The first time you launch the game, you choose a basic plane from among three classes--weapons for more aggressive attacks, armor for a defensive approach, and agility. You have a few credits on hand to upgrade your weapons and defenses as well.

You earn more credits for upgrades and customization by racking up experience points within the game itself. But more impatient players can also use in-app purchases to buy credits for upgrades. It's the same freemium model that Z2Live uses in its other iOS game, the social city building sim .

Controls in MetalStorm include a blend of screen controls for acceleration and firing your weapons and using the built-in accelerometer on your iOS device to make your jet ascend, descend, and turn. Z2Live has incorporated some swipe controls as well for flips and barrel rolls--ideal for evasive maneuvers when you've got a bogie on your tail.

The game offers two modes of play--a dogfight against an opponent or a survival mode in which you face an endless wave of aerial adversaries. MetalStorm features single-player capabilities, but it's the game's multiplayer functionality--which relies heavily on Game Center--where MetalStorm really shines. The game lets you challenge friends to dogfights or join forces with them in the survival mode. MetalStorm also features an in-game voice chat system for communicating with friends and foes.

MetalStorm: Online is a free download from the App Store. You'll need to run iOS 4.1 or later on your iPad, iPhone 3GS or 4, or second-generation or later iPod touch.