limited rotation?

Hey guys, i'm trying to make a game where you fly in a little air plane, so i made a script to steer it. But now, it won't rotate more than 180 degrees. i.e. when i press the left button, it will rotate to the left but it stops at 180 degrees. Here's a copy of the script:

var moveSpeed = 1.0;
var turnSpeed = 1.0;

function Update()
{
transform.position += transform.forward * moveSpeed * Time.deltaTime;

    if(Input.GetButton("Forward"))
    {
        transform.rotation.x += turnSpeed * Time.deltaTime;
    }
    if(Input.GetButton("Backward"))
    {
        transform.rotation.x += -turnSpeed * Time.deltaTime;
    }
    if(Input.GetButton("Left"))
    {
        transform.rotation.y += -turnSpeed * Time.deltaTime;
    }
    if(Input.GetButton("Right"))
    {
        transform.rotation.y += turnSpeed * Time.deltaTime;
    }
}

Thanks!

Use Transform.Rotate instead of directly setting the rotation.