In order to fully understand the motion of a turning aircraft, let’s look at 3 scenarios.
First, imagine an aircraft that is flying a flat turn with no bank. The aircraft’s wings are level throughout the turn and the motion is entirely yaw – there is no roll or pitch.
Next, imagine an aircraft turning with a 90° angle of bank. The motion here is only pitch – there is no roll or yaw.
A properly coordinated turn is somewhere between these two, with a moderate angle of bank and the motion throughout the turn is a combination of pitch and yaw.
To fly a turn properly, the rate of pitch and yaw should be matched (which is much easier said than done!) A turn flown in this manner is considered to be a coordinated turn or balanced turn.
Let’s look at an aircraft in a turn to the left.
If the turn is coordinated, the balance ball will remain centered throughout. Note the nose (longitudinal axis) is in line with the white flight path line in this image.
If the yaw rate in the direction of the turn increases, the balance ball will deflect to the right and the pilot feels a sideways force away from the turn. The pilot should either apply less rudder or more bank.
If the yaw rate in the direction of the turn decreases, the balance ball will deflect to the left and the pilot feels a sideways force in the direction of the turn. The pilot should either apply more left rudder or less bank.