You have a suicide button but I can't move in "Y"?
First off, good menus, very easy to navigate. The graphics were very clean and blended well together. While there wasn't much difference between the levels I could see an increase in difficulty. I'm glad I could place my shots on the screen but it became very easy to anticipate and destroy targets before they were even visible.
One area that needs major attention. Your bounding box. In flash the bounding box is made to encompass every pixel and point in your graphic, this could lead to very huge bounding boxes, like in this case. I found myself getting shot even though my plane was well out of the influence of the bullets. One way to fix this is to create a new bounding box. This can be very simple, like drawing a rectangle. The shape shouldn't be complicated or you'll run into the same problem as before. Once you have this new bounding box, parent it to the main character and set its location to 0, 0 (origin). Turn off the new bounding box's visibility by setting it's alpha to zero. Now you can use this smaller, more reasonable bounding box to deal with your hit detection.
I thought the game was a tad short but the initial difficulty held my attention through all six levels. Having only one weapon ensured the game would be challenging as the difficulty progressed.
Overall a 6, the bounding box cost you two points and short levels, lack of variation in enemies and environments, and the inability to move along the y-axis cost you another two. But! The game was very solid, had good programming, and got me hooked, which is far better then most games I've seen. Good Job.