COMMUTER ACFT DSNDS BELOW ASSIGNED ALT ON ILS APCH.

Narrative

I HEARD THE CTLR SAY: 'MAINTAIN 4000 UNTIL INTERCEPTING LOC.' IMMEDIATELY AFTER; THE LOC CAME ALIVE; AND I CENTERED THE NEEDLE. BELIEVING I WAS CLRED FOR THE APCH; I BEGAN DSCNT IAW THE APCH PLATE. APCH CTLR CALLED US AS WE WERE PASSING 2800 FT AND ADVISED WE WERE SUPPOSED TO STILL BE AT 4000 FT. SHE GAVE US A TURN OFF THE LOC AND WE WERE VECTORED TO THE APCH FOR THE OTHER RWY. AFTER LNDG; I REALIZED THE CTLR HAD NEVER SAID 'CLRED FOR THE APCH;' AND I REALIZED THAT I HAD FLOWN WHAT I HAD EXPECTED TO HEAR; NOT WHAT I ACTUALLY HEARD. HOWEVER; THE WORD SEQUENCE I HAD HEARD IS THAT WHICH WOULD LEAD INTO...'CLRED FOR THE APCH;' I WAS TOLD TO CALL THE APCH SUPVR; WHICH I DID; AND HE SAID HE WOULD HAVE TO SUBMIT PAPERWORK TO EXPLAIN THE LTSS WHICH OCCURRED WITH A LIGHT PLANE AS A RESULT OF OUR DSCNT. I DID NOT KNOW THE CTLR WAS WORKING THIS ACFT; AS WE WERE VMC ABOVE AN UNDERCAST AND DID NOT SEE IT; NOR DID WE HEAR ANY RADIO CALLS TO IT. SHE WAS ALSO WORKING MIL ACFT IN THE RADAR PATTERN AND WAS FAIRLY BUSY. COCKPIT WORKLOAD WAS HIGH AT THIS POINT; AND MY COPLT WAS INVOLVED IN CHANGING ACFT CONFIGN AND OPERATING THE PRESSURIZATION SYS. MY COPLT WAS NEW TO THE COMPANY AND THE ACFT; AND WAS SLIGHTLY BEHIND DURING CRITICAL PHASES OF FLT. HE HAD SOME DOUBT ABOUT THE DSCNT I INITIATED; BUT I FEEL HE WAS NOT EFFECTIVE IN COMMUNICATING THAT HE THOUGHT WE WERE NOT CLRED FOR THE APCH. PERHAPS HE WAS RELUCTANT TO QUESTION A MORE SENIOR PLT. PERHAPS HE WAS UNSURE OR CONFUSED BY THE WORDING OF THE CTLR'S INSTRUCTIONS. I DISMISSED HIS QUERY ABOUT THE ALT ASSIGNMENT BY SHOWING THAT I WAS ESTABLISHED ON THE LOC COURSE. IF HE HAD BEEN MORE EXPERIENCED OR IF I HAD FLOWN WITH HIM BEFORE; MAYBE I WOULD HAVE LISTENED CLOSER AND PAID MORE ATTN TO WHAT HE WAS TRYING TO TELL ME. YOU CAN BET I'LL LISTEN BETTER NEXT TIME; NO MATTER HOW GREEN THE COPLT; AND I WILL VERIFY SUCH CLRNCS WITH THE CTLR IF EITHER OF US HAS THE SLIGHTEST DOUBT OR QUESTION!