Incident Overview
During the departure preparation, the flight crew opted to use the lateral navigation mode of the Flight Management System (FMS) to provide flight path guidance, rather than the more appropriate heading mode. After takeoff, the FMS directed a turn that exceeded the assigned heading of 335°M, continuing to a heading of 025°M. The flight crew initially followed this guidance, incorrectly turning the aircraft to a heading of 013°M before selecting heading mode and detecting the error.
ATC Interaction
Around the same time, Air Traffic Control (ATC) observed the aircraft turning beyond the assigned heading and contacted the flight crew. The error was then managed by both ATC and the flight crew, and the flight proceeded without further incident.
System Use and Crew Actions
The choice to use lateral navigation mode, rather than heading mode, was made by the flight crew during pre-departure planning. This decision led to the FMS commanding a turn that was not aligned with the assigned heading. The crew's initial adherence to the FMS guidance resulted in a heading deviation that was later corrected after switching to heading mode and receiving ATC communication.
Conclusion
The event highlights a deviation from standard operating procedures regarding mode selection during departure. The flight crew's subsequent actions, along with ATC assistance, ensured the safe continuation of the flight. No further issues were reported.