When in doubt, be sure to [read the documentation for the methods you're using][1].

Here you'll find out that if you give `transform.Translate` three float parameters and no `Space`, it defaults to translating in local coordinates. ie. in this case, it moves along the object's Z axis, not the world's Z axis.

Once you rotate the plane, that direction might point up or down, changing the global y coordinate of the object.

Because this change is going through the `Transform`, it completely bypasses the constraints you imposed on the Rigidbody.

So, possible fixes:

 - Specify `Space.World` if you want to move forward in world space, not local space

   `transform.Translate(0, 0, translationSpeed * Time.deltaTime, Space.World);`

 - If you want to steer left/right, just not up/down, compute a desired translation in world space, then clamp-out the y:

   `var desiredTravel = transform.forward * translationSpeed * Time.deltaTime;`
   `desiredTravel.y = 0f;`
   `transform.Translate(desiredTravel, Space.World);`

 - Or, route all movement via the Rigidbody by setting its velocity instead of teleporting with `Transform` changes. This lets the physics engine maintain the constraints you've configured:

   `rigidbody.velocity = transform.forward * translationSpeed;`

  [1]: https://docs.unity3d.com/ScriptReference/Transform.Translate.html