UNAUTH DSCNT BELOW GS DURING IMC; IAP ILS APCH.

Narrative

WHILE ON AN ILS APCH TO RWY 5R IN CLE I DSNDED BELOW THE GS TO THE POINT OF RECEIVING A GPWS AND TERRAIN ALERT. WE WERE GIVEN A POOR VECTOR TO JOIN THE LOC; SO THE CTLR GAVE US AN ADDITIONAL VECTOR TO JOIN WHICH OCCURRED AT 'HARRI' THE LOM. OUR PREVIOUS AIRSPD ASSIGNED WAS 210 KTS PRIOR TO BEING CLRED FOR THE APCH SO I FOUND MYSELF QUITE BUSY TRYING TO REJOIN THE LOC AND REDUCE MY AIRSPD. UPON XING THE LOM ON THE LOC MY GS ON MY ATTITUDE INDICATOR SHOWED FULL SCALE BELOW WITH NO FLAGS. I MENTIONED TO THE FO THAT I SHOWED FULL SCALE AND THAT I WOULD CORRECT. MY FO CONCURRED ON THE FULL SCALE DEFLECTION; BUT WAS SHOWING THE OPPOSITE; FULL-SCALE ABOVE. WITH THE TWR CTLR INTERRUPTION AND MISCOM ON WHERE WE ACTUALLY WERE ON THE GS THE DECISION TO GO MISSED APCH WAS DELAYED. UPON THE GPWS ALERT I ADDED FULL PWR AND EXECUTED A MISSED APCH. ON THE DOWNWIND FOR OUR SECOND APCH WE CONCLUDED THAT IT WAS MY GS INDICATION THAT WAS AN ERROR. WITH THE TIGHT TURN AT THE MARKER AND BAD VECTOR TO JOIN I WAS FOCUSED ON MY ATTITUDE INDICATOR AND FLT DIRECTOR AND DID NOT XCHK MY HSI; WHICH WAS INDICATING THE CORRECT INFO. MY FO KEPT HEARING ME SAY FULL SCALE AND CORRECTING WHENEVER QUESTIONED AND ASSUMED I WOULD PITCH UP; BUT WITH MY INDICATIONS I WAS DOING THE OPPOSITE. I SUPPOSE THAT BECAUSE IT WAS OUR FIRST FLT TOGETHER SHE WAS NOT MORE ASSERTIVE IN DEMANDING A GAR/MISSED APCH.