The big players in aviation are pinning their hopes on twin-engine jetliners like the Airbus A350 and Boeing 787 Dreamliner, which promise greater efficiency due to their groundbreaking composite designs. But with these planes set to take over more and more transoceanic routes, those two engines could be a problem. To follow the "great circle" routes that are the most direct routes between cities, which can save hours and lots of fuel on ultra long-distance flights, planes can fly far from land over remote stretches of ocean. With twin-engines planes, however, losing one engine could leave the jet searching for a place to make an emergency landing as quickly as possible.

And the closest airfield could be hundreds of miles away. Just this past weekend, a United Airlines flight from Honolulu to Guam diverted to the remote island of Midway, the site of an historic WWII battle, after the pilot reported smoke in the cockpit. This wasn't an engine failure; the cause was a malfunctioning equipment cooling fan, United said after it flew in a replacement plane to pick up the 348 passengers and crew. But this kind of problem isn't totally uncommon. Just two months earlier, a Delta 767 made an emergency landing on Wake Island, another WWII site with a rarely used runway left over from its days as a naval base. That time, the airborne emergency was caused by an engine problem, just the kind of scenario that's sparking worries about how far a twin-engine jet can stray from land.

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Just six weeks ago, the two-engine 787 won FAA approval to fly as far as five and half hours from the nearest suitable landing strip, the same as the 777. That was a major coup for Boeing, especially after the jetliner's much-documented troubles with its lithium batteries. In fact, after the plane was grounded in 2013, the FAA hinted that it might not even grant the Dreamliner a more conservative 180-minute certification for extended twin-engine operations (ETOPS). That would have ruled out many routes over the Pacific, where distances are so vast that airlines also depend on a network of far flung diversion airports like Wake and Midway to comply with ETOPS requirements.

Years ago, airlines were permitted to fly twin-engine planes no more than sixty minutes from the nearest airfield. Once jets like the 767, 777 and A330 started replacing 747s and DC-10s on the North Atlantic, Boeing and others argued successfully that twin-engine jets were so reliable that it made little sense to restrict them. The costs saved by flying twin engines on long stretches are so compelling that many airlines made the switch on the longest routes.

Of course, any plane—even one with four engines—can have a in-flight emergency. A Qantas 747 reportedly once had to land on deserted Johnston Atoll in the southern Pacific, a chemical waste site that was decommissioned as a working airfield in 2007. Midway, about 1,300 northwest of Hawaii, is a protected sanctuary for the albatross, which apparently are such a danger to planes that landings can only be made safely at night for a good part of the year. Others include Shemya and Adak, two treeless and tundra-covered islands in the Aleutians chain off Alaska where winter temperatures can dip well below freezing. Flights taking polar routes can also avail themselves of some landing strips in Siberia in an emergency, where conditions are even harsher. Farther south in the Pacific and near the South Pole, there are even fewer alternatives, which is why Airbus was recently reported by the Wall Street Journal to be seeking an ETOPS of 420 minutes, or seven hours, to make possible non-stops on routes like Australia to Brazil or New Zealand to South Africa.

To fly the super long routes under ETOPS, the specific aircraft has to get a special certification that requires extra maintenance and rigorous checks as well as enhanced fire suppression and emergency systems onboard the plane. Among things, the jet has to demonstrate that it can safely fly on one engine for the maximum number of hours approved.

John Cox, a pilot and head of an aviation safety consultancy, says the ETOPS program actually has made long-distance flying safer overall. The twin jets that get this rating "are actually more robust than the earlier generation three- or four-engine airplanes," he says, and the latter can effectively operate without any restrictions on their distance from land. "We are now routinely flying over vast oceans with a better safety record."

But what if an engine does conk out? "Nobody likes losing an engine," Cox notes, no matter where they are, but this isn't the doomsday scenario most people assume. With the lower power generated by just one engine, the plane would go into what pilots call "driftdown", or flying at a lower altitude at slower speeds. As fuel burns off, the plane would be lighter and could ascend to higher altitudes. There are few routes in the world where the plane would be so far from an airport that it would need to take advantage of the maximum time allotted under the expanded ETOPS rules, he says.

Aircraft manufacturers, the FAA, and many safety experts say the record for ETOPS operations is excellent. But the public may take bit more convincing. After all, the industry's inside joke is that ETOPS stands for "Engines Turn or Passengers Swim."