Narrative:

VNAV function on FMS was used to comply with an altitude crossing clearance at STAR waypoint. After crossing; vectors and altitude changes brought me on right 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 localizer was intercepted; I had not been given an approach clearance; so I questioned ATC if I was cleared for the approach. Although I was at 2000 ft MSL; the controller issued 'maintain 6000 ft until established; cleared for ILS xx circle-to-land xy.' I hesitated for a second before reading back '2000' thinking the controller; who was busy with multiple airports; had given me a mistaken altitude. 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 localizer approach and began my descent to MDA. I broke out and leveled at 900 ft MSL. The controller then advised I was still outside the marker and below altitude. I checked position; confirming his transmission; but was close to the fix and in VMC; so I elected to maintain altitude instead of climbing back to IMC. The remainder of the approach was uneventful. After landing 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 approach allows a descent to 1800 ft after intercepting the localizer before intercepting the GS. The indication passed through quickly and I did not notice it was an FMS indicator due to the confusing clearance that had me distraction. On nov/xa/08; I flew the airplane with a cfii at our local field to try and duplicate the situation. When the approach 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.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of May 2009 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.