FLC OF A CARGO B727 INADVERTENTLY CLBED FROM ASSIGNED ALT DURING INTERCEPT FOR A CAT II ILS APCH WHEN THE FO INADVERTENTLY ENGAGED THE AUTOPLT COUPLER IN THE MANUAL GS MODE INSTEAD OF THE AUTO APCH MODE. CORRECTION WAS MADE BACK TO ASSIGNED ALT AND THERE WAS NO COMMENT FROM ATC AS IT WAS EARLY MORNING HRS WITH NO OTHER TFC.

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ON A SCHEDULED FLT FROM IAH-AFW WITH FO FLYING; WE WERE CONDUCTING A CAT II SYS CHK TO RWY 16L AT AFW. WE WERE VMC AND LEVEL AT 3000 FT MSL WHEN GIVEN THE FINAL INTERCEPT HDG TO INTERCEPT THE LOC AND TOLD TO MAINTAIN 3000 FT UNTIL ESTABLISHED. AT THIS POINT WE BOTH SWITCHED OUR FLT DIRECTORS TO APCH MODE. THE FO SWITCHED TO APCH MODE ON AUTOPLT SELECTOR. ACFT STARTED A VIOLENT PITCH UP AND I TOLD FO TO DISCONNECT AND WATCH AIRSPD. HE WAS SLOW TO DO SO AND WE HAD CLBED 450 FT (3450 FT) BEFORE WE HAD ACFT LEVELED OFF. WE RETURNED ACFT TO 3000 FT AND RE-ENGAGED AUTOPLT. AUTOPLT PERFORMED NORMALLY AND A SUCCESSFUL APCH WAS COMPLETED. I THINK THE FO MAY HAVE SWITCHED AUTOPLT SELECTOR THROUGH THE AUTO APCH MODE TO MANUAL GS MODE CAUSING THE ACFT TO PITCH UP TO TRY AND GET TO GS (WE WERE BELOW IT). I CAUTIONED HIM TO BE CAREFUL ABOUT THIS ON RE-ENGAGE AND HE ACKNOWLEDGED HE WOULD AND THAT HE MAY HAVE SWITCHED TO MANUAL GS THE FIRST TIME. SINCE IT WAS A TEST OF THE AUTOPLT; I THINK WE BOTH ALLOWED AUTOPLT TO PITCH ACFT MORE THAN NORMAL TO SEE WHAT IT WOULD DO AND SEE IF IT WOULD CORRECT BACK. THERE WAS NO OTHER TFC IN IMMEDIATE AREA AND APCH MADE NO COMMENT ABOUT ALT. AS CAPT; NEXT TIME I WILL NOT ALLOW ACFT TO DEVIATE SO MUCH; EVEN IN A TEST SIT.