Narrative:

The first officer was flying on autoplt with LNAV and VNAV engaged. We had been cleared to cross aldan at 10000' and 250 KTS, and put this restriction into VNAV on the rtes legs page. At FL180 I initiated the approach descent checklist and noted that the aircraft was in a descent that would satisfy the above restriction. A short time later I noticed that the VNAV function and commanded a premature speed slowdown and this was causing the aircraft to start going high on the proper descent profile. I informed the first officer what I was doing, overrode the VNAV control with the MCP commands, extended the speed brakes and expedited the descent. After this correction was initiated, the ATC controller asked us to call level at 10000'. We complied with the crossing restriction (just barely, however!), then I took a few seconds to assure we were in a stable and proper flight condition, xferred control of the aircraft back to the first officer, and then I reported level at 10000'. (At that time of calling level we had passed aldan even though we had complied with the restriction at aldan.) I discussed the situation with the first officer later, and we were unable to explain why the VNAV function was not complying with the programmed restrictions. In general, I think VNAV descent programming is too tight. Crossing restrictions are complied with exactly, but with no margin for error or deviation. I think the programming should provide for complying with crossing restrictions in dscnts 3 mi early at 10000' and above. This would give the pilot a little room to deal with unexpected deviations.

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Original NASA ASRS Text

Title: ACR ADVTECH MLG FLT CREW UNHAPPY WITH DESIGN OF VNAV DESCENT PROFILE.

Narrative: THE F/O WAS FLYING ON AUTOPLT WITH LNAV AND VNAV ENGAGED. WE HAD BEEN CLRED TO CROSS ALDAN AT 10000' AND 250 KTS, AND PUT THIS RESTRICTION INTO VNAV ON THE RTES LEGS PAGE. AT FL180 I INITIATED THE APCH DSNT CHKLIST AND NOTED THAT THE ACFT WAS IN A DSNT THAT WOULD SATISFY THE ABOVE RESTRICTION. A SHORT TIME LATER I NOTICED THAT THE VNAV FUNCTION AND COMMANDED A PREMATURE SPD SLOWDOWN AND THIS WAS CAUSING THE ACFT TO START GOING HIGH ON THE PROPER DSNT PROFILE. I INFORMED THE F/O WHAT I WAS DOING, OVERRODE THE VNAV CTL WITH THE MCP COMMANDS, EXTENDED THE SPD BRAKES AND EXPEDITED THE DSNT. AFTER THIS CORRECTION WAS INITIATED, THE ATC CTLR ASKED US TO CALL LEVEL AT 10000'. WE COMPLIED WITH THE XING RESTRICTION (JUST BARELY, HOWEVER!), THEN I TOOK A FEW SECS TO ASSURE WE WERE IN A STABLE AND PROPER FLT CONDITION, XFERRED CTL OF THE ACFT BACK TO THE F/O, AND THEN I RPTED LEVEL AT 10000'. (AT THAT TIME OF CALLING LEVEL WE HAD PASSED ALDAN EVEN THOUGH WE HAD COMPLIED WITH THE RESTRICTION AT ALDAN.) I DISCUSSED THE SITUATION WITH THE F/O LATER, AND WE WERE UNABLE TO EXPLAIN WHY THE VNAV FUNCTION WAS NOT COMPLYING WITH THE PROGRAMMED RESTRICTIONS. IN GENERAL, I THINK VNAV DSNT PROGRAMMING IS TOO TIGHT. XING RESTRICTIONS ARE COMPLIED WITH EXACTLY, BUT WITH NO MARGIN FOR ERROR OR DEVIATION. I THINK THE PROGRAMMING SHOULD PROVIDE FOR COMPLYING WITH XING RESTRICTIONS IN DSCNTS 3 MI EARLY AT 10000' AND ABOVE. THIS WOULD GIVE THE PLT A LITTLE ROOM TO DEAL WITH UNEXPECTED DEVIATIONS.

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.