You cannot set a trap for trespassers. You also may have a duty to warn foreseeable trespassers against known dangers. Someone landing on private property may very well be a trespasser, even if it's an emergency (though the trespass is likely excused in that case), but the law will not sanction your deliberately injuring him. However, you do not have to make your property safe for him. So there's a wide gulf between putting up barbed wire (that might impede an emergency landing) in a cow pasture to keep the cows in, and deliberately placing obstacles on a private strip to impede reasonably-anticipated landing traffic. Likely the more obvious the obstacles are, the safer you are, but only a jury can tell you exactly where the line it.
