HAWKER 800XP FLT CREW HAS AN ALT DEV DURING THE ILS 6 TEB.

Narrative

THE ACFT WAS ON RADAR VECTORS AT 2000 FT MSL FOR THE ILS APCH TO RWY 6 AT TEB. THE AUTOPLT WAS ENGAGED IN FMS AND ALT HOLD MODES WITH THE APCH TUNED. WHEN CLRED TO INTERCEPT THE LOC; I ARMED THE APCH MODE. WE WERE THEN CLRED TO CROSS A FIX ON THE LOC COURSE AT 1500 FT. I STARTED A DSCNT IN THE VERT SPD MODE AND THE AUTOPLT CAPTURED THE APCH ALMOST SIMULTANEOUSLY. FROM SEVERAL MI OUT WE HAD BEEN GETTING ERRATIC GS INDICATIONS; BUT AT THIS POINT IT WAS STEADY AND INDICATING THAT WE WERE BELOW THE GS. WHEN THE AUTOPLT CAPTURED THE ILS; THE ACFT BEGAN TO PITCH UP TO INTERCEPT THE GLIDE PATH. I DISCONNECTED THE AUTOPLT AT 2240 FT INDICATED AND STARTED BACK DOWN TO THE ASSIGNED 1500 FT. ATC IMMEDIATELY REQUESTED VERIFICATION OF ALT AND STATED OUR MODE C INDICATIONS ALL THE WAY DOWN TO 1500 FT WHERE WE CONTINUED THE APCH MANUALLY TO LNDG. MY MISTAKE WAS ARMING THE APCH PREMATURELY. SINCE I HAD ARMED IT WITH THE ALT HOLD CAPTURED AT 2000 FT AND THEN SUBSEQUENTLY DSNDED TOWARD 1500 FT; THE AUTOPLT INITIATED A FLY-UP COMMAND TO INTERCEPT THE GLIDE PATH. I USUALLY DO NOT ARM APCH UNTIL 'CLRED FOR THE APCH' BUT DID SO THIS TIME BECAUSE I HAD THE ARPT IN SIGHT. BOTH CREW MEMBERS WERE SCANNING VISUALLY FOR OTHER TFC AND; THEREFORE; DID NOT STOP THE CLB INITIATED BY THE AUTOPLT SOONER. I TRUSTED THE AUTOMATION TOO MUCH.