For those having trouble with level five, the stream on the left of your plant doesn't couse any threats. Put a breezer there and upgrade it a few times. The only thing that's left for you to do is defending.
Having different things you must do assigned to the powerups and having the boost replenish are great at curing the repetiveness normally present in these games. 5/5
It might be tempting to use your engine, but you can fly a lot longer if you steer your plane into bonusses such as golden stars, windmills and crones later on. Use the engine only when you're nearly stalling, and buy the rudder as fast as you can.