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How many engines do you need?
Our columnist ponders the relative merits of one, two, three and four.
Some people tried to convince Charles Lindbergh that he shouldn’t attempt his 1927 New York-to-Paris flight in an airplane with only one engine. His response was that two engines would double his odds of having an engine failure. In the graveyard humor of pilots, the saying goes, “The second engine will take you directly to the scene of the accident.” So Lucky Lindy chose a single Wright J5 Whirlwind to power the Spirit of St. Louis, and the rest is history.
But he wasn’t carrying passengers.
These days, the reliability and efficiency of 21st century turbofan engines is nothing short of astounding. Simplicity of design, strict quality control and impeccable maintenance make them practically immune to mechanical failure. We fly so much that eventually the law of averages will catch up and a flock of geese will precipitate “the unlikely event of a water landing” in full view of New York City. People remember that for a long time. What goes unnoticed are the millions of hours of uninterrupted, routine engine operation before, and since, the Miracle on the Hudson.
The reliability of modern turbine engines is why most airliners today have two of them, rather than three or four as was once the case. Though it’s less expensive for the airlines to feed and care for only two, the world’s aviation authorities wrung their collective hands for a long time before adopting so-called Extended-Range Twin Operations, better known as ETOPS. The rules set strict criteria for reliability that enabled twin-engine jets to operate over water in places that were 60, 90, 120 or more minutes from a suitable airport. The dark-humor “backronym” for ETOPS is “Engines Turn Or Passengers Swim.”
Non-airline twin-engine aircraft, though not required to adhere to ETOPS standards, are equally safe and reliable. The logical truth is, if you’ve got time to worry about the engine failing on a 21st century jet, you’ve got too much time on your hands. But fear often has little to do with logic.
In the early days, some believed that business jet operators wanted as many engines as the airlines. Still, four engines are rare on business jets (other than on converted airliners). Lockheed’s JetStar from the early 1960s is one example. It had its engines mounted on either side of the tail, just like most of today’s business jets, but each pylon supported not one but two fuel-thirsty turbojets. The model was subsequently retrofitted with four more modern (and efficient) turbofans, and many are still flying.
In the 1960s, McDonnell Douglas designed an unusual small jet, the Model 119. Looking like a miniature DC-8, it had four engines mounted on underwing pylons, just like its airliner big brother. Though it received a provisional type certification, it never went into production.
There are reasons for having more than two engines that have nothing to do with reliability. The Falcon 50 was the first of Dassault’s three-engine types; then came the Model 900 series and now the 7X. Properly executed, the trijet configuration is more fuel efficient, making Falcons among the most ecologically green jets in operation. Also, government certification testing for takeoff emergencies measures the aircraft’s performance with one engine not operating. In the case of a three-engine Falcon, that means two-thirds of the power is available, compared with only half for a twin. Unless perhaps an external factor like a bird strike is involved, logic dictates that the odds of losing two engines at once are beyond consideration.
Are engines so reliable that single-engine jets are a viable product? The safety record of single-engine turboprops certainly supports that theory. Risk of an accident due to engine failure among singles from Cessna, Daher-Socata, Pilatus and Piper is no greater than that for multi-engine turboprops. In fact, some argue that singles are safer, since when an engine loses power they are not subject to unbalanced thrust, which can send a twin out of control.
Though the expected tsunami of very light jets (VLJs) never got past a dribble, Cirrus is one manufacturer pressing forward with a single-engine jet project. So we may soon see whether passengers will be comfortable boarding a jet with only one engine.
Mark Phelps is a freelance writer and private pilot.
What our readers had to say
I enjoyed Mark Phelps's "How Many Engines Do You Need?" The original JetStar had two engines, not four, however. The retrofit had four engines. Then the factory adopted the retrofit and manufactured them with four for the last few years of its production (JetStar II). There are fewer than 10 still flying, although a few more than that appear registered.
—Michael Roberts, Jetvend Aviation, Miami
Roberts is correct that the original plan for the JetStar called for two engines but the aircraft was never manufactured that way.—Ed.