You are flying a plane at constant altitude and you wish to make a 90* turn. Why do you bank in order to do so?

To change the direction of the object, it must be accelerated by a force. The acceleration is called the centripetal acceleration and the force is called the centripetal force. Therefore, if the plane has to make a turn, there must be a centripetal force to accelerate him.In case of an aircraft, the horizontal component of the lift that keeps him above the ground provides the centripetal acceleration when it turns. However to turn by large angles, the plane must bank so that it can generate large forces to account for large centripetal acceleration. When the plane banks, the magnitude of the horizontal component of lift increases making it possible to turn.