Narrative:

During the riivr two arrival into lax; we were assigned to cross gramm at [fl]190. We had already completed all briefings. Set 190 into box and started the descent. Just before arriving at gramm we were assigned to descend via the riivr two arrival. At this time I did the mental math and noticed that the donut was giving us a false reading of being too high due to airbus's logic of trying to maintain a constant descent rate and that rate was started to meet the new clearance of gramm at 190. Therefore to prevent the aircraft from entering an unnecessary dive; I set the new cruise altitude to 1. This is a technique that I use every day on almost every arrival. For some reason this time this action caused the FMGC to get confused and drop all of the altitiude constraints on the arrival. I quickly did the mental math again and started the required descent for the next fix. I then put the aircraft into heading mode and asked the first officer to activate the secondary flight plan. He did this action and again the box dropped all the altitude constraints. Knowing I was in the heading mode I was trying my best to keep the aircraft on course and keep doing the mental math for all of the altitude constraints. During this time I then asked the first officer to de-select the runway and then input the correct runway again to see if that would get the altitude constraints back into the box. I was in the turn in order to stay on the arrival; however what I failed to notice was that my map was further out then I thought; therefore I did not turn in time to stay completely on the arrival. I do believe I stayed within the legal limits of the arrival; however ATC noticed I was 'drifting' off course and asked if we were turning to stay on the arrival; which we were. Then ATC asked if the cockpit was still secure; which it was. Good news; during all of this I never missed an altitude; however; it would appear that I did fail in properly navigating the arrival! If anyone can tell me why the aircraft reacted this way to me inputting the new cruise altitude to reset the donut; I would like that information. Again; this is a practice that I do everyday; I even did it twice the following two days of flying; all without a glitch. I am very disappointed that I could not keep the aircraft directly on the arrival. I hand fly this aircraft more then most; and I always do the mental math for my descents. It would appear that I over tasked myself with flying and trying to figure out what went wrong with the FMS. Though I thought I did a good job of delegating; it would appear that I did not do a good enough job of flying the aircraft.

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Original NASA ASRS Text

Title: Because the FMGC was not maintaining the LAX RIIVR TWO Arrival profile after GRAMM; an Airbus Captain set the cruise altitude to 1 resulting in all arrival altitude constraints dropping which then required that he fly the arrival manually.

Narrative: During the RIIVR TWO Arrival into LAX; we were assigned to cross GRAMM at [FL]190. We had already completed all briefings. Set 190 into box and started the descent. Just before arriving at GRAMM we were assigned to descend via the RIIVR TWO arrival. At this time I did the mental math and noticed that the donut was giving us a false reading of being too high due to Airbus's logic of trying to maintain a constant descent rate and that rate was started to meet the new clearance of GRAMM at 190. Therefore to prevent the aircraft from entering an unnecessary dive; I set the new cruise altitude to 1. This is a technique that I use every day on almost every arrival. For some reason this time this action caused the FMGC to get confused and drop all of the altitiude constraints on the arrival. I quickly did the mental math again and started the required descent for the next fix. I then put the aircraft into heading mode and asked the First Officer to activate the secondary flight plan. He did this action and again the box dropped all the altitude constraints. Knowing I was in the Heading Mode I was trying my best to keep the aircraft on course and keep doing the mental math for all of the altitude constraints. During this time I then asked the First Officer to de-select the runway and then input the correct runway again to see if that would get the altitude constraints back into the box. I was in the turn in order to stay on the arrival; however what I failed to notice was that my map was further out then I thought; therefore I did not turn in time to stay completely on the arrival. I do believe I stayed within the legal limits of the arrival; however ATC noticed I was 'drifting' off course and asked if we were turning to stay on the arrival; which we were. Then ATC asked if the cockpit was still secure; which it was. Good news; during all of this I never missed an altitude; however; it would appear that I did fail in properly navigating the arrival! If anyone can tell me why the aircraft reacted this way to me inputting the new cruise altitude to reset the donut; I would like that information. Again; this is a practice that I do everyday; I even did it twice the following two days of flying; all without a glitch. I am very disappointed that I could not keep the aircraft directly on the arrival. I hand fly this aircraft more then most; and I always do the mental math for my descents. It would appear that I over tasked myself with flying and trying to figure out what went wrong with the FMS. Though I thought I did a good job of delegating; it would appear that I did not do a good enough job of flying the aircraft.

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.