Aware of all the attempts at compensation, but simply stated, losing thrust on one whole side is mechanically unstable. At least with one engine still remaining on one side, there is some thrust created on same wing.
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.