As automation increases, pilots are faced with more demanding tasks in actively monitoring flight path. In some very specific operational conditions (depending on coding of navigation database and performance conditions) FMS may compute erroneous trajectory. This error is applicable to flight plan leg that has turn direction and altitude termination.

Issue

Consider a SID where the aircraft is required to fly to waypoint A, then follow a track of 120 degrees until reaching an altitude of 3000 feet, and subsequently track at 100 degrees to waypoint B. In certain cases where the aircraft’s performance allows it to reach the altitude of 3000 feet at or before waypoint A, the FMS will disregard the segment that mandates flying on a track at 120 degrees and instead direct the aircraft straight to waypoint B. This may result in a turn in a completely opposite and unexpected direction in order to reach waypoint B as quickly as possible.

Solution

Check the trajectory at ND during planning stage and again recheck during execution

Use HDG/TRK mode and monitor raw data as appropriate and reengage NAV mode only when lateral flight plan is consistent with the published trajectory.