In May 1985, a Boeing 767 operated by Trans World Airlines became the first twin-engine airliner allowed to fly directly from St. Louis, Missouri, TWA’s hub, to Frankfurt, Germany, without altering its course to comply with an international requirement that it never be more than an hour’s flying time from an airport where it could land. The rule harkens back to the days of piston engines, which were so unreliable that at least four were considered necessary for a long flight over the ocean or hostile terrain. Even with four engines, airliners sometimes had to ditch—most notably the Boeing 377...