Narrative:

Shortly before gramm on the RIIVR2 arrival into lax; we were issued a 'descend via' clearance and cleared for the ILS 25L approach. We were also told to plan on holding an airspeed of 280 knots 'well past riivr.' that being said; I hard coded 280 knots on the descent page as well as 250 knots at each waypoint below 10;000 to fuelr (where the FMC usually plans a slower speed and where ATC typically slow us back from 250 knots).we were then issued 250 knots in the vicinity of riivr; which left us high on the profile that I hard coded based on the 280 knots comment above. Then; descending through 11;000 feet outside of krain; we were issued 210 knots.I initially waited a few seconds as we were approaching 10;000 with a planned level off and I did not want to execute a speed command right at an altitude level off. However; the airplane was not slowing fast enough (even with speed brakes) and I was concerned that we were gaining on the traffic ahead; so I entered and executed the speed change. Shortly after I entered the 210 knots into the FMC; I looked at the FMA to confirm we were still in VNAV path. To my surprise; the pitch mode changed to cws P and the airplane began descending below the 10;000 foot altitude restriction between luvyn and krain (speed was at 250 and thus ok).I immediately disconnected the autopilot to recover; but we had already descend 150 below the minimum altitude. I hand flew the level off; re-engaged the autopilot and reselected VNAV. At that point; the VNAV started reducing the airspeed (possibly because we were high and past the newly calculated descent point). I again turned the autopilot off and hand flew the remainder of the approach.as I reflect back on the flight; I am somewhat at a loss for what I could have done differently. I have flown that approach many times; in different airplanes; and feel quite comfortable with the arrival and approach. I do occasionally have trust issues with the smiths FMC on the 737 as it seems like it gets confused with either too many inputs or inputs at critical times.in this case; neither the first officer nor myself recall bumping the control wheel and I believe the autopilot must have 'given up' when it concluded that it could not keep up with the commands. Though I have been on this airplane 1 1/2 years; I have never seen the autopilot quietly revert to cws P with no physical contact or apparent reason. We did not receive a 'des path unachievable' nor did the MCP revert to VNAV speed or lvl chg. While I believe my vvm and cross checking were heightened at this time; it all happened at a particularly busy time at 10;000 feet while also double chiming the cabin and the first officer preparing for the approach checklist.all this being said; I believe this situation could have been avoided had we not been issued two speed changes; in a short period of time; after being told to expect to keep the speed up on the arrival; all while trying to descend via the arrival in an aircraft that does not have the speed intervene button. As a final piece of irony; the first officer and I were just discussing the recent safety report that featured an ATC section on this very topic of close-in clearance changes on complex descend via stars!however; I plan on being much more vigilant on this arrival in the future and will likely consider vertical speed with incremental altitude resets on the MCP if I get more than one speed or altitude change during a busy part of the arrival/approach.

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Original NASA ASRS Text

Title: B737 flight crew reported descending below a crossing restriction when they failed to notice the FMC mode change from VNAV to CWS (Control Wheel Steering).

Narrative: Shortly before GRAMM on the RIIVR2 arrival into LAX; we were issued a 'descend via' clearance and cleared for the ILS 25L approach. We were also told to plan on holding an airspeed of 280 knots 'well past RIIVR.' That being said; I hard coded 280 knots on the descent page as well as 250 knots at each waypoint below 10;000 to FUELR (where the FMC usually plans a slower speed and where ATC typically slow us back from 250 knots).We were then issued 250 knots in the vicinity of RIIVR; which left us high on the profile that I hard coded based on the 280 knots comment above. Then; descending through 11;000 feet outside of KRAIN; we were issued 210 knots.I initially waited a few seconds as we were approaching 10;000 with a planned level off and I did not want to execute a speed command right at an altitude level off. However; the airplane was not slowing fast enough (even with speed brakes) and I was concerned that we were gaining on the traffic ahead; so I entered and executed the speed change. Shortly after I entered the 210 knots into the FMC; I looked at the FMA to confirm we were still in VNAV PATH. To my surprise; the pitch mode changed to CWS P and the airplane began descending below the 10;000 foot altitude restriction between LUVYN and KRAIN (speed was at 250 and thus ok).I immediately disconnected the autopilot to recover; but we had already descend 150 below the minimum altitude. I hand flew the level off; re-engaged the autopilot and reselected VNAV. At that point; the VNAV started reducing the airspeed (possibly because we were high and past the newly calculated descent point). I again turned the autopilot off and hand flew the remainder of the approach.As I reflect back on the flight; I am somewhat at a loss for what I could have done differently. I have flown that approach many times; in different airplanes; and feel quite comfortable with the arrival and approach. I do occasionally have trust issues with the Smiths FMC on the 737 as it seems like it gets confused with either too many inputs or inputs at critical times.In this case; neither the First Officer nor myself recall bumping the control wheel and I believe the autopilot must have 'given up' when it concluded that it could not keep up with the commands. Though I have been on this airplane 1 1/2 years; I have never seen the autopilot quietly revert to CWS P with no physical contact or apparent reason. We did not receive a 'Des Path Unachievable' nor did the MCP revert to VNAV SPD or LVL CHG. While I believe my VVM and cross checking were heightened at this time; it all happened at a particularly busy time at 10;000 feet while also double chiming the cabin and the First Officer preparing for the Approach Checklist.All this being said; I believe this situation could have been avoided had we not been issued two speed changes; in a short period of time; after being told to expect to keep the speed up on the arrival; all while trying to descend via the arrival in an aircraft that does not have the Speed Intervene button. As a final piece of irony; the First Officer and I were just discussing the recent safety report that featured an ATC section on this very topic of close-in clearance changes on complex descend via STARs!However; I plan on being much more vigilant on this arrival in the future and will likely consider Vertical Speed with incremental altitude resets on the MCP if I get more than one speed or altitude change during a busy part of the arrival/approach.

Data retrieved from NASA's ASRS site 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.