private int shootTimer = 0;
    boolean invincible = false;
    private int invincibleTimer = 0;
    private int cooldownTimer = 0;
    /**
     * Act - do whatever the Player_1 wants to do. This method is called whenever
     * the 'Act' or 'Run' button gets pressed in the environment.
     */
    public void act() 
    {
        //The Planes actions
        move();        
        shootTimer++;
        shoot();
        hit();
        invincible();
        invincibleTimer++;
        cooldownTimer ++;
    }   


public void invincible()
    {
        if (Greenfoot.isKeyDown("i") && cooldownTimer >= 120)
        {
            invincible = true;
            this.setImage(new GreenfootImage("Player 1 Inv.png"));
            invincibleTimer = 0;
        }
        if (invincibleTimer >=90)
        {
            invincible = false;
            this. setImage(new GreenfootImage("Player 1.png"));
            //cooldownTimer = 0; <-- as soon as bring it in it disables the powerup
        }
    }