The passenger fistfights seen on viral videos aren’t inevitable. Here are the warning signs and tips on how to dial down the tension. They just keep coming: “air rage” incidents, like the  fist fight videos  on planes that go viral every few months. Just this year, a passenger  spit on a woman , another threatened to  crash a plane  mid-flight and Ryanair instituted a fine — 500 pounds (about US$675, S$867) in Britain, 500 euros (about US$583; S$750)  in Europe  — for those offloaded from