B350 PLT UNDERSTANDS CLEARED TO 2000 VICE 6000 MSL. MISINTERPRETS VNAV PATH FMS INDICATOR AS A GLIDE SLOPE INDICATOR AND STARTS DOWN TO MDA.

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VNAV FUNCTION ON FMS WAS USED TO COMPLY WITH AN ALT XING CLRNC AT STAR WAYPOINT. AFTER XING; VECTORS AND ALT CHANGES BROUGHT ME ON R DOWNWIND AT 2000 FT MSL FOR THE ILS AT ZZZ. BASE TURN WAS NORMAL AND TURN TO INTERCEPT FINAL WAS SOMEWHAT LATE BUT STILL MANAGEABLE. AS LOC WAS INTERCEPTED; I HAD NOT BEEN GIVEN AN APCH CLRNC; SO I QUESTIONED ATC IF I WAS CLRED FOR THE APCH. ALTHOUGH I WAS AT 2000 FT MSL; THE CTLR ISSUED 'MAINTAIN 6000 FT UNTIL ESTABLISHED; CLRED FOR ILS XX CIRCLE-TO-LAND XY.' I HESITATED FOR A SECOND BEFORE READING BACK '2000' THINKING THE CTLR; WHO WAS BUSY WITH MULTIPLE ARPTS; HAD GIVEN ME A MISTAKEN ALT. I RECEIVED NO REPLY FROM MY 'DIFFERENT' READBACK. WHILE THIS WAS OCCURRING; I NOTICED A GS INDICATION PASS THROUGH THE INDICATOR WINDOW ON THE PFD. THINKING I HAD FLOWN THROUGH THE GS; I DECIDED TO EXECUTE A LOC APCH AND BEGAN MY DSCNT TO MDA. I BROKE OUT AND LEVELED AT 900 FT MSL. THE CTLR THEN ADVISED I WAS STILL OUTSIDE THE MARKER AND BELOW ALT. I CHKED POS; CONFIRMING HIS XMISSION; BUT WAS CLOSE TO THE FIX AND IN VMC; SO I ELECTED TO MAINTAIN ALT INSTEAD OF CLBING BACK TO IMC. THE REMAINDER OF THE APCH WAS UNEVENTFUL. AFTER LNDG AND SOME TIME TO THINK ABOUT WHAT HAD OCCURRED; I SURMISED THAT THE GS INDICATION I RECEIVED MUST HAVE COME FROM THE VNAV FUNCTION STILL ENGAGED ON THE FMS. THE APCH ALLOWS A DSCNT TO 1800 FT AFTER INTERCEPTING THE LOC BEFORE INTERCEPTING THE GS. THE INDICATION PASSED THROUGH QUICKLY AND I DID NOT NOTICE IT WAS AN FMS INDICATOR DUE TO THE CONFUSING CLRNC THAT HAD ME DISTR. ON NOV/XA/08; I FLEW THE AIRPLANE WITH A CFII AT OUR LOCAL FIELD TO TRY AND DUPLICATE THE SITUATION. WHEN THE APCH WAS SET UP WITH THE VNAV STILL FUNCTIONING; I RECEIVED THE SAME INDICATION I HAD RECEIVED AT ZZZ CONFIRMING MY SUSPICION AS TO WHAT HAD HAPPENED. IN THE FUTURE; I WILL ENSURE VNAV FUNCTION IS DISENGAGED WHEN NOT IN USE.