Narrative:

On cowby 4 departure; turning over rbell; we encountered jet wash severe enough that I grabbed the yoke although I was pilot not flying. At the same time; we were given directions from departure control. Plane rolled right and then left winding up in 30-degree left bank. Autopilot remained engaged as did LNAV and VNAV; but the plane continued turning left. Departure control's instruction was; 'delete the altitude at roppr but maintain current speed.' I had to ask for a retransmit due to radio congestion. We were in a left turn and were probably about 30 degrees left of a heading that would have taken us to roppr; the next SID fix. Began the correction at which time departure control queried whether we were proceeding to roppr; they said; 'appeared you turned early.' I confirmed our clearance and that we were proceeding to roppr. Five to ten seconds later; he asked if we had traffic (arrival traffic) at '10 o'clock; 1;000 high'. I responded affirmatively and he came back with; 'proceed on course; maintain visual on the traffic.' I responded and we continued toward roppr turning short of it so as to proceed as quickly as possible to the SID course's next fix of cesar. No further problems and no TCAS alerts. Note; autopilot and LNAV/VNAV remained engaged but the aircraft did not correct to course; almost as if it went to cws. [I] discussed at length with first officer after reaching altitude. Perhaps a more aggressive correction to course but that's obvious. We just got set up with the simultaneous radio call/congestion and the turbulence induced heading change/automation error which neither of us caught quickly enough to avoid the course deviation to the extent that it occurred. [We] should have disengaged autopilot sooner than we did. It just appeared that the auto pilot was correcting or was set up to do so but did not come back to course.

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Original NASA ASRS Text

Title: B737-700 encountered wake vortex departing LAS that caused a deviation from the departure track.

Narrative: On COWBY 4 Departure; turning over RBELL; we encountered jet wash severe enough that I grabbed the yoke although I was pilot not flying. At the same time; we were given directions from Departure Control. Plane rolled right and then left winding up in 30-degree left bank. Autopilot remained engaged as did LNAV and VNAV; but the plane continued turning left. Departure Control's instruction was; 'Delete the altitude at ROPPR but maintain current speed.' I had to ask for a retransmit due to radio congestion. We were in a left turn and were probably about 30 degrees left of a heading that would have taken us to ROPPR; the next SID fix. Began the correction at which time Departure Control queried whether we were proceeding to ROPPR; They said; 'Appeared you turned early.' I confirmed our clearance and that we were proceeding to ROPPR. Five to ten seconds later; he asked if we had traffic (arrival traffic) at '10 o'clock; 1;000 high'. I responded affirmatively and he came back with; 'Proceed on course; maintain visual on the traffic.' I responded and we continued toward ROPPR turning short of it so as to proceed as quickly as possible to the SID course's next fix of CESAR. No further problems and no TCAS alerts. Note; Autopilot and LNAV/VNAV remained engaged but the aircraft did not correct to course; almost as if it went to CWS. [I] discussed at length with First Officer after reaching altitude. Perhaps a more aggressive correction to course but that's obvious. We just got set up with the simultaneous radio call/congestion and the turbulence induced heading change/automation error which neither of us caught quickly enough to avoid the course deviation to the extent that it occurred. [We] should have disengaged autopilot sooner than we did. It just appeared that the auto pilot was correcting or was set up to do so but did not come back to course.

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.