Two is the Perfect Number
In the early days of gas turbine engines, available power per engine was lacking so aeronautical engineers needed to install enough engines to achieve sufficient thrust to make their designs fly. The second reason for three or more engines was that decades ago aircraft engines, particularly the radial piston engines, weren’t all that reliable, so pilots would occasionally lose one—or even two—engines on a single flight. More engines provided a measure of safety.
Thus many years ago, long-range airplanes had three, four or sometimes even more engines—the Boeing B-52 was designed with eight engines. Pilots liked to brag about having a “fist full of throttles,” but that was the only good thing about airplanes with lots of engines. In the real world, extra engines reduce fuel-efficiency, decrease range, increase drag and saddle the airplane owner with extra costs for fuel, maintenance and parts.
The good news is that the multiple engine jet airplane is ancient history, or at least should be. Boeing’s magnificent 747 and the enormous double-decker Airbus A380 still leave the factory with four engines, but that’s about it in the world’s modern business aircraft and commercial airline fleet. The much more popular, and nearly as large, Boeing 777 and Airbus A330 fly the world on two engines.
But what about flight safety? Are two engines enough? Hard data show the answer is emphatically yes.
It took many years, but global aviation regulators and researchers collected inflight engine shutdown rates for modern jet airplanes and reached the conclusion that more than two engines are unnecessary. This regulatory concept is called extended overwater twin-engine operations (ETOPS).
The primary concern for safety experts was, naturally, what happens if an engine fails while a long-range airplane is over the open ocean with no nearby runway? Having three or more engines appeared to provide an answer. But as the real-world flying experience of modern jet airplanes was examined, it became clear that engine failures are extremely rare. And modern twin-engine airplanes with thrust requirements carefully matched to the airframe weight and drag coefficient can continue to fly safely with the remaining engine for very long distances.
The regulators cautiously introduced ETOPS to commercial flying by initially limiting twin-engine overwater routes to remain within one hour flying time of a suitable alternative runway. As more experience was gained, that time requirement was extended, and now most airline twins can fly routes that are as much as four hours from a runway. That essentially means airline twins can fly any open-ocean route people want to travel.
The FAA and other major aviation regulatory bodies have determined that the loss of both engines on a single flight in a twin is improbable—approximately one in a billion.
But what about takeoff safety? It would seem that an airplane with three or more engines would perform better after an engine failure during takeoff than the twin. But that’s not necessarily true because of the way modern airplanes are certified.
Every takeoff in a jet must be planned considering aircraft weight, runway length, air temperature, wind, airport elevation, obstructions and every other critical condition to continue safely in the unlikely event that one engine fails at the worst possible moment.
Two is the perfect number of engines for the modern jet, no matter how far you fly and over what remote route.
The key words in the requirement are “one engine.” That means if an engine on a twin fails and the airplane loses half its available power, it must still have demonstrated during certification that it can take off and climb to safety on the available thrust from the one remaining engine. And with no safety disadvantage, the twin-engine jet reaps large gains in efficiency, which translate into longer range, higher speed, shorter runway requirements and increased payload—all Gulfstream hallmarks.
For a variety of obvious reasons, namely drag and maintenance cost reduction, it is more efficient to get the same amount of thrust from fewer engines.
No matter where you locate the engines on the airplane, there is “interference drag” where the high-velocity slipstream rams into engine-mounting pylons and is forced to squeeze in between the nacelle and fuselage, or nacelle and wing. Engine nacelles, no matter where you put them, are one of the biggest drag headaches for any jet airplane designer, so the bottom line is that fewer of them always creates less drag to slow the airplane.
Locating a third engine, for example, in the center of the fuselage poses particularly difficult drag issues. Though the center engine is typically buried in the fuselage tail cone, the engine air inlet atop the fuselage adds drag and creates inefficiency. No matter how carefully it is designed and crafted, the long and curving inlet duct that is necessary to feed air into a center engine retards airflow to that engine. Because that engine is having less air rammed into it by the forward motion of the airplane, it produces less power than the other two engines though it does not benefit from a commensurate reduction in fuel burn.
Engines do more than propel the airplane. They also generate electrical power and pump up pressure in the hydraulic system. So don’t you want more than two sources of power for those essential systems in a long-range airplane?
The answer is, of course, yes. That’s precisely why Gulfstream airplanes have redundant sources of system power. For example, the auxiliary power unit (APU)—the small turbine engine that cools or heats the airplane cabin on the ground and supplies power for engine start—can also operate in flight, providing an independent third source of electrical power.
There is also a generator powered by the hydraulic system for even more redundancy and independence. That means that with two highly reliable and powerful engines, Gulfstream jets fly the world with four independent sources of enough electricity to power the airplane while boasting the economy of two efficient engines.
The debate is long over. The data is in and the real-world experience of millions of jet flight hours has confirmed it. Two is the perfect number of engines for the modern jet, no matter how far you fly and over what remote route. And if you’re lucky enough to be in a Gulfstream G650ER you can cover 6,400 nautical miles of that flight at a speed of Mach 0.90, something no other airplane has been able to accomplish. Further proof, if needed, that two of the optimum engines are just right.
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