GPWS BELOW GS WARNING SAVES THE DAY FOR AN ACR CREW.

Narrative

THE DAY PRIOR TO THIS INCIDENT THE FLC HAD BEEN UP SINCE XA00 AT HOME AND WORKED UNTIL XP40 PM (STARTED AT XB50 AM). WE FLEW 7 LEGS (7 HRS AND 40 MINS). WE WERE FLYING FROM DES MOINES TO CINCINNATI WITH 8 HRS OF REST. CAPT WAS FLYING. ON BASE LEG FOR THE ILS RWY 36L AT CINCINNATI; CAPT DISCOVERED A PROB WITH THE FLT SPOILERS. HE WAS DISTRACTED WITH THAT AND I HAD THE EMER CHKLIST OUT IN CASE. WE WERE CLRED FOR THE APCH. THE CAPT DID NOT ARM THE APCH MODE ON THE AUTOPLT. HE HIT THE NAV BUTTON. THE AUTOPLT CAPTURED THE LOC BUT NOT THE GS. I STATED THAT 'THE GS IS BELOW US' (WE WERE HIGH). THE CAPT SAID 'UN HUH' AND DIALED IN A STEEPER DSCNT ON THE VERT SPD MODE. (THIS WON'T CAPTURE THE GS.) MAKING THINGS WORSE; HE HAD DIALED A HIGHER ALT IN THE AUTOPLT (5000 FT?) FOR THE MISSED SO WE DIDN'T HAVE ANY WARNING THAT WE WERE GOING BELOW ALT; UNTIL THE PLANE YELLED 'GS; GS.' WE WERE AT THE BASE OF THE CLOUDS AT 700 FT AGL MAYBE 2-3 MI FROM THE RWY. WE WENT AROUND. WE INFORMED THE FLT ATTENDANT AND PAX THAT WE DID A GAR FOR TFC. WE SHOT ANOTHER ILS RWY 36L. THIS ONE WAS UNEVENTFUL. THIS WOULDN'T HAVE HAPPENED IF WE HADN'T BEEN DISTRACTED AND SLEEPY DURING A BUSY PHASE OF FLT.