DURING AN IOE CHK; FLC OVERSHOT ASSIGNED ALT AND WERE BROUGHT BACK ON ASSIGNED COURSE BY ATC INTERVENTION.

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ON MAR/XX/96 AT XA00Z I WAS ON IOE AND FLYING IN THE L SEAT WITH A CHK AIRMAN; IN THE R SEAT; ON A FLT FROM RDU TO ATL. THE SEQUENCE OF EVENTS ARE AS FOLLOWS: 1) THE CLRNC WAS THE PACKK 4 DEP (PACK4.RDU). THE RTE OF FLT WAS INBOUND TO THE LIB VOR VIA THE 089 DEG RADIAL THEN OUTBOUND ON THE LIB 278 DEG RADIAL FOR 31 DME TO AZELL INTXN FOLLOWED BY DIRECT CAE VOR. 2) JUST PRIOR TO REACHING LIB VOR THE CHK AIRMAN ADVISED ME HE WAS TUNING IN CAE VOR ON THE #2 NAVAID WHICH I INTERPRETED THAT I SHOULD ALSO CHANGE TO CAE AND ADVISED HIM I WAS CHANGING THE #1 NAVAID. 3) I PROCEEDED TO TURN THE ACFT TOWARDS CAE WHEN I SHOULD HAVE REMAINED ON LIB AND PROCEED OUTBOUND ON THE LIB 278 DEG RADIAL TO 31 MI. 4) CTR ADVISED US OF AN 8 MI S OF COURSE ERROR AT THE SAME MOMENT WE SHOULD HAVE BEEN LEVELING AT FL230. 5) WE WERE BOTH DISTRACTED BY OUR POS ERROR FOR SEVERAL SECONDS AND ALLOWED THE ACFT TO CLB THROUGH OUR ASSIGNED ALT. 6) UPON RECOGNITION OF THE ERROR; I IMMEDIATELY CORRECTED BACK TO OUR ASSIGNED ALT. 7) AS I APCHED OUR ASSIGNED ALT; ATC ADVISED US OF A POSSIBLE ALT EXCURSION AND TO GIVE THEM A CALL UPON LNDG. 8) WX WAS VFR AND VISIBILITY GREATER THAN 20 MI. THERE WERE NO TCASII ADVISORIES OR CONFLICTS AND ATC DID NOT ADVISE US OF ANY CONFLICTS EITHER. HUMAN PERFORMANCE CONSIDERATIONS: LACK OF ATTN TO DEP PROC INSTRUCTIONS AND FOLLOWING A SID TO COMPLETION BEFORE TRANSITIONING TO ASSIGNED ROUTING. HOW DOES THIS HAPPEN? I CAN ONLY ANALYZE THAT POSSIBLY; DUE TO SO MANY TIMES WE ARE VECTORED OR CLRED OFF A DEP; THAT HEARING A NEW VOR BEING TUNED IN THAT IT WAS MISTAKENLY INTERPRETED FOR A COURSE CHANGE. OBVIOUSLY; THIS WAS A WRONG ASSUMPTION WHICH NEITHER OF US CAUGHT BEFORE ATC INTERVENTION. FORTUNATELY; WX WAS NOT A FACTOR AND THERE WERE NO TFC CONFLICTS. IT PROVES THAT THE ALERTNESS AND PROFESSIONALISM OF ATC IN THIS INSTANCE HELPED OUR FLT GET BACK ON COURSE SAFELY AND WITHOUT INCIDENCE.