gravity gets too strong and plane falls to the ground help

i have an aeroplane and when i fly it it works normally but after a while the gravity gets really strong and it just crashes, i am using transform.translate to move, so the rigidbody is probably falling down and the transform.translate doesnt stop it. can someone change this script to one that will work?

 var movespeed = 0;
 var turn = 0;
 var noseup = 0;
 var F22Model : GameObject;
 var explosion : GameObject;
 var crashed = 1;
 var maxspeed = 600;
 var abletomove = 1;
 var eject = 0;
 var stopeject = 0;
 
 function Update () {
 
 var lift = movespeed * 0.5;
 
 transform.Translate(Vector3.right * Time.deltaTime * movespeed);
 transform.Rotate(Vector3.left * Time.deltaTime * turn);
 transform.Translate(Vector3.up * Time.deltaTime * lift);
 transform.Rotate(Vector3.forward * Time.deltaTime * noseup);
 transform.Translate(Vector3.up * Time.deltaTime * eject);
 
   if (Input.GetKey("up"))
   if (abletomove == 1)
   movespeed += 1;
   
   if (Input.GetKey("down"))
   if (abletomove == 1)
   movespeed -= 0.01;
   
   if (Input.GetKey("a"))
   if (abletomove == 1)
   turn -= 15;
   
   if (Input.GetKey("d"))
   if (abletomove == 1)
   turn += 15;
   
   if (turn <= 0)
   turn += 5;
   
   if (turn >= 0)
   turn -= 5;
   
   if (noseup <= 0)
   noseup += 5;
   
    if (noseup >= 0)
   noseup -= 5;
   
   if (Input.GetKey("s"))
   if (abletomove == 1)
   noseup += 10;
   
   if (Input.GetKey("w"))
   if (abletomove == 1)
   noseup -= 10;
   
   if (movespeed >= maxspeed){
   movespeed = maxspeed;
   }
   if(stopeject >= 300){
   eject = 0;
   }
   
   if (crashed == 0)
   if (stopeject <= 300){
   eject += 20;
   }
   }
 
 function OnCollisionEnter(thing : Collision) {
     if (thing.collider.tag == "Terrain") {
     if (crashed == 1) {
     Destroy (F22Model);
     Instantiate (explosion, transform.position, transform.rotation);
     crashed = 0;
     movespeed = 0;
     stopeject += eject;
     turn = 0;
     noseup = 0;
     maxspeed = 0;
     abletomove = 0;
     transform.Rotate(Vector3.forward * -90);
     }
     }
     }

thanks