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Why airlines no longer use rear-engine planes
- redm 5y agoIt was a compelling read up to the end, then It left me wanting. I think its because it reads like an article, which should make a point or conclusion but was infact more like a Quora entry.
- t0mas88 5y agoThe main point is that airplane design is a trade off, and airliners are very heavily focused on fuel economy while smaller private jets have other considerations. For fuel economy you want high bypass engines and those have a big diameter (hence why the 737 MAX had issues getting the larger engines to fit). All other design limitations are getting less relevant for airliners because airports by now are well equipped. All of this isn't really news, the industry has been moving in this direction for a long time.
- z991 5y agoIf you're looking for more, I strongly recommend "The Road to the 707." A thoroughly well researched book that explains why under-wing engines don't cause the apparent wing thickness to be too large (engines are in fact in front of the wings [1]). It's one of my favorite technology development books. [1] https://en.m.wikipedia.org/wiki/Boeing_707#/media/File%3AB_707.svg https://en.m.wikipedia.org/wiki/Boeing_707#/media/File%3AB_7...
- 238475235243 5y agoOrdered a copy, thanks for this
- FridayoLeary 5y agoVery informative. I was wondering what drives the decision where to mount the engine.
- scrumper 5y agoBAe 146 deserves an honorable mention - 4 engines mounted under wing, but the wing is on top of the fuselage (“high wing”) so benefits from lower FOD risk and avoids many of the issues the article calls out with tail-mounted engines. Commonly used (still?) for short haul flights out to the Scottish islands and as transport for the royal family.
- smartbit 5y agohttps://en.m.wikipedia.org/wiki/British_Aerospace_146#Engines https://en.m.wikipedia.org/wiki/British_Aerospace_146#Engine...
- pedrocr 5y agoThere's also the HondaJet with the engines mounted on pylons above the wings: https://en.m.wikipedia.org/wiki/Honda_HA-420_HondaJet https://en.m.wikipedia.org/wiki/Honda_HA-420_HondaJet
- lstodd 5y agoEh. An-72/74 is the definitive example https://en.wikipedia.org/wiki/Antonov_An-72 https://en.wikipedia.org/wiki/Antonov_An-72 But maintenance is a bitch on them, so they redesigned it to underwing pods - An-148/178.
- Robotbeat 5y agoNot uncommon for military utility jets. The issue is the engines are higher, making them a bit more expensive to service.
- virtue3 5y agoYes but makes them more reliable for less maintained runways etc. AKA ground debris/dust is less likely to enter the intakes
- Agathos 5y agoThe failure of the center engine on a trijet can be scary. https://en.wikipedia.org/wiki/United_Airlines_Flight_232 https://en.wikipedia.org/wiki/United_Airlines_Flight_232 "The uncontained manner in which the engine failed resulted in high-speed metal fragments being hurled from the engine; these fragments penetrated the hydraulic lines of all three independent hydraulic systems on board the aircraft..."
- sneak 5y agoThe story of UA232 is up there with AC143 as one of my favorite things to read.
- laurent92 5y agoBut that was a DC-10. It’s good share of the 400 Air Crash Investigation episodes. The first reason is that it has had a long life. The second is that it was comically badly engineered. Again, perhaps because AutoCAD didn’t exist in 1970 engineering. But the mistakes are still funny: - They forgot to include a compas in the cockpit, so they put it in the overhead space and added mirrors to see it. It often resulted in mechanics mounting it backwards (since the mirror showed a reverse view). You have certainly see that in the movie Airplane, thinking the rearview mirrors were part of a joke. - The MD-11, its successor, was too long, so pilots didn’t feel when they touched down, and they tended to slam the nose because of that, resulting in at least one filmed accident. The FAA shouldn’t have approved the elongation, at least not without adaptation, this was milking too much of the same cow, - It’s a plane which loses pieces. Its successor the MD-11 lose the famous piece that broke the Concorde. This is in addition to the DC-10 losing doors in flight, due to badly engineered mechanism, and getting FAA approval to not have to replace them immediately, which caused another crash. Two plane crashes for the same cause, does it ring a bell? and FAA approving the defective aircraft again, sounds twice familiar? In the later days, DCs were only used for postal services and not carrying passengers. Boeing bought MD circa 2003.
- touisteur 5y agoI actually felt sick when I read the post-accident description of the damages the exploding rolls-royce engine from QF32, did to its hydraulic, electronic systems... https://web.archive.org/web/20150112234930/http://www.atsb.gov.au/media/2888854/ao-2010-089%20preliminary%20report.pdf https://web.archive.org/web/20150112234930/http://www.atsb.g... and it's a nacelle-based design. They couldn't even control the other engine under that wing afterwards, couldn't shut it down on the tarmac... Uncontained engine failure must be one of the most impressive things to witness. Especially on engines as powerful as the RR Trent 900.
- thefourthchime 5y agoI haven’t read the article. But I thought it was well-known that the death of the 3 engine plane was because the FAA dropped the requirement for 3 engine plans for over ocean flight. The rule made sense at the time as propeller engines are far more likely to fail. Overtime it became apparent that modern jet engines are extremely reliable and the need for the third engine wasn’t necessary.
- t0mas88 5y agoThere are also two engine aircraft with the engines mounted at the tail instead of under the wings. The change there is that larger diameter engines are more fuel efficient and those fit better under the wing than at the tail.
- Robotbeat 5y agoY’all should just read the article. It addresses these things.
- Clewza313 5y agoThat's the ETOPS (Extended-range Twin-engine Operations Performance Standards) rule: https://en.wikipedia.org/wiki/ETOPS https://en.wikipedia.org/wiki/ETOPS It's mentioned in the article, and it didn't just kill off 3-engine planes, but 4-engine planes like the 747 as well. The Airbus A350 XWB has a ETOPS-370 rating, meaning it can fly up to 370 minutes (over 6 hours) on a single engine, or between any two points across 99.7% of the world.
- mikro2nd 5y agoAlso (I'm told) colloquially referred to as, "Engines Turn Or Passengers Swim"
- tialaramex 5y ago> meaning it can fly up to 370 minutes (over 6 hours) on a single engine It can fly almost indefinitely† on one engine. Performance is reduced, but it can even still climb. That ETOPS rating means this plane is allowed to fly routes planned at up to 370 minutes over water with two engines in the expectation that when sooner or later it experiences an engine failure the one remaining engine can get them to safety so often that regulators are willing to accept the residual risk. If you call up the regulators and say "OK, I have an A350 here, but one of the engines doesn't work, can I fly that over an ocean to get it fixed?" they're not going to issue you a ferry exemption justified by ETOPS, they're going to say "No". ETOPS is about what you can do with two engines knowing what will happen if one fails. † Obviously it will eventually run out of fuel. And engines can fail, although that's rare, the core idea of ETOPS is that so long as these failures are rare enough we don't need three engines.
- mazugrin2 5y agoThis article has an error. It claims that an advantage of a fuselage-mounted engines is that the wings don't "have to support the extraneous weight" but that's completely false. Mounting the engines on the fuselage requires stronger wings because the fuselage is now a lot heavier, requiring a significantly stronger wing root. Putting as much of that weight in the middle of each wing means that the weight is closer to the center of lift. Imagine a bridge with a span between two supports - placing the load above each support is a lot different from placing that load right in the middle of the span.
- SkyPuncher 5y agoThis article is really bad. I ride a rear-engine plane every time I fly. We live in a small town and the airline only flies small planes into larger hubs.
- tobmlt 5y agoMaybe I missed something but the article mentions that the rear engine design lives on in smaller jets for reasons similar to why it appeared in the first place.
- tobmlt 5y agoYep. I saved that text as I was passing through the article to say something along these lines. To bend over backwards for the article anyway, there may be some merit to the idea that “ Wings on an aircraft with rear-mounted engines can be simpler since they didn't have to support the extraneous weight.” —- but they left out the language: “when on the ground”! Everything you say is right, and captures the driving concerns of the design much better. But perhaps there is some possibility of optimizing (the given overall rear engine design) a bit more for a no lift load case when the engines aren’t there on the wings because when producing no lift, you have no “dead load” to counterbalance with attendant stresses going opposite to what they do under lifting conditions. Eh? I know it’s a stretch.
- Sebb767 5y ago
- kayodelycaon 5y agoThis is only in reference to larger international jets. Many smaller jets still use rear-mounted engines because there is no other place to mount them.
- Ecco 5y agoThis is mentioned at the end of the article.
- kibwen 5y agoThe article mentions that mounting the engines under the wings comes with downsides, but what prevents the engines from being mounted on top of the wings?
- loonster 5y agoProbably something to do with pitch. Imagine the center of lift(?) on the plane. If the engine is below that point, it would want to pitch up, if above, it would want want to pitch down. Edit: Remember the whole Boeing Max problem was caused by placing the engines in a non optimal spot. (This is pure conjecture)
- maxcan 5y agoThis is a big part of the answer. Usually when you're adding power, its because you want to go up so the last thing you want to happen is for the nose to go down. The higher up the engines are relative to the center of mass the more of a downward pitching moment you'll get. Also, I believe (and I'm very much not sure about this) but the above wing mounts like the Honda Jet can have nasty implications for flutter or some other structural issues if you aren't really careful. Its like the dynamic instability of trying to hold a stick at the bottom vs the top.
- galgot 5y agoYes, the reason for the engine position on the 737 Max is that newer hi bypass ratio jet engines takes a lot of place. As the original 737 was designed for flying with much smaller diameter P&W JT8D, it stands quite low on the ground with relatively short landing gear . Thus, with newer and newer hi bypass engines , the engine pods had to be moved further up and front of the wing, which in turn changed the flight characteristics compare to the older versions. With the Max ,enough that it would have required client airlines to have to re-train their pilots on a new 737 flying differently than the original 737 they were trained for. And here enters the infamous MCAS which was designed to keep the original 737 flight characteristics on the 737 Max, it was selling argument for the 737 Max, airlines buying it could have a « new » plane with state of the art economic engines, without having to pay for re-training their pilots. Not a total success…
- Robotbeat 5y agoI think about ways that electric aircraft may be different. The pros/cons don’t really apply much. Gravity fed fuel is irrelevant for battery aircraft. The motors typically don’t require as much service (if direct drive, at least). The reliability of electric motors is kind of a new variable as in principle they could be more reliable (or the failures might occur in other places than the motor), and also there’s much less incentive to keep the number of motors to just 2 (although that may be a good idea anyway), and coaxial motors is also something you can do with electric aircraft but isn’t practical for jets. Also, the most efficient wings are high aspect ratio, so you wouldn’t even want to put the batteries/fuel in there (well, maybe a little bit). Also, simplifying the wing topology makes it easier to maintain laminar flow (for a few reasons). Then again, this article is primarily about large jets for long haul international travel, which will be one of the last areas to electrify. My daydream most efficient electric aircraft is a sailplane-like vehicle with extremely long wings, maybe a v-tail with a coaxial pusher dual motor propulsor. That way, you’re ingesting air that has been slowed by skin friction already, giving you an efficiency advantage (although it can be tricky to pull this off). And since propellers are more efficient for Mach 0.5 propulsion (or even transonic, etc), it’s good to put the loud propeller in the rear, far from the passengers.
- ISL 5y agoAirlines sure use a lot of CRJs.
- xattt 5y agoCame to reflect on this. Bombardier went as far as making the CRJ1000 which is a moderate capacity regional jet, which approaches that of a smaller 737.
- virtue3 5y agoI mean... older 737 sure. The Max / NG (most of) are bigger. The max also uses significantly more efficient engines (which is what led to all the issues).
- amelius 5y ago> if the fuel pump fails there is no possibility of relying on gravity to keep the engines turning. Ok, so an emergency hand pump in the cabin?
- marcus0x62 5y agoWho would crank the pump? Narrow body twin jets can burn in the range of 5 gallons of fuel per minute. That’s a lot of fuel to move with a hand pump.
- dralley 5y agoA better explanation: https://www.youtube.com/watch?v=QqjZSHIKWzE https://www.youtube.com/watch?v=QqjZSHIKWzE
- nofinator 5y agoMy Dad told me rear-engine planes were popular because they were quieter for most of the passengers in the cabin, except for those near the back. But that was the 80s. The post doesn't talk about that. Maybe engines are generally quieter now so this is a non-issue?
- lostapathy 5y agoOr maybe airlines just plain don't care about passenger experience in any other way today, so why would they care about this?
- pixl97 5y agoThe number one passenger experience is cost.
- takenpilot 5y agoIs this why they put first-class in the front, even though they're exposed to all the poor people walking past them?
- photoGrant 5y agoNo, it's so they get off first