Flight script problem

Hey having a small problem with my flight script, it's pretty basic, but when I rotate left or right on the Z, 90 degreas, I can no longer rotate on the X at all. It's become a problem with the flight script, because that's how it turns.

If you can't visualize the problem, imagine a plane banking left and unable to curve outwards to turn.

Coded in C#

 using UnityEngine;
 using System.Collections;
 
 public class PlaneMovement : MonoBehaviour {
 
     public float fThrust;
     public float rLeft;
     public float rRight;
     public float rUp;
     public float rDown;
     
     
     // Update is called once per frame
     void Update () {
         
         if(Input.GetKey(KeyCode.Space))
         {
             rigidbody.AddRelativeForce(0,0,-fThrust);
         }
         
         if(Input.GetKey(KeyCode.A))
         {
             rigidbody.AddRelativeTorque(0,0,-rLeft);
         }
     
         if(Input.GetKey(KeyCode.D))
         {
             rigidbody.AddRelativeTorque(0,0,rRight);
         }
         
         if(Input.GetKey(KeyCode.W))
         {
             rigidbody.AddRelativeTorque(-rUp,0,0);
         }
     
         if(Input.GetKey(KeyCode.S))
         {
             rigidbody.AddRelativeTorque(rDown,0,0);
         }
     
     }
 }