Narrative:

Recently denver has added a series of RNAV stars that seem to be taxing the FMC's capability on 737NG aircraft. What is new to these approaches is that they have numerous step-down and speed restrictions. These I assume are [intended] to improve aircraft flow by better coordinating aircraft separation and reducing separation distance by eliminating some of the variables in altitude and speed that were possible in the less restrictive RNAV approaches previously used. My experience on three separate recent occasions is that 737NG aircraft are too often unable to meet the restrictions by relying on the FMC. On one approach VNAV path dropped into pitch/cws three times; on another the aircraft leveled at an arbitrary altitude that was neither on the MCP or CDU and; most recently; VNAV path failed to make most altitudes and made none of the speed restriction. The CDU didn't display drag required; nor did the FMC drop into cws; or display any [directed] control of the aircraft['s vertical path] although I should note that ATC started us down late on that last arrival. My first officer and I had a long conversation over these and other arrivals we have had into denver and agree that; on these approaches; crews will need to inform ATC that we [are able to] meet altitude restrictions or speed restrictions; but not both. [Flight crews can utilize the 'classic'] three miles per thousand feet crutch to monitor autoflight performance; of course; but isn't that meant to be a backup to automation? With these new denver approaches three per thousand is going to have to be the primary descent path methodology and; with many more step-downs and speed restrictions; won't this further task the flight crews instead of easing their workload? Do we really want pilots in level change spinning down the altitude window five or six times and reloading speeds on the descent page every few minutes on an approach that should be flown in VNAV path?

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Original NASA ASRS Text

Title: A B737NG Captain advised of several recent failures of the aircraft's autoflight system to comply with altitude and airspeed restrictions on the newly implemented Optimized Profile Descent RNAV STARs at DEN.

Narrative: Recently Denver has added a series of RNAV STARs that seem to be taxing the FMC's capability on 737NG aircraft. What is new to these approaches is that they have numerous step-down and speed restrictions. These I assume are [intended] to improve aircraft flow by better coordinating aircraft separation and reducing separation distance by eliminating some of the variables in altitude and speed that were possible in the less restrictive RNAV approaches previously used. My experience on three separate recent occasions is that 737NG aircraft are too often unable to meet the restrictions by relying on the FMC. On one approach VNAV PATH dropped into PITCH/CWS three times; on another the aircraft leveled at an arbitrary altitude that was neither on the MCP or CDU and; most recently; VNAV PATH failed to make most altitudes and made none of the speed restriction. The CDU didn't display Drag Required; nor did the FMC drop into CWS; or display any [directed] control of the aircraft['s vertical path] although I should note that ATC started us down late on that last arrival. My First Officer and I had a long conversation over these and other arrivals we have had into Denver and agree that; on these approaches; crews will need to inform ATC that we [are able to] meet altitude restrictions or speed restrictions; but not both. [Flight crews can utilize the 'classic'] three miles per thousand feet crutch to monitor autoflight performance; of course; but isn't that meant to be a backup to automation? With these new Denver approaches three per thousand is going to have to be the primary descent path methodology and; with many more step-downs and speed restrictions; won't this further task the flight crews instead of easing their workload? Do we really want pilots in level change spinning down the altitude window five or six times and reloading speeds on the descent page every few minutes on an approach that should be flown in VNAV PATH?

Data retrieved from NASA's ASRS site as of July 2013 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.