If the enemy plane is hit by a bullet, then the enemy plane should be destroyed.
Create a new object explosion1. The only thing we need to do is add an Animation End event so that once the explosion animation has played, it destroys itself.
Go into your bullet object and add the following event:
Test it out - if you hit your enemy1 plane with a bullet:
Still in the bullet object
Test it out - you should now be able to hit all the enemy planes.