Dr. Who's Daleks to rocket powered pine trees. Subsequent levels have you fighting against carnival octopus and evil refrigerators while avoiding surface to air lollipops and lava balls. As you progress through each level there are two major concerns, your fuel level and life meter. The speed of your plane determines how much fuel used. The more you use, the more you have to pick up.
Fuel is held in tanks, mostly hidden around the level in obscure places.
Once you find one, you shoot it and the precious fuel awaits you in a hot air balloon. Hit the balloon and the fuel is yours. Run out of fuel and you end up a burning wreck on the ground. The other major concern is your life meter, the more hits you take the more your life meter goes down. Luckily, your life can be replenished in much the same way as your fuel, whenever you shoot an enemy a small balloon appears, hit the balloon and you get a small life boost, thus allowing you continue.