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.
Synopsis
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.
Second reporter narrative
At 1000 FT I called VNAV. The Captain selected VNAV and LNAV. At 1200 FT I selected autopilot and the aircraft started a 30 degree left turn. I do not know the Captain selected LNAV. I noticed the left turn and disconnected the autopilot and returned the aircraft to a 185 degree heading.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.