FMS EQUIPPED MD80 FLC ALTDEV ALT OVERSHOT IN DSCNT WHEN PIC; THE PNF; IS FLC DISTR TASK AND FAILS TO BACK UP FO IN DSCNT PROCS.

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WE WERE BEING VECTORED TO INTERCEPT THE LOC FOR AN ILS RWY 35L AT COS. CLRED TO DSND TO 8100 FT. PRIOR TO THE FAF; APCH CTL PROVIDED A WX UPDATE WHICH WAS BELOW OUR LNDG MINIMUMS. I ADVISED APCH WE COULD NOT LAND AND REQUESTED HOLDING INSTRUCTIONS. WE WERE TOLD TO HOLD ON THE LOC AT PETEY AT 8100 FT WITH R TURNS. I WAS PROGRAMMING THE FMS FOR THE HOLDING PATTERN AND OBSERVED THE FO (PF) DSND BELOW 8100 FT. I ADVISED HIM TO LEVEL OFF AT 8100 FT AND REACHED UP AND ENGAGED ALT HOLD AS WE RECEIVED AN AURAL ALT ALERT. WAS CORRECTING BACK TO 8100 FT WHEN APCH CTL ADVISED US TO MAINTAIN 8100 FT. DSCNT STOPPED AT 7850 FT. THE ALT ARM WAS ILLUMINATED WHEN THE 1000 FT ABOVE CALL WAS GIVEN. THE DSCNT RATE WAS VERY GRADUAL (APPROX 500-600 FPM). I THINK THAT WORKLOAD SATURATION WAS A CAUSE FOR THE EXCURSION IN SETTING UP FOR THE HOLDING SIT JUST A COUPLE OF MI AWAY FROM THE HOLDING FIX. AS SOON AS I NOTICED THE DSCNT BELOW 8100 FT AND ADVISED THE FO TO LEVEL OFF I REALIZED HE WAS DSNDING IN THE IAS MODE OF THE AUTOPLT. I SUSPECT THAT HE ATTEMPTED TO SLOW THE ACFT USING THE IAS WHEEL WHILE THE AUTOPLT WAS IN THE ALT CAPTURE MODE AND THIS WOULD HAVE CANCELED THE CAPTURE.