A B727 CREW; ON APCH TO MEM; DSNDED BELOW THE INITIAL APCH ATTITUDE WITHOUT BEING CLRED FOR THE APCH.

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ON APCH TO RWY 36R AT MEMPHIS WE WERE GIVEN THE CLRNC; 'TURN L TO 030 DEGS; DSND TO 2000 FT AND INTERCEPT THE RWY 36R LOC.' WE HAD BEEN TOLD EARLIER TO CALL THE FIELD IN SIGHT FOR VISUAL; BUT WE HAD NOT YET DONE THAT. AS THE GS INDICATOR WAS COMING DOWN WE HAD NOT YET BEEN CLRED FOR THE APCH. AS THE GS INDICATOR CTRED; THE FO BEGAN TO DSND. I WAS ABOUT TO ASK FOR CLRNC WHEN 2 OR 3 ACFT TRIED TO XMIT AT ONCE; BLOCKING THE FREQ. WHEN THE FREQ CLRED WE HEARD 'ACR X TFC AT 11 O'CLOCK POS; 6 MI.' WE RESPONDED 'LOOKING.' NEXT WE HEARD; 'ACR X TURN L 270 DEGS MAINTAIN 2000 FT DID YOU EVER CALL THE FIELD IN SIGHT?' I RESPONDED 'NEGATIVE.' THE CTLR SAID 'WE WERE SHOWING YOU AT 1500 FT.' I RESPONDED 'NEGATIVE.' THE LOWEST ANY OF US SAW WAS 1750-1800 FT. WE WERE VECTORED AROUND FOR AN UNEVENTFUL APCH AND LNDG. THE PROB AROSE WHEN I ALLOWED THE FO TO BEGIN DSCNT ON THE GS. MY THINKING WAS THAT WE WOULD HAVE APCH CLRNC BEFORE WE WERE 100 FT OUT OF ASSIGNED ALT. I WON'T DO THAT AGAIN. THE RADIOS WERE BUSY AND I SHOULD HAVE ANTICIPATED A PROB GETTING MY XMISSION OUT. WHEN TFC WAS CALLED OUT I WAS DISTRACTED FROM MY ORIGINAL GOAL OF OBTAINING APCH CLRNC. MY PLANS FOR AVOIDING THIS IN THE FUTURE ARE SIMPLE -- STICK WITH THE BASICS. DON'T BEGIN AN APCH OR ANYTHING ELSE STRICTLY ON ANTICIPATION. OUR COMS WITH EACH OTHER IN THE COCKPIT WEREN'T AS GOOD AS THEY SHOULD HAVE BEEN. THE FO HAD NOT REALIZED THAT WE HAD NOT BEEN CLRED FOR THE APCH. THE FE WAS BUSY WITH THE BEFORE LNDG CHKLIST. WE HELD A DEBRIEFING AFTER WE LANDED AND CONCLUDED THAT THIS COULD HAVE BEEN AVOIDED BY STICKING WITH THE CLRNC THAT WE HAD ACTUALLY RECEIVED -- NOT THE ONE WE EXPECTED.