CAPT OF A BEECHCRAFT KINGAIR 90 FAILED TO MAINTAIN ASSIGNED ALT AND DSNDED PREMATURELY DURING A BACK COURSE APCH. IN ADDITION; THE CAPT FAILED TO MAINTAIN THE FINAL APCH COURSE AND CORRECTED ONLY AFTER DSNDING BELOW THE OVCST TO PICKUP THE RWY VISUALLY.

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ON AUG/XX/96; SMT WAS FLYING FROM UNV TO ABE AT APPROX XA00 PM. AFTER ENTERING ALLENTOWN'S CLASS C AIRSPACE AND BEING VECTORED FOR THE LOC BACK COURSE RWY 24; APCH CTL INSTRUCTED SMT TO TURN R TO A HDG OF 220 DEGS; MAINTAIN 2000 FT UNTIL ESTABLISHED ON THE LOC BACK COURSE RWY 24 TO INTERCEPT THE COURSE BTWN NAZAR AND WILEY. PROBS AROSE WHEN CAPT ENGAGED THE AUTOPLT TO SHOOT THE COUPLED APCH; RESULTING IN THE ACFT TURNING AND DSNDING TO THE L. I REPEATED THE INSTRUCTIONS GIVEN BY APCH CTL. THE ACFT CONTINUED THROUGH THE BACK COURSE AND DSNDED THROUGH 2000 FT MSL. AT THIS POINT; CAPT DISCONNECTED THE AUTOPLT AND MADE A R HAND TURN TO FLY DIRECT TO THE ARPT AND CONTINUED TO DSND. I INDICATED THAT THE ACFT WAS LOW AND L OF COURSE (CDI INDICATED FULL SCALE DEFLECTION). APCH CTL INFORMED US THAT SMT WAS 400 FT BELOW GS (INDICATING 1400 FT MSL) AND TO MAINTAIN 1800 FT UNTIL REACHING WILEY. I RESPONDED THAT WE WERE CORRECTING. CAPT DID NOT RESPOND TO APCH'S DIRECTIONS; PROMPTING ME TO REPEAT THE INSTRUCTIONS. CAPT ONLY RESPONSE WAS THAT THE ACFT WAS 1 MI FROM WILEY; THEREFORE THE ALT WAS NOT A PROB. AFTER PASSING WILEY; THE CAPT DSNDED TO 1100 FT MSL (STILL FULL SCALE DEFLECTION TO THE L) AND LEVELED OFF. AT 1100 FT THE ACFT BROKE OUT OF THE CLOUDS AND THE CAPT WAS ABLE TO TAKE OVER VISUALLY. THROUGHOUT THIS APCH; THE CAPT NEITHER GAVE NOR SOLICITED INFO FROM THE COPLT. THE CAPT; ALSO; DID NOT ACKNOWLEDGE OR TAKE CORRECTIVE ACTION WHEN PROMPTED BY THE COPLT OR APCH CTL. THIS ISSUE HAS BEEN DISCUSSED WITH BOTH THE DIRECTOR OF OPS AND CHIEF PLT AT OUR COMPANY AND THEY ARE TAKING CORRECTIVE ACTIONS WITH THE HELP OF THE CAPT.