Travel during the holidays comes with an expected amount of stress, but under ideal circumstances, passengers never impart on the comfort of others by lashing out physically or verbally. United Airlines has been in the news twice in recent weeks due to violent outbursts from flight patrons, most recently on flight 502 from Austin to L.A. (about three hours and 25 minutes non-stop). During a Nov. 16 trip, when the plane was just 40 minutes from reaching its final destination, shocked…