FLC OF ACR LTT ACFT DSNDED PREMATURELY DURING THE TRANSITION PHASE ROUTING FOR AN INST APCH DUE TO A MISUNDERSTANDING THAT DSCNT WAS AUTHORIZED TO THE MIN ALT SHOWN FOR THE TRANSITION RTE EVEN THOUGH ATC HAD NOT PROVIDED THIS AUTHORIZATION.

Narrative

ACR FLT #X WAS BEGINNING APCH (LOC BACK COURSE RWY 31) AT JOPLIN REGIONAL ARPT IN JOPLIN; MO. ZKC CLRED FLT X TO DSND TO 5000 FT WHILE PROCEEDING DIRECT TO NEOSHO FOR THE NEOSHO TRANSITION TO LOC BACK COURSE 31 AT JOPLIN. APPROX 10 MI S OF NEOSHO CTR CLRED X TO 4100 FT AND SAID FOR X TO MAINTAIN 4100 FT UNTIL ESTABLISHED ON THE APCH; CLRED BACK COURSE RWY 31 AT JOPLIN. MY CAPT REPLIED BY SAYING X CLRED BACK COURSE 31 APCH AT JOPLIN WE ARE GOING TO DO THE NEOSHO TRANSITION. CTR DID NOT RESPOND. AFTER WE PASSED NEOSHO VOR WE BEGAN DSCNT FROM 4100 MSL DOWN TO 2800 FT (LABELED ON APCH - NEOSHO TRANSITION NOPT MIN ALT PRIOR TO FAF). WHEN WE WERE APPROX 5 MI NE OF NEOSHO ON THE NEOSHO 023 DEG RADIAL; CTR ASKED US TO 'SAY ALT.' CAPT RESPONDED '3300 FT.' CTR RESPONDED BY SAYING OUR ASSIGNED ALT WAS 4100 UNTIL ON FINAL AND THAT AN LTT WAS IN THE AREA ON THE APCH. CAPT THEN STATED THAT 'ON THE APCH PLATE WE WERE CLRED TO 2800 FT BUT WE WOULD CLB BACK TO 4100 FT.' THE APCH CONTINUED AND WE LANDED WITHOUT INCIDENT. I BELIEVE THAT THE CAUSE OF THIS WAS A MISCOM BTWN CTR AND FLT X ON WHEN WE WERE CLRED TO LEAVE 4100 FT. THE CTR WANTED US TO DSND WHEN ON FINAL; WHILE WE THOUGHT WE WERE CLRED TO LEAVE 4100 WHEN WE WERE ESTABLISHED ON THE TRANSITION. MY SOLUTION IS SIMPLY TO CLARIFY BTWN PARTIES WHERE ON THE APCH IS CONSIDERED TO BE 'ESTABLISHED.'