Scene contains flat terrain, and a cube.
I can fly the cube around! (yay)
The cube is realy hard to fly around, unless the Y axis is locked. (Otherwise the cube can turn backwards, and the controls get all crazy!)
So far, Every method of locking the Y axis has had a problem in it.
If you guys can think of another method, that would be fantastic!

Note: Every axis is a Local axis to the Cube.

Heres what I've tried:

Locking Y rotation in the Rigidbody Constraints section
This, strangely, does absolutely nothing.

transform.eulerAngles.y = 0;
This Works, however the cube Cannot go upside-down, and gets stuck at exactly half way

rigidbody.rotation.y = 0.0;
This is the closest I've come to success. Everything in the air works perfectly, however, The cube gets stuck in the ground whenever i touch it, and is no longer under my control.

Thanks for Reading this! Heres some code:

 #pragma strict
 public var Enginepower : float;
 public var TiltPower : float;
 public var EnginePlate : GameObject;
 public var Particles : GameObject;
 function Start () {
 EnginePlate = GameObject.Find("EngineOn");
 Particles = GameObject.Find("Particles");
 }
 
 function Update () {
     
     
     //******************************************************************************BEGIN PROBLEM CODE AREA *************************************************************************************
     
     
     
     //Several lines, Each of which attempt to Lock Y axis Rotation
     //transform.eulerAngles.y = 0;
     rigidbody.rotation.y = 0.0;
 
 
 
     //*******************************************************************************END PROBLEM CODE AREA **************************************************************************************
     
     
     //Fire the Main Engine Code Block
     if(Input.GetButton("Engine1")){
         //Add virticle force along the local Y axis
         rigidbody.AddRelativeForce(Vector3.up * Time.smoothDeltaTime * Enginepower);
         //Activates Rendering of the Engine plate and Particle Effect
         if(!EnginePlate.renderer.enabled){
             EnginePlate.renderer.enabled = true;
             Particles.renderer.enabled = true;
         }
     }else{
         //Hides engine plate and Particles if engine not on
         if(EnginePlate.renderer.enabled){
             EnginePlate.renderer.enabled = false;
             Particles.renderer.enabled = false;
         }
     }
     
     
     
     if(Input.GetButton("BackwardsTilt")){
         rigidbody.AddRelativeTorque(-Vector3.right * Time.deltaTime * TiltPower);
     }
     
     if(Input.GetButton("ForwardsTilt")){
         rigidbody.AddRelativeTorque(Vector3.right * Time.deltaTime * TiltPower);
     }
     
     if(Input.GetButton("LeftTilt")){
         rigidbody.AddRelativeTorque(Vector3.forward * Time.deltaTime * TiltPower);
     }
     
     if(Input.GetButton("RightTilt")){
         rigidbody.AddRelativeTorque(-Vector3.forward * Time.deltaTime * TiltPower);
     }
 }