See the figure below for a diagram of forces.
The plane is flying on a circular path, with constant speed. There are three forces that acts on it. First one is its weight, G that is vertical downwards. Second force is the centrifugal force, due to the circular motion. This force is horizontal towards the right side of the plane (since the center of the curved plane path is to the left). The last force is the air lift force that acts perpendicular to the plane wings. The plane is banking (rolling on a coast with a certain angle), until the resultant force `F_r` of its weight `G` and the lift force `L` , is equal to the centrifugal force `F_cf`. Thus `F_r =F_(cf)` and the sum of all three forces acting on the plane becomes zero.
`G + L + F_(cf) =0`
Therefore the resultant force acting on the plane is zero.