What happened
During the departure phase of a flight, the crew chose to utilize the lateral navigation (LNAV) mode of the Flight Management System (FMS) to guide the aircraft's path. This decision was made instead of using the more suitable heading mode for the specific departure procedure. Following takeoff, the FMS provided guidance that directed the aircraft to turn toward a heading of 025°M, which deviated from the assigned departure heading of 335°M. The crew initially adhered to this FMS-generated path, resulting in the aircraft turning to a heading of 013°M. The deviation was identified by the crew when they switched to heading mode, and simultaneously, Air Traffic Control (ATC) noticed the unassigned turn and alerted the pilots.
The investigation
Investigators examined the use of navigation modes during the departure sequence. The investigation determined that the FMS mode selected by the crew was not appropriate for the assigned departure procedure, as the guidance provided by the system did not align with the Standard Instrument Departure (SID). The crew followed the automated guidance provided by the FMS until the error was manually detected and corrected.
Findings
- The selection of the FMS lateral navigation mode was inappropriate for the assigned departure procedure.
- The FMS provided flight path guidance that was inconsistent with the requirements of the SID.
- The flight crew followed the erroneous guidance provided by the FMS prior to the error being identified.