Narrative:

I was operating in class B airspace, descending from 9000 ft down to 6000 ft and the field was reported 'in sight.' the controller responded by saying 'proceed visually to the field using your own navigation.' I understood this to mean a 'visual approach.' I descended through my original altitude assignment of 6000 ft. At 5500 ft (approximately) the approach controller said maintain 4000 ft. I then realized that he did not intend for the previous clearance to mean 'cleared for a visual approach.' no conflict occurred and there was no traffic on my TCASII screen. Contributing factors were the VFR clear conditions with the field being called in sight so early, an ambiguous clearance given by ATC, and my misunderstanding of the clearance. The altitude deviation occurred or I should say was discovered when the descent to 4000 ft was issued and I realized the clearance for a visual approach was not given. Corrective actions could be to issue crossing/altitude restrs on visual approachs (ie, 'cleared for the visual, maintain at or above 4000 ft until 10 DME from the field'), and to clear up any ambiguous clrncs when they are issued.

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Original NASA ASRS Text

Title: AN ACR E120 FLC MISUNDERSTOOD A CLRNC TO NAV TO THE FIELD AS CLRNC FOR A VISUAL APCH AND STARTED A DSCNT OUT OF THEIR ASSIGNED ALT. THE CTLR ALERTLY ASSIGNED THEM A LOWER ALT.

Narrative: I WAS OPERATING IN CLASS B AIRSPACE, DSNDING FROM 9000 FT DOWN TO 6000 FT AND THE FIELD WAS RPTED 'IN SIGHT.' THE CTLR RESPONDED BY SAYING 'PROCEED VISUALLY TO THE FIELD USING YOUR OWN NAV.' I UNDERSTOOD THIS TO MEAN A 'VISUAL APCH.' I DSNDED THROUGH MY ORIGINAL ALT ASSIGNMENT OF 6000 FT. AT 5500 FT (APPROX) THE APCH CTLR SAID MAINTAIN 4000 FT. I THEN REALIZED THAT HE DID NOT INTEND FOR THE PREVIOUS CLRNC TO MEAN 'CLRED FOR A VISUAL APCH.' NO CONFLICT OCCURRED AND THERE WAS NO TFC ON MY TCASII SCREEN. CONTRIBUTING FACTORS WERE THE VFR CLR CONDITIONS WITH THE FIELD BEING CALLED IN SIGHT SO EARLY, AN AMBIGUOUS CLRNC GIVEN BY ATC, AND MY MISUNDERSTANDING OF THE CLRNC. THE ALTDEV OCCURRED OR I SHOULD SAY WAS DISCOVERED WHEN THE DSCNT TO 4000 FT WAS ISSUED AND I REALIZED THE CLRNC FOR A VISUAL APCH WAS NOT GIVEN. CORRECTIVE ACTIONS COULD BE TO ISSUE XING/ALT RESTRS ON VISUAL APCHS (IE, 'CLRED FOR THE VISUAL, MAINTAIN AT OR ABOVE 4000 FT UNTIL 10 DME FROM THE FIELD'), AND TO CLR UP ANY AMBIGUOUS CLRNCS WHEN THEY ARE ISSUED.

Data retrieved from NASA's ASRS site as of July 2007 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.