ACR LGT ALTDEV EXCURSION FROM CLRNC ALT. STARTED VISUAL APCH BEFORE BEING CLRED.

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I WAS THE FO ON AN LGT ON APCH TO ORLANDO INTL. THE CAPT WAS FLYING WITH THE AUTOPLT OFF. FLT CONDITIONS WERE VMC AND APCH CTL WAS VECTORING US ON A R DOWNWIND AND BASE FOR A VISUAL APCH TO RWY 18R. WE WERE CLRED TO MAINTAIN 4000 FT; WHICH I SET IN THE ALT ALERT WINDOW. WE WERE GIVEN SUCCESSIVE R TURNS ONTO A BASE LEG AND APCH CALLED OUT LIGHT AIRPLANE TFC SBOUND AT 3000 FT WHICH I RPTED IN SIGHT. WE PASSED ABOVE AND BEHIND THE TFC. WE WERE CLRED TO INTERCEPT THE RWY 18R LOC AND THE LIGHT AIRPLANE WAS TO FOLLOW US. BECAUSE WE WERE HIGH ON THE DOWNWIND AND BASE LEG; THE CAPT CALLED FOR THE LNDG GEAR DOWN AND FINAL DSCNT CHKLIST. WHILE ACCOMPLISHING THE CHKLIST; APCH CALLED ADDITIONAL TFC FOR RWY 17; WHICH I RPTED IN SIGHT. ALSO; I WAS COMPLYING WITH THE CAPT'S CALLS FOR FLAPS; SETTING SUCCESSIVE HDG CLRNCS IN THE FLT DIRECTOR MODE CTL PANEL; AND SETTING FLT DIRECTOR/AUTOTHROTTLE SPD COMMANDS AS CALLED FOR BY THE CAPT. (COMPANY SOP REQUIRES THE PNF TO MAKE ALL INPUTS TO THE MODE CTL PANEL WHEN THE AUTOPLT IS OFF.) AFTER INTERCEPTING THE LOC; APCH ASKED WHAT OUR ALT WAS AND I REPLIED '3100 FT.' APCH TOLD US OUR CLRNC WAS TO MAINTAIN 4000 FT; THEN ASKED THE PLT OF THE LIGHT AIRPLANE IF HE STILL HAD US IN SIGHT. HE DID. AT THAT TIME WE HAD OVERTAKEN THE LIGHT AIRPLANE AND IT WAS AT OUR 3-3:30 O'CLOCK POS AT ABOUT 1/2 TO 1 MI. NO EVASIVE ACTION WAS REQUIRED BY EITHER ACFT. APCH SUBSEQUENTLY CLRED US FOR A VISUAL APCH; AND WE CONTINUED WITHOUT FURTHER INCIDENT. FACTORS WHICH I BELIEVE CONTRIBUTED TO THIS SITUATION: THE CAPT STATED AFTERWARDS THAT HE THOUGHT WE HAD BEEN CLRED FOR A VISUAL APCH; NOT JUST TO INTERCEPT THE LOC AT 4000 FT. I FAILED TO NOTICE THE ALTDEV WHILE ACCOMPLISHING OTHER DUTIES. ON THE LGT; THE ALTDEV ALERTING SYS (AURAL WARNING AND MASTER CAUTION LIGHTS) IS INHIBITED WHEN THE LNDG GEAR ARE DOWN. HIGH RATE OF DSCNT NECESSITATED BY BEING HIGH DOWNWIND. SUGGESTIONS ON PREVENTING A RECURRENCE: PF KEEP THE AUTOPLT ON UNTIL ESTABLISHED ON FINAL DSCNT. IN THIS CASE THE AUTOPLT WOULD HAVE LEVELED OFF AT 4000 FT. ALSO; THE PNF WOULD HAVE A MUCH LOWER WORKLOAD AT AN ALREADY VERY BUSY TIME. RECONFIGURE ALTDEV SYS TO GIVE AN AURAL WARNING EVEN WHEN THE GEAR ARE DOWN.