Answer by Statement

An alternative approach to the same problem.

var velocity = 3.0;

var minXAngle : float = -45;
var maxXAngle : float = 45;
var minZAngle : float = -45;
var maxZAngle : float = 45;

private var xAngle : float = 0.5f;
private var zAngle : float = 0.5f;

function Update () {
    var vdt = velocity * Time.deltaTime;
    var xtrans = Input.GetAxis("Horizontal") * vdt;
    var ytrans = Input.GetAxis("Vertical") * vdt;
    var ztrans = Input.GetAxis("Fire1") * vdt;

    transform.Translate(-ytrans, ztrans, xtrans, Space.World);
    Rotate(xtrans, ytrans);
}

function Rotate (x : float, z : float) {
    xAngle = Mathf.Clamp01(xAngle + x);
    zAngle = Mathf.Clamp01(zAngle + z);

    var euler = transform.eulerAngles;
    euler.x = Mathf.Lerp(minXAngle, maxXAngle, xAngle);
    euler.z = Mathf.Lerp(minZAngle, maxZAngle, zAngle);
    transform.eulerAngles = euler;
}