Create an instance of aa mxml in bb.mxml and try to call that function..
// In your bb.mxml file
var aaObj:aa = new aa();
I suggest you that if you are calling the function fullScreen() in various places of your app..you can put this function in your main application..and call it in any file using..Application.application
Note: Your function should be scoped to public access modifier in both cases..
You can also acheive this by using events...this is besy approach you can always use..
thanks for your reply