Why airlines no longer use rear-engine planes | Hacker News Reader

Why airlines no longer use rear-engine planes

traveller.com.au

135 pointsStratoscope70 comments

This 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.

This 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.

Maybe 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.

Yep. 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.

> but they left out the language: “when on the ground”!

Even on the ground this is not true when the gear is mounted below the wings.

Also, the engines are very light relative to the weight of the payload.

> Even on the ground this is not true when the gear is mounted below the wings.

Above or below, it would seem to be the same amount of stress where the wing joins the fuselage?

They meant as opposed to under the fuselage. If the wings are directly supported, the root does not bear the weight.

...how exactly are you imagining gear above the wing?

This is the sort of problem that some astute design calculations should be able to disentangle. ;)

Maybe they were using a topology optimizer driven by reinforcement learning to come up with the initial Pareto front.

It's also why the wings are gas tanks.

This comment pointed out a misconception I didn’t even know I had. I imagined the wings hanging off the plane like cantilevers. But the wings are the ones where the lift is. Logically the middle is the thing hanging off the wings when in flight.

Neat to find a cognitive error like that that I didn’t even know I had. Thanks.

Perhaps the wings are simpler because they don't have the engine pylon structures and the associated wiring/hoses for the engines, not because of the weight of the engines as the article states.

The pylons for aft-mounted engines are anything but simple also.

On the other hand, if we change the word from 'weight' to 'thrust' it becomes an accurate-ish statement.

But wings are not only major producer of lift, they produce the most of drag as well.

I was wondering what the basis of the article is: personal expertise? research? It's not the former:

> Sydney-based writer and photographer, Michael Gebicki has been writing travel articles since 1982 and is Traveller's resident Tripologist.

The failure of the center engine on a trijet can be scary.

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..."

The story of UA232 is up there with AC143 as one of my favorite things to read.

But 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.

I 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.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.

I 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.

BAe 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.

https://en.m.wikipedia.org/wiki/British_Aerospace_146#Engine...

There's also the HondaJet with the engines mounted on pylons above the wings:

https://en.m.wikipedia.org/wiki/Honda_HA-420_HondaJet

Eh. An-72/74 is the definitive example

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.

Not uncommon for military utility jets. The issue is the engines are higher, making them a bit more expensive to service.

Yes but makes them more reliable for less maintained runways etc. AKA ground debris/dust is less likely to enter the intakes

My 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?

Or maybe airlines just plain don't care about passenger experience in any other way today, so why would they care about this?

The number one passenger experience is cost.

Is this why they put first-class in the front, even though they're exposed to all the poor people walking past them?

No, it's so they get off first

Airlines sure use a lot of CRJs.

Came 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.

I 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).

It 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.

The 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.

If 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_7...

Ordered a copy, thanks for this

A better explanation: https://www.youtube.com/watch?v=QqjZSHIKWzE

The 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?

Probably 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)

This 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.

Yes, 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…

It's been done on the Honda HA-420 HondaJet.

https://en.wikipedia.org/wiki/Honda_HA-420_HondaJet

Maybe the lower pressure, and particularly at close to/at stall speed, turbulent air above the wing reduces the effectiveness of the engines? So you probably need the engines mounted further from the wing (longer, heavier pylons) than an equivalent under-wing mounting?

That and center of gravity is why they stick out in front of the wing already for under-wing engines. You'll also notice the cowling slopes down and is not flat to the airflow for the same reason, to ingest air when climbing.

Some decent responses, all of which may be true, but something not mentioned is that it's simply easier to inspect & maintain engines that hang down.

The pylons would have to be stronger (heavier) to account for column buckling loads. Servicing the engines would be harder. The under-wing engines are designed to fall away if they fail catastrophically, this would be more problematic with an over-wing engine.

Noise

I 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.

There 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.

Y’all should just read the article. It addresses these things.

That's the ETOPS (Extended-range Twin-engine Operations Performance Standards) rule: 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.

Also (I'm told) colloquially referred to as, "Engines Turn Or Passengers Swim"

> 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.

ETOPS planning ensures that the aircraft will always be able to reach a suitable alternate airport, even if the aircraft experiences loss of an engine and loss of cabin pressure, at the worst possible time during the flight. It’s not a ”so often” thing.

Loss of an engine.

If you lose both engines mid-way across the Atlantic, I assure you that your twin engine ETOPS qualified airliner is not going to reach to an airport. The Azores Glider made it over 100km after losing its second engine which sounds very impressive -- until you look at a map of typical ETOPS authorised routes. If they'd continued on their original course when they at first didn't understand what the problem was, instead of diverting, they'd have been forced to ditch in the ocean.

Now, ditching an airliner in the ocean isn't certain death. But it's very dangerous and there is no margin for error. While the Hudson River is not exactly a welcoming experience, being very cold and more than deep enough to drown, it's much more favourable than an ocean.

So, ETOPS is focused on the very low but non-zero probability of losing another engine and so likely loss of life from what may then become a forced ditching of the aircraft.

You reminded me of the tale of the Ford trimotor. The way I recall it from aerospace design class (20 years ago) was that any single engine out meant the aircraft couldn’t climb. Hence, the probability of a serious incident was 3 times as likely as for a single engine design. I tried to google this just now and came up blanks.

Oh well, I thought it was going to be entertaining and factually accurate enough. Now I am questioning my memory.

They still fly these 92 year old planes out at Oshkosh, and I took a short flight in one while I was there. Now I wish I’d asked more questions since their volunteers are pretty knowledgeable.

Also interesting to note that a two-engine craft ordinarily has 200% the power it needs to operate, whereas a three-holer might only have 150%.

That might have been what I am mis-recalling then, thanks! Our proff. still managed to turn it into something like a proverb of wisdom. (More engines not necessarily = more reliability)

Ironically, the more engines the more likely engine failure and collateral damage is.

Very informative. I was wondering what drives the decision where to mount the engine.

> 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?

Who 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.

Passengers in economy class could take turns :)

Like on 17th century sea voyage, passengers would have to take a turn on the pumps if the ship started to founder in a storm…

This 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.

This is mentioned at the end of the article.

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