Narrative:

We were on the riivr 2 arrival just outside of the gramm fix. Our previous clearance was for a 25L arrival. Approach control was clearing us to transition to a 24R arrival. I was the pilot not flying and had to read back the clearance two or three times. I was not sure what I might have read back that was in question so I was being as deliberate and clear as possible. The pilot flying had the FMC loaded and was waiting for me to verify the transition to 24R before executing. It was at that time that we encountered jet wash and the aircraft rolled 20 to 30 degrees to the left. My focus immediately went to what was causing it; to what extent we had been upset; and if we needed to take any action to avoid further upset. After a few seconds of scanning; the airplane settled down and I returned my focus to our navigation. We were now tracking about 15 degrees left of what we had been before the upset. I then checked the magenta navigation line and it appeared to be slightly to the right of the aircraft track. I checked the panel and realized that the LNAV had disengaged but the autopilot had not. Therefore there had been no warning horn. I instructed the pilot flying to correct to the right. He began a general correction to the right immediately. Of course he was still waiting for me to verify the transition change. When I then looked at the legs page we had passed the fix which had been loaded several seconds earlier while I was communicating with approach control. I believe that was gramm. I tried to select rustt which is the next fix but we were so close the FMC would not take that change. We selected riivr; which is further ahead; and the FMC took that. Meanwhile socal had noticed our track was off and called us several times to verify that we were making the transition to 24R. The controller's first call was slightly casual and simply asked if we were making the change. I answered in the affirmative. He immediately called back and informed us of traffic at our 7 o'clock position that would be tracking in for 25L. I acknowledged that traffic. He immediately called again and restricted our descent to 12;000 feet. After he saw that we were tracking to 24R; he released us for the approach. We were never told that separation was compromised. It is important to note that the entire event occurred in about a minute. At least six radio transmissions; aircraft upset and recovery; navigation disengagement; recognition and correction. I thought later that I should have told the controller that we had temporarily lost our lateral navigation and I think I would have but I am not sure I could have fit it in before he noticed we were not making the turn for 24L.

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Original NASA ASRS Text

Title: Medium large transport flight crew suffers track deviation when a wake encounter coincided with a runway change on the RIIVR 2 STAR to LAX.

Narrative: We were on the RIIVR 2 Arrival just outside of the GRAMM fix. Our previous clearance was for a 25L arrival. Approach Control was clearing us to transition to a 24R arrival. I was the pilot not flying and had to read back the clearance two or three times. I was not sure what I might have read back that was in question so I was being as deliberate and clear as possible. The pilot flying had the FMC loaded and was waiting for me to verify the transition to 24R before executing. It was at that time that we encountered jet wash and the aircraft rolled 20 to 30 degrees to the left. My focus immediately went to what was causing it; to what extent we had been upset; and if we needed to take any action to avoid further upset. After a few seconds of scanning; the airplane settled down and I returned my focus to our navigation. We were now tracking about 15 degrees left of what we had been before the upset. I then checked the magenta navigation line and it appeared to be slightly to the right of the aircraft track. I checked the panel and realized that the LNAV had disengaged but the autopilot had not. Therefore there had been no warning horn. I instructed the pilot flying to correct to the right. He began a general correction to the right immediately. Of course he was still waiting for me to verify the transition change. When I then looked at the LEGS page we had passed the fix which had been loaded several seconds earlier while I was communicating with Approach Control. I believe that was GRAMM. I tried to select RUSTT which is the next fix but we were so close the FMC would not take that change. We selected RIIVR; which is further ahead; and the FMC took that. Meanwhile SoCal had noticed our track was off and called us several times to verify that we were making the transition to 24R. The Controller's first call was slightly casual and simply asked if we were making the change. I answered in the affirmative. He immediately called back and informed us of traffic at our 7 o'clock position that would be tracking in for 25L. I acknowledged that traffic. He immediately called again and restricted our descent to 12;000 feet. After he saw that we were tracking to 24R; he released us for the approach. We were never told that separation was compromised. It is important to note that the entire event occurred in about a minute. At least six radio transmissions; aircraft upset and recovery; navigation disengagement; recognition and correction. I thought later that I should have told the Controller that we had temporarily lost our lateral navigation and I think I would have but I am not sure I could have fit it in before he noticed we were not making the turn for 24L.

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.