airplane collision detection problem

hi their , i am doing a simple top-down view airplane game where u pilot a small airplane then u can land it on the airport , this is where i have the problem bcz the airplane is rigid body here is the scene settings:

1-a flat ground with a box collider(if over this it must crash)

2-a flat rectangle plane (Airport) with a box collider ,if over this it must not crash ever.

3-airplane that is controlled via physics that have box collider that prevents it from falling.

the problem : when landing hard (i want to damage it only) it crashes as if it is over the flat ground , because the colliders intersect with each other and pass each other for a split second.

note:the airport is over the flat ground.

Script:

 #pragma strict
 var hits : RaycastHit[];
 var airport : Collider;
 var ground : Collider;
 var height : float;
 
 var boomObject : GameObject;
 var airplaneObject : GameObject;
 var planeWreck : GameObject;
 
 var isLanded : boolean;
 
 
 function OnGUI(){
     if(isLanded == true){
         GUI.Box(Rect(Screen.width / 2 - 50 , 10 , 100 , 25),"Landed!");
     }
 }
 
 function FixedUpdate () {
     hits = Physics.RaycastAll (transform.position, -transform.up, 0.5);
     for( var hit : RaycastHit in hits ) {
         if(hit.collider == airport){
             height = transform.position.y - hit.point.y;  
             if( height <= 0.5){
                 isLanded = true;
                 return;
             }
         }
         if(hit.collider == ground){
             height = transform.position.y - hit.point.y;
             if( height <= 0.5){
                 Instantiate(boomObject, transform.position, transform.rotation);
                 Instantiate(planeWreck, transform.position, airplaneObject.transform.rotation);
                 Destroy(airplaneObject);
                 Destroy(GetComponent(PlaneControl));
                 return;
             }
         }
     }
 }

please advice and thanks for any help i can get.