[AS3] Modifying Aid Raid game

[Random490]

I'm trying to add a few things to an game I got from "ActionScript 3.9: Game Programing University". The game basically has planes fly across the top of the screen and you try to shoot them with a ground tank. What I want to do is make it so after you hit a certain number of planes it would move on to another frame. Also I would like to have one plane that deducts points when shot.

Any help would be great, thanks.

package {
import flash.display.*;
import flash.events.*;
import flash.utils.Timer;
import flash.text.TextField;

public class AirRaid extends MovieClip {
private var aagun:AAGun;
private var airplanes:Array;
private var bullets:Array;
public var leftArrow, rightArrow:Boolean;
private var nextPlane:Timer;
private var shotsLeft:int;
private var shotsHit:int;

public function startAirRaid() {
// init score
shotsLeft = 20;
shotsHit = 0;
showGameScore();

// create gun
aagun = new AAGun();
addChild(aagun);

// create object arrays
airplanes = new Array();
bullets = new Array();

// listen for keyboard
stage.addEventListener(KeyboardEvent.KEY_DOWN,keyD ownFunction);
stage.addEventListener(KeyboardEvent.KEY_UP,keyUpF unction);

// look for collisions
addEventListener(Event.ENTER_FRAME,checkForHits);

// start planes flying
setNextPlane();
}

public function setNextPlane() {
nextPlane = new Timer(1000+Math.random()*1000,1);
nextPlane.addEventListener(TimerEvent.TIMER_COMPLE TE,newPlane);
nextPlane.start();
}

public function newPlane(event:TimerEvent) {
// random side, speed and altitude
if (Math.random() > .5) {
var side:String = "left";
} else {
side = "right";
}
var altitude:Number = Math.random()*50+20;
var speed:Number = Math.random()*150+150;

// create plane
var p:Airplane = new Airplane(side,speed,altitude);
addChild(p);
airplanes.push(p);

// set time for next plane
setNextPlane();
}

// check for collisions
public function checkForHits(event:Event) {
for(var bulletNum:int=bullets.length-1;bulletNum>=0;bulletNum--){
for (var airplaneNum:int=airplanes.length-1;airplaneNum>=0;airplaneNum--) {
if (bullets[bulletNum].hitTestObject(airplanes[airplaneNum])) {
airplanes[airplaneNum].planeHit();
bullets[bulletNum].deleteBullet();
shotsHit++;
showGameScore();
break;
}
}
}

if ((shotsLeft == 0) && (bullets.length == 0)) {
endGame();
}
}

// key pressed
public function keyDownFunction(event:KeyboardEvent) {
if (event.keyCode == 37) {
leftArrow = true;
} else if (event.keyCode == 39) {
rightArrow = true;
} else if (event.keyCode == 32) {
fireBullet();
}
}

// key lifted
public function keyUpFunction(event:KeyboardEvent) {
if (event.keyCode == 37) {
leftArrow = false;
} else if (event.keyCode == 39) {
rightArrow = false;
}
}

// new bullet created
public function fireBullet() {
if (shotsLeft <= 0) return;
var b:Bullet = new Bullet(aagun.x,aagun.y,-300);
addChild(b);
bullets.push(b);
shotsLeft--;
showGameScore();
}

public function showGameScore() {
showScore.text = String("Score: "+shotsHit);
showShots.text = String("Shots Left: "+shotsLeft);
}

// take a plane from the array
public function removePlane(plane:Airplane) {
for(var i in airplanes) {
if (airplanes[i] == plane) {
airplanes.splice(i,1);
break;
}
}
}

// take a bullet from the array
public function removeBullet(bullet:Bullet) {
for(var i in bullets) {
if (bullets[i] == bullet) {
bullets.splice(i,1);
break;
}
}
}

// game is over, clear movie clips
public function endGame() {
// remove planes
for(var i:int=airplanes.length-1;i>=0;i--) {
airplanes[i].deletePlane();
}
airplanes = null;

aagun.deleteGun();
aagun = null;

stage.removeEventListener(KeyboardEvent.KEY_DOWN,k eyDownFunction);
stage.removeEventListener(KeyboardEvent.KEY_UP,key UpFunction);
removeEventListener(Event.ENTER_FRAME,checkForHits );

nextPlane.stop();
nextPlane = null;

gotoAndStop("gameover");
}

}
}