Why must the gear suddenly pop out when you are in the air and shut your engine off? It also happens in the icon when you have landed in the water, but beached the plane and shut your engine off.

Are you talking about the 'B' button? That does a whole list of things, basically runs the engine startup checklist all in one pop.If you just turn the ignition off that should shut down the engine without running the other items on the checklist.See the engine shutdown checklist for more info.