Narrative:

The first officer and I had discussed the procedures for flying the RNAV SID for which we had been filed. On takeoff from the west runway system; the tower told us to fly a heading of 185 degrees rather than the SID. At 400 ft AGL; the first officer called for heading select 185 degrees which I engaged for him. Then just above 500 ft AGL he engaged an autopilot. At 1000 ft AGL he called for VNAV which I engaged. However; the aircraft began an abrupt turn to the left away from the 185 degree heading it was flying. The first officer quickly disengaged the autopilot and turned the aircraft back to its assigned 185 degree heading. We both noticed that rather than engaging the VNAV switch I had mistakenly engaged the LNAV switch. The aircraft responded properly by trying to intercept the RNAV departure track. The mistake was mine in that I did not carefully select the proper switch to engage when the first officer called for VNAV. In retrospect; the first officer should have selected the switch himself as the autopilot was flying the aircraft and company procedures call for the flying pilot to engage vertical and lateral navigation modes himself with the autopilot engaged. Nonetheless I was the responsible pilot for making the error. I called tower to advise them of our deviation. The controller acknowledged the call and told us to maintain the 185 degree heading and then handed us off to departure control. I will endeavor to be more aware of selecting the proper navigation mode switches in the future. While there was no traffic conflict on our departure as a result of our deviation; there could have been a serious conflict had there been a departure leaving the east runway system.

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Original NASA ASRS Text

Title: A B757 Captain inadvertently selected LNAV after the First Officer; flying pilot; asked only for VNAV because TRACON assigned a heading after takeoff instead of the filed SID. A track deviation resulted when the autopilot was selected on.

Narrative: The First Officer and I had discussed the procedures for flying the RNAV SID for which we had been filed. On takeoff from the west runway system; the Tower told us to fly a heading of 185 degrees rather than the SID. At 400 FT AGL; the First Officer called for heading select 185 degrees which I engaged for him. Then just above 500 FT AGL he engaged an autopilot. At 1000 FT AGL he called for VNAV which I engaged. However; the aircraft began an abrupt turn to the left away from the 185 degree heading it was flying. The First Officer quickly disengaged the autopilot and turned the aircraft back to its assigned 185 degree heading. We both noticed that rather than engaging the VNAV switch I had mistakenly engaged the LNAV switch. The aircraft responded properly by trying to intercept the RNAV departure track. The mistake was mine in that I did not carefully select the proper switch to engage when the First Officer called for VNAV. In retrospect; the First Officer should have selected the switch himself as the autopilot was flying the aircraft and company procedures call for the flying pilot to engage vertical and lateral navigation modes himself with the autopilot engaged. Nonetheless I was the responsible pilot for making the error. I called Tower to advise them of our deviation. The Controller acknowledged the call and told us to maintain the 185 degree heading and then handed us off to Departure Control. I will endeavor to be more aware of selecting the proper navigation mode switches in the future. While there was no traffic conflict on our departure as a result of our deviation; there could have been a serious conflict had there been a departure leaving the east runway system.

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.