Narrative:

This was our third leg of the morning. During approach, we were cleared to descend and maintain 8000 ft MSL. Approaching the level-off at 8000 ft, the controller issued us a speed reduction from 250 KTS to 180 KTS, if we're able. The controller repeated the instructions as the first officer was a little slow in verbally responding. Suddenly, the workload increased. We had to quickly reconfigure the aircraft in order to slow to 180 KTS, as there was a sense of urgency in the controller's voice. The communications with ATC became a little disjointed as we attempted to advise the controller that we understood and could comply with the speed reduction. This distraction compounded the workload. During this period, the aircraft's altitude arm feature became disarmed. We descended 200 ft below our assigned altitude and we corrected upon noticing the deviation. The controller 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 flight guidance system that allows for easy pilot induced disarming of altitude hold system. I suspect that I may have selected a reduced vertical speed (to assist in reducing aircraft speed) just as the aircraft was entering the 'altitude capture' phase, thus disarming the altitude capture. Some of my company's aircraft continue to utilize an old flight 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 departure. Having flown with IRS's FMC's for over 7 yrs, I feel reasonably comfortable in commenting about our user unfriendly system. The system is great (FMS/IRS) at midnight, when no one else is around, but other than going someplace direct (coast to coast) the system provides us with a magnificent platter of information, but operationally, in cannot be used. They teach nothing but using the FMS for all 'in flight' changes from speeds to climbs to dscnts, etc. In the chkrides I have observed, 90% of the pilots of these new fangled system 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 altitude change in the computer. If it is associated with an airspeed change, and an immediate descent, another page must be selected and the computer seems to be quite slow in this regime. Sometimes over 1 min elapses before the aircraft finally responds. To me, this is unacceptable in today's crowded airspace.

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Original NASA ASRS Text

Title: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.