Need Help for AirPlane Script

I have written below script for the Airplane this script allows airplane to move right or left and up and down but I want air plane to move forward and back also …May I get help for the same…

using UnityEngine;
using System.Collections;

public class MoveAirPlane: MonoBehaviour {

public float    _init_x                 = 0.0f;         //inital x position
public float    _init_y                 = 0.0f;         //initial y position
public float    _horizontal_extent      = 20.0f;        //max horizontal distance achieved on each side
public float    _vertical_extent        = 10.0f;        //max vertical distance achieve on each side
public float   varspeed = 50.0f;

// Use this for initalizing setting
/*void initSettings()
{
	
}*/

// Use this for initialization
void Start () {

	//	initSettings();
}


// Update is called once per frame
void Update () {
	
	Quaternion temp_rotate0 = new Quaternion((Input.GetAxis("Vertical") / 4) * -1, 0.0f, (Input.GetAxis("Horizontal") / 4) * -1, 1.0f);
	 //set the new rotation with a tween so that the craft returns back to normal in a slow motion
	transform.rotation = Quaternion.Lerp(transform.rotation, temp_rotate0, 0.05f);
	//0.05f will limit the rotation to maximum 45 degrees on each side, 1 will make it 90
			
	
	transform.position = new Vector3(Mathf.Clamp(transform.position.x, _init_x - _horizontal_extent, _init_x + _horizontal_extent), 
										Mathf.Clamp(transform.position.y, _init_y - _vertical_extent, _init_y + _vertical_extent), transform.position.z);

 transform.Translate(Input.GetAxis("Horizontal") * 50, Input.GetAxis("Vertical") * 50, 0, Space.World);	
	
	rigidbody.AddForce(0,0,5 * varspeed);
	
	}

}

apply the same logic you have for the X and Y axis to the Z axis.

(I’m pretty new as well, just mark this as the correct answer i guess)