THE FO (PF) THOUGHT THAT THE FLC WAS CLRED FOR A VISUAL APCH AND BEGAN A DSCNT WITHOUT ATC CLRNC. THE PNF CAUGHT THE ERROR AND THE FLC RETURNED THE ACFT TO THEIR ASSIGNED ALT. APCH CTLR INTERVENED; ALSO.

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FO FLYING AND CAPT; PNF. BURBANK APCH ASKED US TO CALL THE 'ARPT IN SIGHT' AFTER ASSIGNING US 5000 FT. I ACKNOWLEDGED THIS WHILE ATTEMPTING TO LOCATE ARPT VISUALLY. FO ASSUMED THIS TO BE CLRNC FOR A VISUAL APCH AND BEGAN DSCNT. AS I RETURNED MY SCAN TO COCKPIT AND NOTICED ALT 4700 FT DSNDING; CALLED ALT AND THE CORRECTION BACK TO 5000 FT. THE CTLR THEN STATED 'YOU WERE CLRED ONLY TO 5000 FT.' I ROGERED THIS AS WE WERE AGAIN AT 5000 FT. WE CALLED THE ARPT IN SIGHT AND WERE CLRED FOR A VISUAL TO RWY 15. NO FURTHER COMMENTS WERE MADE REGARDING OUR ASSIGNED ALT. VISIBILITY WAS 20 MI. FOG WAS FORMING TO THE SW OF ARPT AND SUN COMING UP FROM E DID NOT LET US SEE ARPT UNTIL APPROX 8 MI FROM ARPT. BETTER COMS (CREW LEADERSHIP RESOURCE MGMNT) BTWN CAPT AND FO WOULD HAVE PRECLUDED THIS. ALSO; THE USE OF THE AUTOFLT SYS WOULD HAVE ALERTED CAPT WHEN DSCNT WAS INITIATED; OR THE ALT WINDOW WOULD HAVE TO HAVE BEEN CHANGED. SUPPLEMENTAL INFO FROM ACN 330836: FO (PF) UNDERSTOOD THAT FLC WAS CLRED FOR THE FOUR STACKS VISUAL APCH.