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rivalarrival t1_ja7auq9 wrote

The planes with single engines in the vertical stabilizer are due to a law that required twin-engine aircraft to always be within 60 minutes of an airport.This regulation came to be known as ETOPS: Extended range Twin engine Operation Performance Standards. (Or, "Engines Turn Or Passengers Swim)

This law did not apply to jets with 3 or more engines. If carriers wanted to fly longer, over-water routes, they had to follow coastlines, or fly planes with more engines. It's cheaper to operate 3-engined aircraft than 4-engined, so the market made 3-engined aircraft.

Planes with an odd number of engines can't divide them between the wings; they need the odd one in the fuselage. It has to point aft, so the tail is the natural choice.

With improved jet engine reliability, ETOPS standards have been extended from 60 minutes to 120 minutes in 1985, and 180 minutes in 1988. Some twin-engine aircraft are now certified to fly up to 370 minutes from a diversion airport. The era of trijets is rapidly coming to a close.

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ace5762 t1_ja8rljs wrote

Seems strange. If you have an engine failure in a 4 engine aircraft, why not just shutdown the opposing engine to match the thrust? Presumably if one of the wing engines failed in a trijet, it would be the same thing.

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epic1107 t1_ja8zum5 wrote

You don't need to balance out the thrust in the event of an engine failure. You can fly with unequal configurations, it's just not efficient to do so by choice.

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rivalarrival t1_ja9hzzm wrote

Aircraft are designed with engine-out operation in mind. The vertical stabilizer provides sufficient yaw authority to operate with an engine out.

You don't shut down a good engine if you don't have to, because you might need it, and it might not start again. You might increase thrust on another engine on the same side, or reduce thrust in an opposing engine, but you're not going to shut a second engine down after one has failed.

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ace5762 t1_ja9mlag wrote

You kind of missed the point and just compounded on what I was saying- surely if the other engines and the control surfaces can compensate for the failed engine the rule you laid out makes even less sense?

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Bozzzzzzz t1_ja9rc33 wrote

Using fewer engine worse, more engine better.

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rivalarrival t1_ja9z706 wrote

ETOPS is (was) for planes with two engines. The thinking was that if one engine failed, you only have one engine in reserve. If it, too, fails, the passengers will be swimming for hours or days before help can arrive. Overland, the loss of both engines leaves the pilots with one final option before passengers are endangered: A dead-stick landing on the ground (See: "Gimli Glider"), or near enough to land for the passengers to be quickly rescued (See: "Miracle on the Hudson"). Flying out to sea, you can't expect a quick rescue after a forced landing. Regulatory authorities don't like it when you are operating with no redundancy left. They really hated the idea of twin engine aircraft flying over the ocean, just two failures away from catastrophe.

ETOPS initially said that If you wanted to fly a route that took you more than 60 minutes from land, you couldn't use a twin-engine plane. You had to find a plane with more than two engines.

You don't want to fly a three engines plane because it is less economical, but you are required by law to use at least three engines. You could use four, but that's even less economical. There was no practical need for three-engine aircraft (outside of extraordinarily rare double-emergency situations), but the law regulating twin-engine planes prohibited them from making these flights. Airlines needed three-engine planes to bypass this law.

ETOPS certification was incrementally expanded from 60 minutes to 370 minutes for some twin-engine aircraft and airlines. If you could prove your engines were reliable enough, and your maintenance program thorough enough, you could acquire additional certification to fly farther from land. With such certification available, more routes can be legally operated with twin-engine aircraft. Fewer routes legally require three engines. Presently, 99.7% of the Earth's surface is within 370 minutes single-engine flight time of a major airport. 99.7% of all possible flights can be undertaken with twin-engine aircraft. There was never much practical use for a three-engine aircraft in the first place; now there is only 0.3% of a legal requirement left. It's now cheaper to slightly alter one of the few remaining routes that twins can't legally perform than to maintain fleets of tri-jets to perform them. (Basically, they have to fly around the south pole, rather than directly over it, so they stay within 370 minutes of airports in South America, Africa, Australia, etc.)

Even 4-engine aircraft are being replaced with twins. Boeing ended production of the four-engined 747 in favor of the twin-engine 777 with nearly the same capabilities, because carriers prefer jets with two huge engines instead of jets with four smaller engines. Twins are simply more efficient to operate. The only thing that stopped them from using twins was the law, and the law has stepped out of the way.

You are correct that I don't understand your point, but I think the problem is that I failed to adequately explain the law. Your criticism doesn't seem to address a situation the law was trying to regulate.

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Bensemus t1_ja9ok26 wrote

You misunderstand. Four engines is more expensive to operate than three which is more expensive to operate than two. Airlines what planes that are cheap to operate. Due to ETOPS regulations the cheapest two engine planes weren't the cheapest to operate on certain routes due to having to take longer routes to stay within x minutes of an airport. So they went one engine up. Three engines made the plane more expensive but this was countered by being able to fly more direct routes.

Four engines were used only on the largest aircraft due to needing all that thrust to fly. Modern engines are powerful enough to only need two and ETOPS regulations have greatly relaxed with increased engine reliability.

When a multi-engine plane loses and engine they never shut down an opposing one if they can avoid it. Planes can fly with asymmetric thrust, it's just not efficient. So while you would never design a plane with asymmetric thrust you don't crash when it happens.

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Spank86 t1_ja8zl5m wrote

Exactly. The aircraft could continue with just the tail engine. In a 2 engine aircraft however you've run out of engines, unless you want to try to crab your way to landing (hence staying close to an airport)

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decisionisgoaround t1_jabt9yk wrote

What?

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Spank86 t1_jabtd84 wrote

Which bit?

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decisionisgoaround t1_jabtv50 wrote

All of it. If you lost one engine, you would never unnecessarily shut down another. And if you lose one engine on a twin, it absolutely will continue on the remaining engine.

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