AN ACR FLC FLYING A B777 TO SFO RPTS DEPARTING FROM THEIR ASSIGNED ALT AFTER INTERCEPTING A RADIAL FOR THE FMS VISUAL.

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I WAS OPERATING THE RADIOS AND THE CAPT WAS FLYING. THE LAST XMISSION FROM SFO APCH WAS TO PROCEED DIRECT TO ARCHI INTXN AND INTERCEPT THE FMS BRIDGE VISUAL TO RWY 28R AT SFO. JUST PRIOR TO ARCHI; I ASKED THE CAPT TO MONITOR THE RADIOS; SO I COULD GO TO THE PA AND ADVISE THE FLT ATTENDANTS TO 'PREPARE FOR LNDG.' WHEN I TOLD THE CAPT I ONCE AGAIN HAD TO RADIO; WE WERE PASSING ARCHI INTXN. HE THEN REQUESTED THAT I SELECT 6000 FT IN THE ALT WINDOW; WHICH WAS THE XING ALT AT TRDOW; OUR NEXT FIX. I COMPLIED WITH HIS REQUEST AND SET THE ALT. SHORTLY AFTER THE CAPT INITIATED THE DSCNT; SFO APCH INQUIRED AS TO OUR ALT. I IMMEDIATELY REALIZED THAT WE MUST NOT HAVE BEEN CLRED FOR THE FMS APCH; EVEN THOUGH WE WERE ESTABLISHED ON THE APCH AND INSIDE OF ARCHI INTXN. AT THE TIME OF THE INQUIRY; WE WERE AT 6700 FT; I REACHED UP AND SELECTED ALT HOLD SO AS NOT TO DSND FURTHER. SFO APCH THEN CONFIRMED THAT WE HADN'T BEEN CLRED FOR THE FMS APCH AND THAT WE WERE LAST CLRED TO 'INTERCEPT ONLY' AND MAINTAIN 7000 FT. I MISTAKENLY THOUGHT WHEN I LEFT THE RADIO TO DO THE PA ANNOUNCEMENT THAT THE CAPT MUST HAVE BEEN CLRED FOR THE FMS APCH; OTHERWISE HE WOULDN'T HAVE REQUESTED ME TO SET IN THE LOWER ALT. I SHOULD HAVE QUERIED HIM AT THAT POINT; BUT DUE TO THE INCREASING WORKLOAD AT THE TIME; FAILED TO DO SO.