Moments-Milestones Old Faithful

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 Stratocruiser, three of which had engine and propeller problems that forced them to land in the water.

With the arrival of turbojet propulsion on the de Havilland Comet and Boeing 707, both of which had four engines, the airlines began to build a record of engine reliability that is now nearly perfect. Engineers could calculate the odds that more than one engine might have to be shut down in flight, and determined that with three engines, that possibility was so remote as to be virtually nonexistent. In 1964, all three-engine jets were exempted from any sort of time rule, and the International Civil Aviation Organization expanded the rule for twin-engine jets from 60 minutes to 90. Engines are rated by dispatch reliability—a measurement of how many flights are delayed or cancelled because of engine problems—and today, it is not unusual for an engine to have a rating in excess of 99.9 percent.

As reliable as jet engines are, they are also expensive to buy, operate, and maintain. It’s no coincidence that the Airbus A300 and the Boeing 757 and 767, all long-distance designs, have twin engines. Boeing’s big 777 is powered by two GE90s, the largest turbofan ever to fly. Out of the experience with the first generation of twins emerged a consensus among regulators that if a twin-engine jetliner had certain redundant equipment on board, a trained crew, and a proving period of, say, a year, it would be able to operate safely up to 120 minutes from an airport. The stipulations form a rule known as Extended-(range) Twin-(engine) Operational Performance Standards, which became crunched into the acronym ETOPS. In its earliest form, the rule was aimed at transatlantic routes that forced airlines to fly doglegs to stay within an hour’s flight on the remaining engine to diversion airports in Iceland or Greenland. Still, regulators wanted 12 to 24 months of proof of reliability before granting further extensions.

With the 777, Boeing pressed the Federal Aviation Administration to approve the airplane-engine combination, together with its list of redundant equipment such as fire extinguishing systems and electrical generators, for ETOPS up to 180 minutes on the day it was delivered. European regulators balked at the “ETOPS out of the box” policy, so European operators had a 120-minute limit for one year at delivery, and after the trial period, they could apply for a 180-minute limit. Today, with some exceptions for weather conditions in the polar regions and the vast Pacific Ocean, airlines can use their twin-engine fleets to fly direct routes between almost any two cities on the planet, saving time and fuel while delivering passengers safely to their destinations.

George C. Larson | READ MORE

George C. Larson served as editor of Air & Space from 1985 to 2005. He is currently an inactive pilot, but holds a commercial pilot's license, with instrument and multi-engine ratings. He is between airplanes at this time, but has owned or operated a Grumman American AA-5B Tiger and a Mooney 201. He has been writing about aviation since 1972, when he joined the staff of Flying Magazine.