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Why not 4 engines?

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Other than for fuel reasons, I understand why some airlines choose the 777 over the 747 (in the case of US airlines). I was reading an article about the 747-8 and realized that Continental has only a 777 in it's long-haul category, so I am kind of curious, why not 747? I know Continental ventured into the 4 engine market years ago with the -200 model I believe, but discontinued it. I understand the fact that the thief known as Lorenzo took the airline into financial hardships, and ultimately destroyed Eastern (but that's another point). Does anyone think that Continental is a viable customer for the 747-8? If not, why?-completely out of curiosity-

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maintenance cost.An engine is extremely expensive to buy and maintain (If I remember correctly over half the cost of a 737-800 is the engines for example).If you reduce the number of engines you reduce the amount of time needed for maintenance, the number of spare parts you need, etc. etc.And it's not just the engines themselves, the aircraft structure also gets more complex (read, more expensive) when you have more engines. More fuellines, pumps, electrical circuits, etc. etc.All that needs to be maintained as well, costing even more.

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Also, the airline might be trying to reduce the number of types of aircraft, since each additional type requires extra training for maintenance and flight crews. I think Southwest took this approach to the extreme (at least for awhile) when they went solely with 737's.- Martin

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Aha! Thanks for the replies, I didn't really think of structural side of things. That does make sense with all of the extra parts and maintenance, might also increase turn around time because of the longer preflight.

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It may also be to do with traffic loads. The 777 has a slightly lower capacity than the 747, so why operate aircraft that can't be filled. This then means you can operate smaller aircraft, which are lighter and therefore save money on fuel etc... but as said earlier probably the main reason is maintenance of an engine

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All of this reminds me of the retired senior airline captain who said when asked as to why he always flew the Atlantic on four engined jets and not the new twin engined 777 said " Because they don't make them with five"

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I too wondered about how airlines have opted for really huge aircraft with just two engines and the question of engine failure came to mind and how it effects aircraft structural/frame integrity.Can anyone working as an aircraft engineer, mechanic, pilot or other technica professional enlighten us about the following: If an engine failure occurs on a twin-engine heavy like the 777 or the A330 series, how likely is it that (1) the structural stress may cause a failure in the frame and/or wings of the aircraft ESPECIALLY if by some rare chance, failure occurs in mid-flight at cruise altitude over the ocean (e.g. an oceanic crossing like North America to Europe); (2) that the pilots will lose control of the aircraft (though this is probably not that likely as long as no other system failure occurs)? The amount of TWISTING tendency (for lack of a technical term) of the plane must be overwhelming after an engine fails on a twin compared to a 4-engine Jumbo or A340. It's pretty obvious 4-engines are a lot safer due not only to redundancy but in terms of balance of power distribution in physical terms, whereby there are 4 POINTS of "stress" on the wings with 2 points on each side of the aircraft, not just 2 with one on each side. I know that the statistics apparently show many more engine failures on 4-engine aircraft compared to twins but still, one down and only one to go is scary for safety reasons where losing an engine does not just become an incovenience as it does with a 4-engine, but probably a (near?) emergency.John

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>I too wondered about how airlines have opted for really huge>aircraft with just two engines and the question of engine>failure came to mind and how it effects aircraft>structural/frame integrity....The airframe and wings are plenty safe when it comes to stress failure due to one engine being inoperative. This is something that the engineers plan for. You might argue that an older airframe might have some cracks or metal fatigue, but again, this is a problem on any aircraft no matter how many engines it has. There used to be a video on Boeings website of a stress test on a wing, where they mechanically flexed a wing until it broke. It was pretty amazing to watch how much that wing would bend before it finally snapped, and the force exerted was far greater than that which could be supplied by an engine. I think that there is a bigger problem in terms of control (yaw), but pilots are trained for this. This is more of an issue at takeoff and landing when more thrust is applied and airspeed is lower.The 2 vs. 4 engine debate occaisionally pops up here. If an engine fails on any aircraft, I think it should land (if I was onboard I would want it to), but even if it is in the middle of the ocean, a 777 can still continue to fly on one engine and make it to an airport safely. Somebody can probably chime in here with the specific regulations. Some time ago, a BA 747 took off from Los Angeles to London, and had an engine shut down on them. The flight crew decided to continue to London, (but had to stop somewhere else, due to fuel). BA and the flight crew were critisised in the media for continuing on despite the failure - the suggestion was that BA was putting profit ahead of safety.I guess the moral is don't believe the 2 vs. 4 hype. It would be interesting to see actual statistics on how many single engine failures have brought airliners down on both types.- Martin

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Depending on certification of the aircraft/airline/crew combination a twin can be allowed to fly for 1, 2, and now I think 4 hours from the nearest runway capable of accepting it for landing.For an aircraft with more than 2 engines there are no such limits (at least none that would ever matter in practice).At current the only place where 4 engines are still almost a requirement is long Pacific crossings, and twin restrictions are being widened to allow that as well (if they haven't been already).The trijet era was a direct result of the original 1 hour restriction (before the 2 hour ETOPS restriction was created).Engines were capable of using 2 of them for a commercial airliner on Pacific and Atlantic routes but they weren't allowed to be used.3 engines were allowed so aircraft manufacturers put a third engine in the tail in the hope of saving the customers money.But then Boeing came with the far larger 747 which they'd not counted on (the DC-10 and Tristar were designed to compete with the smaller 707 and DC-8 style aircraft).I think the maintenance problems introduced by having an engine that high up in the air and enclosed like the tail engines of a trijet are were also misunderstood at the time.Special rigs are needed to get at the engine, the equipment required to get it out and in again is far more elaborate than for wingmounted engines, etc.That's the main reason you don't see more of them and airlines are replacing them.

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