So everyone talks about fuel efficiency as the reason for downsizing to twins from quads. But twins are a relatively new phenomenon and even then, there is still a healthy number of pax quads flying around but zero pax trijets. By that logic, shouldn't quads have disappeared first? Trijets are more efficient than similarly sized quad counterparts so what gives? A340's, 747's, and A380's are rare but not that rare. But DC10's, L1011's, and 727's by comparison are a thing of the past and almost seem to have disappeared too quickly. Is it just me or have trijets vanished before the true end of their usable lifespan?

Modern turbofans get larger in diameter as the bypass ratio increases. The weight penalty of mounting the vertical tail above the engine then increases.

Adding new engines to a trijet is many times more difficult than a twin or quad.

Most quads built in the same period when the DC10's, L1011's were produced are also long retired, so nothing to see there. The MD11 is the only trijet that could be compared to the A340-300 and 747-400 quads. The issues with the MD11 didn't make it a popular plane in the 1st place. With the MD - Boeing merger it was simply replaced by the 777, so it was replaced by a twin, not a quad. In the meantime a very strong conversion market had emerged for the MD11, making it easier for airlines to sell them compared to the A340 which doesn't have a conversion program. Therefor you will see A340s lasting longer as a passenger plane then the MD11. Now the only quads still in production are the significantly larger 747-8 and the A380, they are still too large to be replaced by twins, although the 777-9 comes close.

As for the 727, Boeing replaced it with the 737 and 757, there's no comparable quad, so I don't know why you even mentioned it.

neomax wrote:

But twins are a relatively new phenomenon

How so? Pax twinjets have basically existed as long as pax trijets have.

DC9s entered service only two years after 727s did.
737s followed just over two years after that.

The A300 first flew the same year that the L1011 entered service, a year after the DC10 and only two years after the 747.

And these are just pax examples; you can go all the way back to the mid-1940s to find wing-mounted 2engine jets in military testing and (limited) service.

neomax wrote:

but zero pax trijets.

There are four TU154s still in scheduled and scheduled-charter pax service:
two with 6R and two with JS.

39 of them still carry pax in military or private/VIP service.

1 serves as a mobile research platform, maintaining its pax capability.

neomax wrote:

Trijets are more efficient than similarly sized quad counterparts so what gives?

How do you figure?

A343s destroyed DC10s/L1011s/MD11s in essentially every measure of efficiency; hence airlines like LX, CX, SK, LH, SQ, AY, etc replacing some of their trijets with quads.

At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.

The real wonder is why we ever went down the dead end that tris were in the first place. The first modestly successful airliner was the Curtiss Condo, which after the noise and odors of the Ford Tri-motor was a revelation. The first real airliner, the DC-2 followed by the DC-3 were twins. Tris won’t work with props, so the 727 was a reinvention of the idea and novel in 1962. WW II brought quads as an answer to the overwater mission, both range and redundancy with pistons.

GF

GalaxyFlyer wrote:

The real wonder is why we ever went down the dead end that tris were in the first place. The first modestly successful airliner was the Curtiss Condo, which after the noise and odors of the Ford Tri-motor was a revelation. The first real airliner, the DC-2 followed by the DC-3 were twins. Tris won’t work with props, so the 727 was a reinvention of the idea and novel in 1962. WW II brought quads as an answer to the overwater mission, both range and redundancy with pistons.

GF

The issue is engines. One couldn't develop a twin big enough to compete at the mid-widebody until the 777. Even then those engines had a disproportionate share of issues at EIS. All three...

Passengers were believed not ready for twins. Honestly, until improvements happened in the JT8D, JT9D and CF6, the engines weren't ready. The first year of 727 operations had quite a few engine swap. Then the engines were debugged allowing the DC-9 and 737 a smoother EIS.

The 747 owned the top of the market. While the L1011 received a custom engine, it was an engine also for the 747 and 767 to ensure volume. Just as the CF6 was intended for those frames.

Pratt missed fuel burn and so did Douglas on the MD-11. If both had made fuel burn, I believe A343 sales as well as initial 777 sales would have been reduced. The plane couldn't do the missions AA asked for nor some LH wanted. While later fixed, that ruined the reputation.

727s we're replaced by the far more efficient 757 which is fading to A321s, 739s, and soon -10s. Twins save 3 to 5 tons of weight.

The 747 fills a larger gauge roll, so not a direct competition. As noted, the 777 and A330 filled the triple niche. The A332 really crimped their sales and then once there were higher takeoff weight A333s, that was that. The 787 came too late to impact them.

Lightsaber

Having flown at EAL, the other fill-in is the A300 with CF-6s was the first twin widebody, albeit with subsidies and gift lease deals at EA. We flew the hell out of the B2s and B4s. The B2s even flew the LGA shuttles. It wasn’t much for long range, but worked well.

The regulatory environment at the FAA when the 727 was designed was also tilted toward tris and quads. Remember the take-off limits were ½ for three and four engine, 1 mile for twins, there were no ETOPS and so on. The builders thought no one would fly cross-country on a twin.

GF

GalaxyFlyer wrote:

The real wonder is why we ever went down the dead end that tris were in the first place. The first modestly successful airliner was the Curtiss Condo, which after the noise and odors of the Ford Tri-motor was a revelation. The first real airliner, the DC-2 followed by the DC-3 were twins. Tris won’t work with props, so the 727 was a reinvention of the idea and novel in 1962. WW II brought quads as an answer to the overwater mission, both range and redundancy with pistons.

GF

There have been / still are three engine props....

https://en.wikipedia.org/wiki/Conroy_Tri-Turbo-Three

...and don't forget the Trislander....

Not that they were big commercial successes....

Trijets can be difficult to load as the OEW CG is well aft on the fuselage. Twins and Quads have OEW CG's in the middle of fuselage and making it easier to manage the loaded CG.

Trijet center engine maintenance can also be a pain given its location.

If you have engines big enough to satisfy mission requirements with a Twin, why bother with the operational disadvantages of a Trijet?

The big tri-jets really only existed because ETOPS rules had not been written into law yet so manufacturers still needed to make jets with >2 motors to cross oceans. Airline demands to improve efficiency made the switch from quads a good idea. Once ETOPS was up and running and ETOPS 180mins at delivery was approved by the FAA there was zero advantage to continuing to produce or purchase a tri-jet.

Quads are lasting longer because of the need for more thrust; the amount of thrust needed for an A380 simply doesn't exist in a twin configuration. And the size of the fans for say a GE90 make it virtually impossible to mount one in a tri-jet configuration even though you could get theoretically get enough thrust with that configuration.

neomax wrote:

But twins are a relatively new phenomenon

I'm not sure how you came to that conclusion since widebody twinjets have been around since the mid-1970s starting with the A300, followed by the B767s, A310s from the early '80s, then the A330s & B777s during the '90s and now the A350s and B787s.

neomax wrote:

But DC10's, L1011's, and 727's by comparison are a thing of the past and almost seem to have disappeared too quickly. Is it just me or have trijets vanished before the true end of their usable lifespan?

What do you consider as usable lifespan, and does it match with aviation reality, i.e. noise reduction, fuel efficiency, maintenance costs (including spare parts), crew costs and so on. Between 1970 and 1988, 446 DC-10/KC-10A have been manufactured, 250 L-1011s between 1971 and 1984 and 200 MD-11s between 1990 and 2001, for a grand total of 896. How many 747s since 1969? Something around 1400 or 1500.

Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.

Seems like trijets suffer for being in the middle. Not enough total thrust of a quad, but not efficient (or as cost effective) as a twin. The worst of both worlds.

Efficiency isn't just about number of engines. Think of all that extra structure (weight) built into the tail to support that heavy third engine. Plus I'm sure someone else will have mentioned how tail engine maintenance is a PITA.

At one point, Boeing was looking at a small third engine on the larger 777s to reduce the required size of the main engines at take off, should one engine fail. This ended up not being necessary. At cruise, the third engine world be shut down, and the aircraft world fly as a twin.

If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?

BlueberryWheats wrote:

Efficiency isn't just about number of engines. Think of all that extra structure (weight) built into the tail to support that heavy third engine. Plus I'm sure someone else will have mentioned how tail engine maintenance is a PITA.

I think putting that third engine in the tail just made it more complex than a quad. As you mention, think of all the extra structural support and associated weight that went along with mounting it in the rear. And then there's the maintenance issues. Not easy to get to and work on an engine that is sitting basically at the equivalent of a third floor building. And then I imagine there are systems and operational considerations. For example, I recall that the fuel pump to that #2 engine was critical - no pump meant no pressure, which meant engine didn't turn. Unlike wing mounted engines that can have the fuel gravity fed, at least to the boost pumps. And there were other intricacies in operations - the thrust reverser on a DC10/MD11 doesn't deploy on that #2 engine until the squat switch is activated on the nose gear; otherwise, thrust reverse activation on the #2 would mean the nose would stay up. I'm sure there are other factors that meant that tail-mounted engine was bastardized.

redflyer wrote:

I'm sure there are other factors that meant that tail-mounted engine was bastardized.

Yes, the lack of need of a tail engine. The latest version of the A330-300 is more or less as capable as the early DC-10-30s/-40s were with just two engines. And for the MD-11 it's the same with more recent twinjets.

trnswrld wrote:

Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.

The 767 had more growth potential than the A300 in terms of rangefor one thing. The A300 was designed to be a “ regional” jet, the region being high traffic intra-Europe routes, so it didn’t have the range that the 767 has. Also, until the -600; there was an engineer required.

GF

NameOmitted wrote:

At one point, Boeing was looking at a small third engine on the larger 777s to reduce the required size of the main engines at take off, should one engine fail. This ended up not being necessary. At cruise, the third engine world be shut down, and the aircraft world fly as a twin.

If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?

I've never heard that about the 777 (only that it was originally envisioned as a true trijet to compete with the DC-10 and L-1011), but the last version of the Hawker Siddeley Trident had a small 5,000lbf engine in the tail that was only used for takeoff.

I also imagine that the maintenance on the tail-mounted engine was quite more difficult than on a quad.

gwrudolph wrote:

At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.

The advent of twins had essentially nil to do with runway length.

NameOmitted wrote:

If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?

What do you think the 779 is?

It can physically hold well over 500 pax with no problem, but no customer actually desired it to, so Boeing hasn't offered the amount of doors necessary for such configurations. That could change, if it's what airlines wanted. For now, they don't.

A lot of people have hit on reasons that I think are valid - it wasn't just one thing. But I think you also have to consider that every trijet was its own unique animal. They weren't all retired en masse, they were retired one by one, for their own reasons, but usually because a more efficient replacement came along. And in the case of existing trijets, those replacements came in the form of twinjets, which are pretty much always going to be more efficient.

That recalibrates the question a little bit to "why didn't new trijets replace old quads?" And the answer there is still mainly "because those quads were replaced with twins", although in some cases, they were replaced with newer quads either because four engines were needed for performance reasons (the A380) or because it was cheaper to adapt an existing design (the later 747 variants) than to create a new twin of the same size.

That just leaves the loose end of why those individual trijet designs were each dead ends while we're still getting updated 737's and 747's, and that question is covered by some of the earlier answers regarding maintenance and other things. One thing I've not yet seen mentioned is that the larger trijets often had/have weight restrictions on various runways at various airports because of that rear center-mounted engine, which concentrates the weight in the center of the airplane. I'm not sure if this could have been overcome in time with different gear designs, but I know that at least at some airports, a much heavier 747 could land with no restrictions whereas an L-1011 could only land at something like 511,000 lbs. or less. And this is also why the later DC-10 variants had that center gear, to spread and carry the weight a bit more evenly. So this is at least a concern with trijets that doesn't exist with quads or twins.

LAX772LR wrote:

What do you think the 779 is?

It can physically hold well over 500 pax with no problem, but no customer actually desired it to, so Boeing hasn't offered the amount of doors necessary for such configurations. That could change, if it's what airlines wanted. For now, they don't.

Yeah, never mind, I just re-red the specs on the 9x (and possible 10x). Thank you for re-directing me.

Lots of reasons, most already mentioned here.

None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.

Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.

Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.

Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.

Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.

Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.

Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.

Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.

Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.

To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...

neomax wrote:

Trijets are more efficient than similarly sized quad counterparts so what gives?

Your premise is wrong. Is it just that you think more engines means higher fuel burn?

I'll add one more factor those already listed: trijet makes your wing heavier. Why? For twins and quads, engine weight decreases the wing's bending moment and the strength/weight required. For tri's the weight of the center engine is shifted from reducing bending moment to increasing it. The already-mentioned heavier tail structure exacerbates this dynamic.

Quads can be very fuel efficient. A343 is very close to 77E despite being an older design. Avro RJ was/is very fuel-efficient. The big quads - A346, 748i, A388 - are all simply terrible designs.

LAX772LR wrote:

The advent of twins had essentially nil to do with runway length.

The requirement for ops at LGA influenced heavily in the inclusion of the 3rd engine IIRC.

LGA drove the wing design, not the engine. UA wanted four engines, EA wanted three and AA wanted two. The easy compromise was three.


GF

trnswrld wrote:

Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.

different product slot and the A300 from an new market entrant. (Then, a 34 years production run isn't bad, is it )
A310 and the 762 are same time frame, same product slot.
Boeing grew the 767, Airbus created the A330 going forward.
While Airbus was busy with the A320 and FBW the 767 sold well.

Two quads -- the A340 and the 747 -- outlasted one trijet -- the MD-11. It all comes down to which companies survived and which didn't. Lockheed had stopped producing commercial jets by the 1990s and McDonnell Douglas was gobbled up by Boeing. Boeing and Airbus had quads and Lockheed and McDonnell Douglas had trijets.

If Airbus ever develops an A380neo, could it be a tri-jet? Three GE9X provides significantly more thrust combined than the current four engines.

reidar76 wrote:

If Airbus ever develops an A380neo, could it be a tri-jet? Three GE9X provides significantly more thrust combined than the current four engines.

And where, pray tell, would you prefer #2 GE9X installed on such a plane? inside the fuselage, like on L-1011, with air supplied via an S-duct?
Or like on DC-10/MD-11?

reidar76 wrote:

If Airbus ever develops an A380neo, could it be a tri-jet? Three GE9X provides significantly more thrust combined than the current four engines.

Let’s summarize basically all of the responses in this thread:

Whatever potential benefit you could possibly derive from having one fewer engine on the plane is more than lost by all of the challenges and design compromises that must be made to put that #2 engine in the tail.

That’s why you don’t (and, as long as we’re talking conventional designs and technology, never will) see trijets anymore.

CanadianNorth wrote:

Lots of reasons, most already mentioned here.

None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.

Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.

Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.

Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.

Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.

Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.

Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.

Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.

Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.

To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...

Thanks for writing this out so nicely! All excellent points & I think you have en capsulized all of the reasons, we are destined to twins for the future. I think the only thing I could add, that I didn't see mentioned before in this thread, is the last 3 holer designed & made was the MD-11 & we all know that both AA & DL had performance issues with them after delivery, which is why they disappeared rather quickly.

Had the MD-11 come out of the gate with stronger than predicted performance number, like several aircraft today do, I think it would have been more popular but in fact the numbers were below what had been predicted & SJC-NRT became SJC-OAK-NRT, while other MD-11 carriers liked them & as a passenger, I liked flying them, it was never going to survive against the 777 family from Boeing.

gwrudolph wrote:

At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.

You forget ETOPS wasn't a thing back then. A third engine got around some regulatory challenges.

I think the better way to look at this question is that why we are still getting new quads today but not tri. 747-8 inherited the 747 family so it's kinda obvious, but why didn't Airbus use trijet design on A380? Was 3 engines not enough for A380, or was 4 engines being more efficient?

Momo1435 wrote:

Most quads built in the same period when the DC10's, L1011's were produced are also long retired, so nothing to see there. The MD11 is the only trijet that could be compared to the A340-300 and 747-400 quads. The issues with the MD11 didn't make it a popular plane in the 1st place. With the MD - Boeing merger it was simply replaced by the 777, so it was replaced by a twin, not a quad. In the meantime a very strong conversion market had emerged for the MD11, making it easier for airlines to sell them compared to the A340 which doesn't have a conversion program. Therefor you will see A340s lasting longer as a passenger plane then the MD11. Now the only quads still in production are the significantly larger 747-8 and the A380, they are still too large to be replaced by twins, although the 777-9 comes close.

As for the 727, Boeing replaced it with the 737 and 757, there's no comparable quad, so I don't know why you even mentioned it.

Did you forget the B720 & smaller DC-8's that were similar size to the 727-200 in capacity. So yes there was a comparable quad to the 727.

trnswrld wrote:

Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.

The early A300 like the tri-jets were 3 person cockpit crewed. The twins & later A300 series plane simply out did the tri-jets based on op cost.

c933103 wrote:

I think the better way to look at this question is that why we are still getting new quads today but not tri. 747-8 inherited the 747 family so it's kinda obvious, but why didn't Airbus use trijet design on A380? Was 3 engines not enough for A380, or was 4 engines being more efficient?

This question was already answered upthread. I gave an overly simplified summary three posts above yours, and CanadianNorth has a more detailed answer further up.

trnswrld wrote:

Someone above mentioned the A300.... Can anyone explain to me why the A300 didn't seem to do anywhere near as well as the 767? I know the A300 came out first, but what made the 767 more successful? I know it was early off in the game for Airbus, did that have anything to do with it? My understanding is that neither the A300 or the early 762 models had spectacular range. Did the A330 come along soon enough to where that was more or less Airbus' answer for their own airplane? Just looking for a little more info on that subject.

As mentioned above, A300 was a "regional" jet and the design showed this - its wings were smaller and there was less capacity for fuel and weight growth. 767 was designed with a larger wing optimized for longer range, although over time it needed upgraded engines and weight ratings in order to make use of it.

As for the general philosophy of twin vs trijet, I think the fundamental issue was always engine technology. In my view, the trijets existed because the desired airframe was too heavy for two engines of the existing technology to manage, so say hello to three or four engines. If a 60 klbs turbofan with decent reliability was available in the mid-1960s, I'm sure Douglas and Lockheed would have made their DC-10 and L1011 as twins. But at that moment it was only a 40 klbs turbofan, so you needed three engines to get that airframe off the ground. The A300 did use those initial turbofans and had to be lighter as a result, and being "regional" then the question of high reliability over water could be deferred for awhile.

CanadianNorth wrote:

Lots of reasons, most already mentioned here.

None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.

Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.

Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.

Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.

Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.

Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.

Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.

Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.

Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.

To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...

On the DC-10/L1011 the original spec request from AA was for a WB trijet to be able to operate from LGA airport. It was not many airlines wanting a trijet. It was an AA RFP.

LAX772LR wrote:

gwrudolph wrote:

At least on the widebody side, I think some of those trijets barely needed three engines for their intended missions even when they were originally launched in the 70s. Some could have been twins but customer requirements for using them at airports that didn't have longer runways (corner cases in my opinion) caused them to go tri vs. twin. I believe, for example, the L1011 was at one point contemplated as a twin. Once the 757/767 family came along and could do takeoff from a reasonably sized runway with a decent range and decent payload, the idea of wb trijets was done.

The advent of twins had essentially nil to do with runway length.

NameOmitted wrote:

If, in 20 years or so, the market wants a VLA, and Boeing wants to use as much 777 engineering as possible, what is the likelihood of such a plan being revisited?

What do you think the 779 is?

It can physically hold well over 500 pax with no problem, but no customer actually desired it to, so Boeing hasn't offered the amount of doors necessary for such configurations. That could change, if it's what airlines wanted. For now, they don't.

I never said it did. I made two main points:

1) The WB trijets were marginally necessary as trijets for most of their missions. The short runway requirement pushed the decision over the edge

2) Once the WB two engine jets such as the 57 and 67 proved that the same missions could be done even on those shorter fields, that requirement was met and no longer required the third engine

How you came up with the idea that I was suggesting that WB twin jets were invented because of runway length is beyond me.

rbavfan wrote:

CanadianNorth wrote:

Lots of reasons, most already mentioned here.

None of them I don't think on their own would make or break the tri-jet concept, but all put together they have lead us to where we are today.

Fuel burn:
A bit of a trap for logic. Quads vs. tri-jet one would think that 25% less engines would be 25% less fuel, but not really because, assuming range, payload, technology, and engine generation are all equal then a quad design although having more engines each individual engine would be smaller and burning less fuel. So overall fuel burn would go up with engine numbers, but not in direct proportion.

Maintenance:
Tri jet vs quad is one less engine to maintain, but the centre engine is harder to maintain. More ladders and work stands and such to buy, more specialized tooling to have available to hoist the engine and large components in and out, more labour cost for the mechanics to get up there and then come back down every time they drop something or need something, longer delays when something breaks down.

Weight:
The centre engine is usually in or near the empenage. This means you have to design the fuselage to take the weight and thrust loads from back there, you have to run fuel lines, fire extinguishing systems, and all that jazz. All that adds weight to your design.

Engineering:
It's quite the thing to design a spot for that centre engine. Airflow, structure to hold up the tail, all that becomes much more complicated. Lockheed had a heck of a time designing their S duct, and from what I've read the fittings that hold the tail up above the DC-10's centre engine were "quite the thing" to design and engineer.

Re-engine options:
On a quad or a twin as we know them today, if you want to re-engine them as long as you have the ground clearance from the landing gear then basically alls you need is a new engine mount and strengthen the wing structure to accommodate your new engines. Once again, that centre engine though. The MD-11 they had one heck of a time re-engineering the tail fittings to accommodate the larger engine, cost them a ton of money to do. Many 727s were re-engined from their original JT8Ds to the more efficient and quieter JT8D-200 series, however only #1 and #3 engines were replaced with new, the centre engine was left as is because the people doing the mod figured re-designing the whole back end to fit the larger diameter engine would cost more than it would to just live with the older generation engine in there.

Safety:
Basically all the same arguments as having three cockpit crew vs two. The third had a time and a place, but technology and reliability have come to the point where a third engine for safety just isn't needed any more. ETOPS wasn't a thing when the DC-10 and L1011 came about, now we do it every day without thinking twice.

Performance:
Many of the great tri-jets had customers that wanted to use them from relatively short runways and specifically asked for extra engines for that purpose. Major airports around the world have since grown to better accommodate large aircraft so the superior runway performance often doesn't justify the extra cost any more.

Engine availability:
Aircraft are sold in much greater quantities than before. Back in the day a production run of a couple hundred aircraft was considered to be a pretty good go. Nowadays it's perfectly normal, almost expected, for jetliners to break the 1000 order mark. Engines can cost billions to design, so you need a couple thousand sales to make any money. With only a couple hundred of each type engine manufacturers needed to spread their engines out across several aircraft. Produce one basic design and you can use two to power an A300/767 sized airplane, three to power a DC-10/L1011 sized airplane, and four to power a 747 size. Now most of those categories has enough of a market engine manufacturers can produce engines specifically for that category so you don't need to add extra engines to get your required thrust.

To a point though. Engines are only so big, and beyond that size the market is too small at the moment to justify the cost of designing engines big enough to power a stretched 747 or A380 sized aircraft as a twin. Thus the four-holers hang on for now...

On the DC-10/L1011 the original spec request from AA was for a WB trijet to be able to operate from LGA airport. It was not many airlines wanting a trijet. It was an AA RFP.

Yes, that is the story I have always heard as well. The third engine was just marginally necessary for most of the intended missions at the time

Why were DC-10 and Tristar ever invented as trijets in the first place? Couldn´t a small widebody quadjet like Il-86 (but with better engines!) have worked?

drgmobile wrote:

Two quads -- the A340 and the 747 -- outlasted one trijet -- the MD-11. It all comes down to which companies survived and which didn't. Lockheed had stopped producing commercial jets by the 1990s and McDonnell Douglas was gobbled up by Boeing. Boeing and Airbus had quads and Lockheed and McDonnell Douglas had trijets.

Actually the A340 was produced for 20 years. The DC-10 was produced for 20 years and the MD11 for 12 years giving the -10 & -11 stretch 32 year production. So the A340 did not outlast the MD.

chornedsnorkack wrote:

Why were DC-10 and Tristar ever invented as trijets in the first place? Couldn´t a small widebody quadjet like Il-86 (but with better engines!) have worked?

Yes but the better engines were not available till after the IL-86 was made. The JT8D-200 would have had to been developed at higher thrust. It came out well after the trijets as well.

rbavfan wrote:

drgmobile wrote:

Two quads -- the A340 and the 747 -- outlasted one trijet -- the MD-11. It all comes down to which companies survived and which didn't. Lockheed had stopped producing commercial jets by the 1990s and McDonnell Douglas was gobbled up by Boeing. Boeing and Airbus had quads and Lockheed and McDonnell Douglas had trijets.

Actually the A340 was produced for 20 years. The DC-10 was produced for 20 years and the MD11 for 12 years giving the -10 & -11 stretch 32 year production. So the A340 did not outlast the MD.

The question was about why the quads were still flying after the trijets had been taken out of service, so the total duration of service by each model isn't really relevant to the question. The last MD-11 was delivered in 2001. The last A340s were produced in 2011.

Momo1435 wrote:

As for the 727, Boeing replaced it with the 737 and 757, there's no comparable quad, so I don't know why you even mentioned it.

There was a comparable quad to the 727, the Boeing 720 which had about the same capacity but longer range. The 727 came after though, and outlasted it as it was more economical and the 707 squeezed it from the upper end.

Beech

Tri-jets existed because one less engine is cheaper than four for the same job. This allowed transcontinental operations with larger aircraft with fewer engines. Then you have to consider ETOPS. The A-300 proved that the twin-jet widebody would work. There was still the problem of extended over-water options. The DC-10 and the L-1011-500 eliminated the need for most DC-8, 707 and 747s across the pond. The 767ER and the 772 and later the A-330 eliminated the need for the tri-jets.

I think 3 engines has the same advantages over a 4 hauler than a twin engine has. The location of a 3rd engine is a major constrain that proved hard to realize in a light, maintenance friendly way. I remember Airbus filled a trijet patent. A decade ago I (& Henry Lam) looked at a VLA trijet with the third engine, offering 35k lbs additional take-off / emergency power next to 2 x115k lbs GE90s.



The idea was to provide the required 150k lbs thrust after an engine failure past V1, without having to come up with new 150k lbs monsters for a twin, or a quad (4x50k lbs) configuration. It would be a CFM sized ATPU with retractable inlets, deactivated above 10000 ft. A CFM sized engine is much lighter to handle in the tail of an engine design.