Stunt Pilot-esque Collisions

Hi, I know how to do various types of collisions and also know how to scroll a background. I’m sort of having trouble putting two and two together for how a game like stunt pilot knows if a plane has gone through a ring vs hitting it… It’s for an iphone game (objective c w/ sparrow framework) just I figured it would be easier to figure out in flash then I can bring that knowledge over to my iPhone. Thanks!