EXCURSION FROM ASSIGNED APCH VECTORING ALT.

Narrative

WE WERE BEING VECTORED AFTER BEING BROKEN OUT OF THE ARR SEQUENCE TO RWY 19 (ILS) BY SDF APCH CTL DUE TO NOT ENOUGH SPACING BTWN OUR ACFT (DC-8) AND A B-757 WITH A 15 KT SLOWER FINAL APCH SPD. THE FO WAS FLYING THE ILS APCH COUPLED TO THE AUTOPLT DUE TO THE RELATIVELY LOW AND RAGGED CEILING. WE WERE AT THE OM WHEN THE GAR WAS INITIATED AT 2800 FT MSL AND INSTRUCTED TO TURN R 90 DEGS AND CLB TO 3000 FT MSL. THE FO COMPLIED WITH THESE INSTRUCTIONS WHILE TURNING OFF THE AUTOPLT AND RETRACTING GEAR/FLAPS. WHEN ATTEMPTING TO RE-CONNECT THE AUTOPLT; HE DID NOT GET AN ALT CAPTURE AND A SLIGHT DSCNT WAS NOTICED BY THE CAPT WHO IMMEDIATELY POINTED THIS FACT OUT. AT 2900 FT MSL; THE CAPT COMMANDED THE FO TO TURN THE AUTOPLT OFF AND GET BACK UP TO ALT; WHILE AT THE SAME TIME ANOTHER 90 DEG RADIAL TURN WAS GIVEN BY APCH CTL. THE FO IMMEDIATELY ROLLED THE ACFT INTO 30 DEGS OF BANK ANGLE WHICH INCREASED THE RATE OF DSCNT AND KICKED OFF THE AUTOPLT. HE ATTEMPTED TO CORRECT THE DSCNT AND BRING THE ACFT BACK UP TO ALT; BUT HIS CORRECTION WAS TOO SMALL AND THE CAPT ASSUMED CTL AT 2800 FT MSL AND IMMEDIATELY BROUGHT THE ACFT BACK TO 3000 FT MSL. PRIMARY CAUSE: THIS WAS A RESERVE CREW WHO WAS EXPERIENCING CHRONIC FATIGUE FROM 2 PRECEDING INTL LEGS IN WHICH THEY WERE DISTURBED IN THE MIDDLE OF THEIR CREW REST BY CALLS FROM SCHEDULING. SECONDARY CAUSE: INEXPERIENCE OF THE FO.