AN A320 CAPT IN TRAINING DSNDS ON GS BELOW THE AUTH ALT WHILE OFF THE LOC COURSE. THE CHK PLT HAD INSISTED ON THE TRAINEE USING ONLY RAW DATA FOR THE APCH AND THE TRAINEE OVERSHOT THE TURN TO INTERCEPT THE ILS. TWR CTLR GAVE THE FLT A VISUAL APCH CLRNC.

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WE WERE ON A REGULARLY SCHEDULED ACR PAX FLT FROM ORD TO ATL. THE FLT WAS AN IOE FLT FOR ME; CONDUCTED AS PART OF MY INITIAL CAPT QUALIFICATION ON THIS ACFT AND WITH A PART 121 ACR. A COMPANY LINE CHK AIRMAN (LCA) WAS ACTING AS MY FO. FOR TRAINING/EVALUATION PURPOSES; HE OPTED TO HAVE ME FLY THE ENTIRE FLT WITHOUT AUTOPLT; AND ONCE ESTABLISHED IN CRUISE AT FL330; THE AUTOTHROTTLES WERE DISCONNECTED FOR THE BAL OF THE FLT. ON THE RADAR DOWNWIND FOR AN ILS TO RWY 26R AT ATLANTA; FOLLOWING A 'ROME NINE ARR' (BWG.RMG9); THE LCA DISCONNECTED MY FLT DIRECTOR. THIS RESULTED IN A MANUAL; RAW DATA FLT SIT. APCH CTL GAVE US A R TURN TO A HDG OF 180 DEGS FOR A BASE LEG; WITH A DSCNT TO 4000 FT. THIS BASE LEG WOULD HAVE INTERCEPTED THE ILS RWY 26R LOC JUST INSIDE OF 'FREAL' AT APPROX 12 DME/ATL. THIS HEADING WAS SOON MODIFIED TO 240 DEGS AND THEN 250 DEGS TO JOIN THE LOC AT 4000 FT AND CLRNC FOR THE ILS RWY 26R. WE WERE ALSO INSTRUCTED TO CONTACT THE TWR AT THAT TIME. AT 4000 FT; PRIOR TO BEING ON THE LOC COURSE (I HAD OVERSHOT THE INTERCEPT HEADING) I BEGAN TO DSND ON THE GS. AT THIS TIME; THE LINE CHK AIRMAN (FO) WAS LOOKING UP THE TWR FREQ. HE LOOKED UP TO NOTICE MY PREMATURE DSCNT AND HEADING OVERSHOOT AND ADVISED ME TO CORRECT BACK TO 4000 FT UNTIL ON THE LOC. THE TWR CTLR; NOTING OUR POS; ASKED WHETHER WE HAD A PROB AND WHETHER WE HAD THE ARPT IN SIGHT. THE LINE CHK AIRMAN REPLIED THAT THERE WAS NO PROB BUT THAT THERE WAS TRAINING IN PROGRESS AND THAT WE HAD THE FIELD IN SIGHT. THE TWR CTLR THEN CLRED US FOR A VISUAL APCH TO RWY 26R. IN THE PREMATURE DSCNT; I NOTICED A MINIMAL ALT OF 3650- 3700 FT MSL; APPROX 15 DEGS N OF THE LOC COURSE AT ABOUT 10-12 DME/ATL. THE APCH WAS FULLY STABILIZED BY 2000 FT MSL (1000 FT AGL). FROM THAT POINT; WE COMPLETED AN UNEVENTFUL VISUAL APCH AND LNDG ON RWY 26R. HUMAN PERFORMANCE: I WAS WELL AWARE OF THE RESTR AND PROCEDURAL REQUIREMENTS TO MAINTAIN 4000 FT UNTIL ESTABLISHED ON THE LOC COURSE. I ALLOWED MYSELF TO BECOME FIXATED ON THE GS INDICATOR; SUBDUING MY AWARENESS OF LOC PROX. THAT FIXATION ALSO PLAYED A PART IN MY OVERSHOOTING THE ASSIGNED INTERCEPT HEADING AND DETAILED SITUATIONAL AWARENESS. CONTRIBUTING FACTORS: FOR THE PAST 5 YRS; I HAVE BEEN A FO ON B75/767/777 ACFT; FLYING ALMOST EXCLUSIVELY TRANS-ATLANTIC OPS WITH AUGMENTED CREWS. LEGS WERE INFREQUENT (2 OR 3 PER MONTH ON AVERAGE). MANUAL/RAW DATA ONLY OPS WERE LIMITED SINCE THE USE OF AUTOFLT IS PREFERRED IN THE OP OF THESE ACFT (AS WELL AS THE A320). AS A RESULT; I HAVE FOUND THAT MY MANUAL/RAW DATA FLYING SKILLS HAVE DETERIORATED SOMEWHAT. THEY ARE COMING BACK BUT THEY OBVIOUSLY STILL NEED IMPROVEMENT.