I tried it this morning. It's a nice little game, and I love the sounds.
Suggestions for gameplay: collision detection is the most important imo. Several times I thought I had moved clear of a bullet, only to have it register as a hit. You could either do a pixel perfect test as said above, or use a better fitting collision rectangle, which might be easier.
Speed felt a little slow for me too. This kind of game is more fun when things happen a bit faster.
Lastly, consider animating the graphics of the plane when climbing or diving.
But overall, it's better than most flash games I've played.