AirPlane Sway Effect

Hello everybody i need little help with this script :

 if(horizontal != 0){
     plane.Rotate(horizontal * Vector3.forward * Time.deltaTime * -rotateInt);
     }
         else{
             Debug.Log("get back the rotation to 0");
         }

i use this script to rotate my plane if i pressed A or D,

but i got to problem

1- how to make back to normal rotation if i didn't press A,D anymore

2-how to make the rotation limited because if i keep hold A or D, he will keep rotate like 360* degree

thanks