DC9 CREW HAD ALT OVERSHOOT.

Narrative

ACR FLT ABCD OPERATED FROM ILN TO LAX WITH A REFUELING STOP IN OKC. THIS WAS MY FIRST TRIP OF IOE AS A DC9 FO. ON THE LEG FROM OKC TO LAX; OUR FLT RECEIVED A LATE CHANGE IN THE STAR PROC TO THE PDZ 3 ARR. THE CHANGE IN THE ARR WAS RECEIVED AFTER WE HAD BEGUN OUR DSCNT; WHICH GAVE THE FLC LITTLE TIME TO STUDY THE PROFILE. THIS ARR OF COURSE CONSISTED OF NUMEROUS STEPDOWNS; WHICH I TALKED THE CHK CAPT (PF) THROUGH. AS WE APCHED THE PDZ VOR; THE ARR CALLED FOR A STEPDOWN TO 12000 FT; FOLLOWED SHORTLY BY A COURSE CHANGE PASSING THE PDZ VOR. AT THIS POINT; ACR ABCD DSNDED THROUGH 12000 FT TO APPROX 11600-11700 FT FOR A PERIOD OF LESS THAN 10 SECONDS TOTAL DEV FROM 12000 FT. AFTER RECOVERING TO 12000 FT; THE FLT PROCEEDED NORMALLY. THE ALTDEV WAS CAUSED BY THE PF BEING DISTR FROM CTLING THE ACFT FLT PATH IN ORDER TO VERIFY/CONFIRM THE STAR AS DESCRIBED BY ME; THE PNF. WHILE MY DESCRIPTION OF THE ARR WAS ACCURATE AND DICTATED IN AN ORGANIZED AND EFFICIENT MANNER; THERE IS A NATURAL RELUCTANCE FOR A CHK AIRMAN TO BLINDLY ACCEPT THIS INFO FROM A NEW FO -- ESPECIALLY WHEN THE PLT HAS NOT HAD MUCH TIME TO PREPARE. MY BACKGND INCLUDED APPROX 3000 HRS AS A FO IN DC8; AND A300'S WITH MANY PREVIOUS TRIPS TO LAX. WHILE I AM NOT CERTAIN IF THE CAPT KNEW OF MY PREVIOUS EXPERIENCE; THIS INFO WAS AVAILABLE TO HIM. NONE OF THE ABOVE SHOULD HAVE RESULTED IN ANY DEV. IN FACT; IT IS CERTAINLY PRUDENT TO CONFIRM AND VERIFY THE INSTRUCTIONS GIVEN BY THE PNF. NORMALLY THIS WOULD NOT CAUSE ENOUGH OF A DISTR TO INTERFERE WITH ACFT CTL. HOWEVER; IN THIS PARTICULAR CASE; PLT FATIGUE WAS THE FACTOR WHICH ALLOWED THIS DEV TO OCCUR. BOTH THE CAPT AND I HAD BEEN UP ALL NIGHT. FIRST FLYING INTO ILM AND OUTBOUND TO LAX. WHILE THE FLT WAS LEGAL IN TERMS OF A DUTY DAY AND FLT TIME LIMITATIONS; I FEEL OUR COGNITIVE ABILITY WAS CERTAINLY REDUCED DUE TO THE CIRCADIAN CYCLE.