Control and restrict object movement

Hi I am trying to make a game where a player controls a plane and i want to have it restricted to move and face in only one direction. I also need to be able to rotate the plane (do barrel rolls).

The problem I am facing is that when ever I adjust the pitch of the plane while doing a barrel roll it no longer faces along the z axis

What I have tried is using the lookAt function and using a cube and moving it up and down to adjust the pitch of the plane which works but dose not allow me to use AddTorque to add spin the plane. I have also tried to adjust the pitch and spin just using AddTorque but I get the front of the plane no longer facing along the z axis.

I am using a Ridgidbody on the plane and have the rotation frozen on the x axis but this did not help. Im sure there is more code needed to get something like this working but this is what I have at the moment.

 #pragma strict
 
 var target: Transform;
 private  var rb: Rigidbody;
 var speed: float;
 var spin: float;
 
 function Start()
 {
     rb = GetComponent.<Rigidbody>();
 }
 
 function FixedUpdate()
 {    
     //pitches the nose down
     if(Input.GetKey(KeyCode.UpArrow))
     {
         rb.AddTorque(Vector3.right * spin);
     }
 
     //pitches the nose up
     if(Input.GetKey(KeyCode.DownArrow))
     {
         rb.AddTorque(Vector3.left * spin);
     }        
 
     //barrel roll right
     if(Input.GetKey(KeyCode.RightArrow))
     {
         rb.AddRelativeTorque(Vector3.forward * spin);
     }
 
     //barrel roll left
     if(Input.GetKey(KeyCode.LeftArrow))
     {
         rb.AddRelativeTorque(-Vector3.forward * spin);
     }
 
     //add force in the direction its facing
     rb.AddRelativeForce(transform.forward * speed);        
 }