I never really thought of Airplane, the Hot Shots, Rambo, PA, Fletch as "bad movies". For me they always fallen under classics. They are all pretty goofy low budget flicks, but I know of few people who don't laugh out loud at them. I would say that sequals most often qualify as "bad movies", because of the likelihood that it will make money regardless of production value. Major League 3 (which I kind of like) and Airplane 2 (which I really don't) are good examples.