ALTDEV ALT EXCURSION DURING A GAR PROC.

Narrative

CAPT (PF) ASKED FOR APCH FLAPS SETTING OF 15 DEGS. WE WERE AT 4000 FT; ON INTERCEPT VECTOR FOR ILS; AND CLRED FOR APCH. I SET 15 DEG FLAPS AND NOTICED 'SLAT DISAGREE' LIGHT. PF SAID; 'I'VE GOT ACFT AND ATC; YOU GET OUT THE BOOK.' AUTO APCH WAS SELECTED AT THE TIME. I BENT DOWN TO MY BOOK BAG AND GOT OUT THE COCKPIT OPERATING MANUAL AND TRIED TO LOCATE THE PROC FOR THIS PARTICULAR PROB. MEANWHILE; THE PF TOLD ATC OF OUR PROB AND REQUESTED VECTORS OFF THE APCH SO WE COULD WORK THE PROC. IMMEDIATELY; ATC CANCELLED APCH CLRNC AND GAVE US A HDG AND TOLD US TO MAINTAIN 4000 FT. AT ABOUT THE SAME TIME THE AUTOPLT INTERCEPTED THE LOC AND THE GS AND STARTED A DSCNT. THE PF TURNED TO THE HDG OK; BUT FAILED TO STOP THE DSCNT. HE WAS PREOCCUPIED WITH SETTING THE PWR; BECAUSE THE AUTO THROTTLES HAD JUST CLICKED OFF WHEN 15 DEGS FLAPS WAS SELECTED. WHEN I LOOKED UP; WE WERE ABOUT 400 FT LOW. I MADE SOME REF TO THE ALT AT ABOUT THE SAME TIME AS ATC. WE RETURNED TO LEVEL FLT; AND RAN THE MALFUNCTION PROC AND LANDED UNEVENTFULLY. LESSONS LEARNED: EVEN WHEN LOOKING IN THE BOOK; XCHK THE OTHER PLT'S FLYING PERFORMANCE. WHEN FLYING AUTO APCHS; PLAN ON FAILURES OF AUTOPLT AND AUTO THROTTLE AT THE WORST POSSIBLE TIME.