PLT FUSSING WITH AUTOPLT GOES BELOW GS UNTIL GIVEN AN ALERT BY TWR AND HE RETURNS TO FLYING THE ACFT.

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COPLT WAS FLYING AND CLRED FOR APCH PHL ILS RWY 27R. AFTER CLRED TO 2100 FT MSL; COPLT WAS TRYING TO GET AUTOPLT TO LOCK ON FOR ILS TO RWY 27R TURNING R TO SPEEZ (OM). DURING LNDG CHKLIST; ACFT STARTED DSNDING THROUGH 2100 FT MSL AFTER SPEEZ AT AROUND 1500 FPM. AFTER I MENTIONED OUR ALT AND BEING 2 DOTS LOW ON GS; HE STARTED LEVELING OFF AT 1200 FT MSL; AT WHICH TIME TWR ASKED US TO CHK OUR ALT BECAUSE THEY WERE GETTING A LOW ALT ALERT. COPLT MAINTAINED 1200 FT MSL UNTIL GS RECAPTURE TO AN UNEVENTFUL LNDG. IT APPEARED THAT WE LOST TRACK OF RATE OF DSCNT WHILE LOOKING AT THE GND FOR A VISUAL REF TO OUR POS. ALSO; HIS ATTEMPTS TO LOCK ON WITH AUTOPLT FAILED FOR SOME REASON (UNNEEDED DISTR). OUR VISIBILITY STRAIGHT AHEAD WAS APPROX 2 SM; BUT WE COULD SEE THE GND CLRLY. I'M NOT SURE IF HE WAS LOOKING AT AIRSPD BUG AND TRYING TO SLOW ACFT OR NOT; BUT HIS SCAN FAILED TO SEE THE DEV (LOW) OF OUR GS. DOING THE LNDG CHKLIST CAUSED ME TO NOT CATCH OUR DEV (LOW) UNTIL 1500 FT MSL; AT WHICH TIME CORRECTIONS WERE PUT INTO PLACE TO LEVEL OFF. I THINK A LITTLE EARLIER PREPARATION ON BASE LEG COULD HAVE HELPED US FROM BEING RUSHED ON FINAL. COPLT USING RAW DATA TO MANUALLY FLY APCH INSTEAD OF WASTING PRECIOUS TIME TRYING TO GET THE AUTOPLT TO CAPTURE THE APCH WAS ESSENTIAL. ONCE HE DID THAT; REMAINDER OF APCH AND LNDG WERE FINE.