ALTDEV ALT OVERSHOT IN DSCNT. POSSIBLE SPD DEV.

Narrative

THIS WAS OUR THIRD LEG OF THE MORNING. DURING APCH; WE WERE CLRED TO DSND AND MAINTAIN 8000 FT MSL. APCHING THE LEVEL-OFF AT 8000 FT; THE CTLR ISSUED US A SPD REDUCTION FROM 250 KTS TO 180 KTS; IF WE'RE ABLE. THE CTLR REPEATED THE INSTRUCTIONS AS THE FO WAS A LITTLE SLOW IN VERBALLY RESPONDING. SUDDENLY; THE WORKLOAD INCREASED. WE HAD TO QUICKLY RECONFIGURE THE ACFT IN ORDER TO SLOW TO 180 KTS; AS THERE WAS A SENSE OF URGENCY IN THE CTLR'S VOICE. THE COMS WITH ATC BECAME A LITTLE DISJOINTED AS WE ATTEMPTED TO ADVISE THE CTLR THAT WE UNDERSTOOD AND COULD COMPLY WITH THE SPD REDUCTION. THIS DISTR COMPOUNDED THE WORKLOAD. DURING THIS PERIOD; THE ACFT'S ALT ARM FEATURE BECAME DISARMED. WE DSNDED 200 FT BELOW OUR ASSIGNED ALT AND WE CORRECTED UPON NOTICING THE DEV. THE CTLR ASKED US IF WE'RE LEVEL AT 8000 FT. BY THE TIME WE RESPONDED; WE WERE JUST LEVELING AT 8000 FT. SEVERAL FACTORS WERE RESPONSIBLE FOR THIS OCCURRENCE. FIRST THE SUDDEN INCREASE IN WORKLOAD. SECOND; A POORLY DESIGNED FLT GUIDANCE SYS THAT ALLOWS FOR EASY PLT INDUCED DISARMING OF ALT HOLD SYS. I SUSPECT THAT I MAY HAVE SELECTED A REDUCED VERT SPD (TO ASSIST IN REDUCING ACFT SPD) JUST AS THE ACFT WAS ENTERING THE 'ALT CAPTURE' PHASE; THUS DISARMING THE ALT CAPTURE. SOME OF MY COMPANY'S ACFT CONTINUE TO UTILIZE AN OLD FLT GUIDANCE SOFTWARE PROGRAM THAT HAS THIS DESIGN FLAW. THIRDLY; FATIGUE PLAYED SOME PART. WE HAD A SHORT LAYOVER; AND I HAD TROUBLE SLEEPING THE PREVIOUS NIGHT. THIS WAS COMPOUNDED BY AN EARLY MORNING DEP. HAVING FLOWN WITH IRS'S FMC'S FOR OVER 7 YRS; I FEEL REASONABLY COMFORTABLE IN COMMENTING ABOUT OUR USER UNFRIENDLY SYS. THE SYS IS GREAT (FMS/IRS) AT MIDNIGHT; WHEN NO ONE ELSE IS AROUND; BUT OTHER THAN GOING SOMEPLACE DIRECT (COAST TO COAST) THE SYS PROVIDES US WITH A MAGNIFICENT PLATTER OF INFO; BUT OPERATIONALLY; IN CANNOT BE USED. THEY TEACH NOTHING BUT USING THE FMS FOR ALL 'IN FLT' CHANGES FROM SPDS TO CLBS TO DSCNTS; ETC. IN THE CHKRIDES I HAVE OBSERVED; 90% OF THE PLTS OF THESE NEW FANGLED SYS SEEM TO LOVE THE GLASS COCKPITS/FMS/IRS/TCASII ET AL. BUT -- THEY CANNOT SAFELY OPERATE THEM; NOR ARE THEY AT ALL PROFICIENT IN OPERATING THEM. LETþS FACE IT; IT TAKES TIME TO ENTER AN ALT CHANGE IN THE COMPUTER. IF IT IS ASSOCIATED WITH AN AIRSPD CHANGE; AND AN IMMEDIATE DSCNT; ANOTHER PAGE MUST BE SELECTED AND THE COMPUTER SEEMS TO BE QUITE SLOW IN THIS REGIME. SOMETIMES OVER 1 MIN ELAPSES BEFORE THE ACFT FINALLY RESPONDS. TO ME; THIS IS UNACCEPTABLE IN TODAY'S CROWDED AIRSPACE.