Move Agent with gravity, let him fall into a hole

Hi

iam trying to move an gravity bound agent/charackter automatically along one axis. if he try to pass a gap/hole i want to make him fall down. but at the moment he is not falling - its more a slow walk into the "deep" - do you have some suggestion or better solutions? hope someone can help me, thank you

here is my code:

 var speed : float;
 var gravity : float = 9.8;
 
 function Update () 
 {
 GetComponent(CharacterController).SimpleMove(Vector3.zero);
 transform.Translate(Vector3(0,0,speed) * Time.deltaTime);
 }
 
 function FixedUpdate()
 {
 rigidbody.AddForce(Vector3 (0, -gravity*rigidbody.mass, 0));
 }