DivideByZeroException problem

Hello my name is yusuf and i very new in Unity… i want to make a realistic aircraft simulation flight but i have a problem… (sorry for my english) if i dont run the game, Unity not say error message but if i played a game it say DivideByZeroException: Division by zero please anyone can help me…
this is my script

 #pragma strict
var model = Transform;
var rigid = Rigidbody;
var maxSpeed = 100;
var accelerate = 0.00;
var deccelerate = 0.00;
var pitchConst : float = 50;
var rollConst = 100;
var yawConst = 10;
static var doublespeed : float;
private var pitchInput :float;
private var rollInput :float;
private var yawInput :float;
private var nosePitch :float;
var afterburner= 0.00;
var airbrake = 0.00;
var speed = 0.00;
var gravity = 9.8;
var levelFlightPercent : int = 25;
var liftConst : float = 7.5;
var maxDiveForce : float = 0.00;
var noseDiveConts : float = 0.00;
var airbrakeAccel : float = 1;
var airbrakeDeccel : float = 1;
var afterburnerAccel : float = 0.5;
var afterburnerDeccel : float = 1;
var maxAfterbuner: float = 5;
var throttle = 0.00;
static var trigerred = 0.00;
private var noseDive : float;
static var trueSpeed: float;
static var trueDrag: float;

function Start()
{
trueDrag = 0;
doublespeed = 0;
}

function Update () {
throttle = (Input.GetAxis("Throttle"));
pitchInput = (Input.GetAxis("Pitch"));
rollInput = (Input.GetAxis("Roll"));
yawInput = (Input.GetAxis("Yaw"));

if (throttle == 1)
{
	speed = Mathf.SmoothStep(speed , maxSpeed ,  Time.deltaTime * accelerate);
}
if (throttle == 0 )
{
    speed = Mathf.Lerp(speed , 0 , Time.deltaTime * deccelerate );
}
if (throttle == -1)
{
	speed = Mathf.Lerp(speed,0, Time.deltaTime * deccelerate *10);
}

if (Input.GetButton("Afterburner"))
{
	doublespeed = Mathf.Lerp(doublespeed , maxSpeed ,  Time.deltaTime * afterburnerAccel);
}
if ((Input.GetButton("Afterburner"))&&(speed !=0))
{
	doublespeed = Mathf.Lerp(doublespeed , 0 , Time.deltaTime * afterburnerDeccel);
}
if (Input.GetButton("Airbrake"))
{
	trueDrag = Mathf.Lerp(trueDrag , trueSpeed , Time.deltaTime * airbrakeAccel);
}
if ((Input.GetButton("Airbrake"))&&(speed !=0))
{
	trueDrag = Mathf.Lerp(trueDrag , 0 , Time.deltaTime * airbrakeDeccel );
}

if (speed <= (maxSpeed /levelFlightPercent))
{
	nosePitch = Mathf.Lerp(nosePitch, maxDiveForce, noseDiveConts* Time.deltaTime);
}
else
{
	nosePitch = Mathf.Lerp(nosePitch, 0, 2* noseDiveConts* Time.deltaTime);
}

if (((trueSpeed - trueDrag) + doublespeed) <= (maxSpeed* levelFlightPercent/100))
{
	noseDive = Mathf.Lerp(noseDive,maxDiveForce,(((trueSpeed - trueDrag) + doublespeed) - (maxSpeed* levelFlightPercent/100))*5* Time.deltaTime);
	transform.Rotate(noseDive,0,0,Space.World);
}
trueSpeed = ((speed/2)*maxSpeed);
rigidbody.drag = liftConst*((speed)*(speed));
rigidbody.velocity = transform.forward * speed;
rollInput *= Time.deltaTime * rollConst;
pitchInput *= Time.deltaTime * pitchConst;
yawInput *= Time.deltaTime * yawConst;

transform.Rotate (0,0,-rollInput);
transform.Rotate (pitchInput,0,0);
transform.Rotate (0,yawInput,0);
}
function FixedUpdate()
{
if (speed <= maxSpeed)
{
	transform.Translate(0,0,((trueSpeed-trueDrag)/100 + doublespeed));
}
rigidbody.AddForce(0,(rigidbody.drag - gravity),0);

}

Please…Please…Please… help me…

If the error is in that script, the only division by zero possible is

if (speed <= (maxSpeed /levelFlightPercent))

Somehow, levelFlightPercent is equal to 0 at some point.