A method determines the optimal turn direction of an aircraft among two directions, right and left, following a lateral trajectory to join an arrival straight charted by an angle of arrival, based on a departure point and angle of departure defining a departure straight oriented along movement of the aircraft, the direction defined by a respectively positive or negative optimal turn sign, comprising: determining a conventional departure sign of the departure point; determining a center value of an angle of change of course equal to the difference between the angle of arrival and angle of departure referred back between −180° and +180°, the center value exhibiting a logical sign corresponding to the center value sign of the angle of change of course; determining the sign of the optimal turn based on comparison between the departure sign and the logical sign, the sign of the optimal turn defining optimal turn direction.