL APCH CLRNC.