ACR MLG ON VNAV INCURRED HEADING EXCURSION ON LOCALIZER INTERCEPT AND IN CORRECTING AZIMUTH MANUALLY ALT CROSSING RESTRICTION WAS OVERSHOT ON DESCENT.

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LOC BACK COURSE R23. CLRED FOR APCH. CROSSED ATELL AT 3000' MSL; BEGAN DSCNT TO CROSS LECAR FAF AT 2300' MSL. 1 1/2 MI PRIOR TO LECAR ACFT BANKED LEFT 20 DEGS. I DISCONNECTED AUTOPLT IMMEDIATELY AND BANKED TO RIGHT AND OVERSHOT CENTERED LOC; BUT DID MAINTAIN LOC INFO AND VALID LOCK ON ENTIRE TIME. WHILE BUSY WITH AZIMUTH CONTROL; I DSNDED TO 1800' MSL AT 7 DME. I IMMEDIATELY LEVELED OFF. I DID HAVE VISUAL CONTACT WITH THE GND AT ALL TIMES AND DID ACQUIRE THE RWY VISUALLY AT 1800'. PROB AROSE DUE TO: CDU WAS PROGRAMMED DIRECT ATELL; DIRECT LECAR; WITH LOC BACK COURSE R23 APCH. LEGS PAGE WAS VERIFIED WITH PROPER ALTS AND HDGS. CAPT WAS MISLED INTO BELIEVING ACFT WAS SET UP FOR APCH. CORRECTIVE ACTION: AT FIRST BANKING OF ACFT; I IMMEDIATELY DISCONNECTED AUTOPLT AND HAND FLEW ACFT. SUGGESTIONS: THE MLG NEEDS A LOC BACK COURSE FUNCTION. REASON: THE CDU WAS PROGRAMMED AND LNAV WAS USED TO FLY DIRECT ATELL; DIRECT LECAR. ACFT DID SO. ON DEP/ARR PAGE; SELECTED LOC BACK COURSE 23. AFTER CLRING ALL ROUTE DISCONTINUITIES; HDGS AND ALTS WERE VERIFIED CORRECT FOR APCH. JUST PRIOR TO LECAR LNAV INFO WAS NO LONGER AVAILABLE AND ACFT BANKED. I ASSUMED WHEN LNAV DISENGAGED; ACFT WOULD REMAIN STRAIGHT AND LEVEL AND I WOULD BE ABLE TO FOLLOW RAW DATA ON THE BACK COURSE APCH. WITH MY ATTN DIVERTED TO LAT CONTROL; I ALLOWED ACFT TO DSND TO 1800'; 500' BELOW REQUIRED ALT AT LECAR. SUGGESTION: CLT USES R23 AS A PRIMARY INSTRUMENT RWY. WITH 2 N/S LONG INSTRUMENT RWYS (36L/R); NOISE ABATEMENT DURING INSTRUMENT CONDITIONS SHOULD NOT DICTATE WHICH RWY WILL BE USED. SUPPLEMENTAL INFO FROM ACN 93437: THIS PROB AROSE DUE TO CDU PROGRAMMED DIRECT ATELL; DIRECT LECAR WITH LOC BACKCOURSE 23 APCH IN COMPUTER. THE LEGS PAGE VERIFIED PROPER ALTS AND HDGS. THE CAPT WAS MISLED TO BELIEVE HE WAS SET UP FOR APCH. CALLBACK CONVERSATION WITH RPTR ACN 93437 REVEALED THE FOLLOWING: RPTR STATED THEY DID THIS APCH AGAIN LATER IN THE DAY AND THE FMC PERFORMED PROPERLY. NOT CERTAIN WHAT CAUSED EXCURSION ON INTERCEPT; BUT WAS CERTAIN APCH WAS NOT ARMED. MAYBE GREMLINS.