Narrative:

We were inbound to lax on the riivr 2 approximately 2 miles from russt. During the descent; ATC issued us a runway assignment different from what we had already programmed in the FMC. I was pilot not flying and began to program the new runway into the FMC. I was taking my time; being careful to load the correct arrival; transition; and runway. As I was in the middle of doing this; ATC issued yet another airspeed change and captain programmed the new speed into his CDU and executed the change. Unfortunately; at the very moment he hit the execute button; I had a discontinuity on the legs page. The changes on the legs page got executed also; with a discontinuity at the top of legs page 1; resulting in LNAV disengaging and the MCP reverting to cws right mode. This happened right about the time we hit the large course change in the STAR but the aircraft was continuing on its previous heading due to the cws right mode. We immediately realized what happened; and captain made an appropriate and timely course correction back to the riivr arrival course. The error was corrected very quickly; so I was surprised when the controller asked us to verify that we were on course; and she offered us vectors to the localizer. Captain and I had already discussed earlier in the flight how easy it is to make an error on these arrivals. I am familiar with the alert handouts and the alert plate regarding the lax stars. From all of these pubs; we knew we had to be super vigilant going into lax on this arrival; yet we still raised the eyebrows of ATC.we're all human; and every one of us has forgotten to verify a change in the FMC before executing it at some time or another. This highlights the need; once again; to verify all changes made on the FMC CDU; no matter how seemingly small. We were a crew that was well rested; had a good rapport; and were both pretty comfortable with the new automation procedures. Yet; we still almost got bitten. It was the worst possible place on the STAR for that execution error to occur. Captain did a great job of making a timely; positive course correction and I think we both did a good job immediately communicating to each other the error made and how to fix it. In the future; when lax ATC makes a runway change at a high workload moment; and any error is made on our part; I will be more proactive in just requesting vectors to join the new localizer if our workload becomes elevated for any reason.

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Original NASA ASRS Text

Title: B737 flight crew experienced a track deviation while executing a runway change during RIIVR 2 arrival to LAX.

Narrative: We were inbound to LAX on the RIIVR 2 approximately 2 miles from RUSST. During the descent; ATC issued us a runway assignment different from what we had already programmed in the FMC. I was pilot not flying and began to program the new runway into the FMC. I was taking my time; being careful to load the correct arrival; transition; and runway. As I was in the middle of doing this; ATC issued yet another airspeed change and Captain programmed the new speed into his CDU and executed the change. Unfortunately; at the very moment he hit the execute button; I had a discontinuity on the LEGS page. The changes on the LEGS page got executed also; with a discontinuity at the top of LEGS page 1; resulting in LNAV disengaging and the MCP reverting to CWS R mode. This happened right about the time we hit the large course change in the STAR but the aircraft was continuing on its previous heading due to the CWS R mode. We immediately realized what happened; and Captain made an appropriate and timely course correction back to the RIIVR Arrival course. The error was corrected very quickly; so I was surprised when the Controller asked us to verify that we were on course; and she offered us vectors to the localizer. Captain and I had already discussed earlier in the flight how easy it is to make an error on these arrivals. I am familiar with the alert handouts and the alert plate regarding the LAX STARs. From all of these pubs; we knew we had to be super vigilant going into LAX on this arrival; yet we still raised the eyebrows of ATC.We're all human; and every one of us has forgotten to verify a change in the FMC before executing it at some time or another. This highlights the need; once again; to verify ALL changes made on the FMC CDU; no matter how seemingly small. We were a Crew that was well rested; had a good rapport; and were both pretty comfortable with the new automation procedures. Yet; we still almost got bitten. It was the worst possible place on the STAR for that execution error to occur. Captain did a great job of making a timely; positive course correction and I think we both did a good job immediately communicating to each other the error made and how to fix it. In the future; when LAX ATC makes a runway change at a high workload moment; and any error is made on our part; I will be more proactive in just requesting vectors to join the new localizer if our workload becomes elevated for any reason.

Data retrieved from NASA's ASRS site as of April 2012 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.