Bjorn’s Corner: Twins or quads?

By Bjorn Fehrm

By Bjorn Fehrm

11 December 2015, ©. Leeham Co: The debate over two or four engines for long range aircraft is as old as the jet airliner. A number of myths have been pedaled over the years over the virtues of the one over the other. The myths have even been presented by airline CEOs as “facts that are known in the industry.”

Having done several in-depth comparisons of two-vs-four engined long range aircraft, we can’t find the patterns that these myths propel: that a quad is less efficient than a twin and should have higher maintenance costs. What we see is that it is all dependent on what one compares and to what technology generation the one or the other aircraft belong.

When we didn’t get the same results as the myths on a number of areas, we started to wonder what could have created the myths in the first place. Looking at what four engined airliners could have been the source of the rumours, we started to see a pattern. It was a pattern of apple-and-oranges being compared and wide ranging conclusions being drawn.

Here is what we found.

How to compare

The two- and four-engined long range aircraft that have been available in the market are all of different size and vintage. To make comparisons worth the name we need to equalize such differences as different transport capacities (passengers, cargo), different range capabilities and different design generations. Only when we can equalize such differences can we compare aircraft in an objective way and thereby build a picture of the characteristics of quads vs twins.

The most fundamental principle is to compare everything by a common yard stick. In our case, it is the transported passenger over nominal distance. This requires that we reduce all data to compare it over the “seat mile.”  This applies to technical data, operational economics and capacity.

It shall be observed that we talk about a statute mile (1,609m) and not a nautical mile (1,852m) for all seat mile comparisons. We also use carefully equalized seating configurations for all aircraft, our normalized LOPAs (Layout Of Passenger Accommodations). Only when all information is prepared in an equal and neutral way, can we expect to see any true patterns between twin and quad aircraft.

We currently have 85 aircraft types stored in this way in our proprietary model. Each aircraft is represented with around 1,000 parameters. As an example, we measure each area of the aircraft: wings, engines, fuselage and empennage. Only then can we be sure to have, e.g., wetted area represented in an apples-to-apples way in the database.

From where stems the quad myths?

As said, through all this data and our comparisons, we can’t see a pattern that jives with the quad versus twin myths. We can’t see that a quad should be more inefficient; we can’t see that a quad should be more expensive in engine maintenance. So from where does all the hype come from?

I think one can trace a lot of the myths from comparisons of the venerable Boeing 747 (in its variants) with twins, which many times are decades younger in their design. The Boeing 747 was a major feat, creating the first true long range aircraft with not only twin aisles but also twin decks. It was designed in the late 1960s with Entry Into Service, EIS, 1970.

Its design carries some marks from the time. It was designed as a combined passenger aircraft and freighter with a front loading capability. Its resulting “humped” twin deck layout is not competitive with a straight tube design like the 777 for passenger transport. It has a larger wetted area per seat than modern designs and therefore higher drag per seat.

In term of engines, the 747, in different variants, has typically been at least one engine generation behind the aircraft it has been compared with. The popular 747-400 had a 1989 EIS with PW4056/CF6-80C/RB211-524 engines and these stayed essentially the same until 2012, when the 747-8i entered service with modern GEnx-2B engines. Over these 23 years, it was compared with 777 and A330 aircraft with at least one generation younger engines (PW4090, GE90, Trent 800 for 777 and PW4168, CF6-80E, Trent 700 for A330).

Inevitably a draggier 747 with older engines is going to lose an efficiency comparison to modern twins. But this is a characteristic of the 747, being from the 1960s and carrying older engines. It is not a fundamental characteristic of quad versus twin. It is very probable the opinion on quads efficiency and cost of operation was to a large part formed by these apples-and-oranges comparisons.

More recent quad versus twin comparisons

There can be made more modern comparisons. The first is the Boeing 777-200ER versus Airbus A340-300. These came into the market within four years of each other. We have just finished a three article series comparing these aircraft in detail as used aircraft. One can summarize the comparisons as follows:

  • There is very little difference in their efficiency. The difference in efficiency and capability can to a large part be explained by the 777-200ER having more advanced long range engines with pressure ratios in the 40s. After the no-show of the Superfan, the A340 adapted the CFM56 short haul engine to a long range engine with pressure ratios of around 30. The A340 use the quads’ installed thrust advantage to compensate for this disadvantage and give the 777-200ER an even match for efficiency, if not for capability.
  • The maintenance costs for the four CFM engines are not higher than the two engines on the 777-200ER, they are lower.
  • The most noticeable difference twin versus quad is that a twin needs more powerful engines. It loses 50% of its power in an engine out situation; the quad only loses 25%. The result is that a twin needs high thrust engines for flight safety. This makes them “over” strong for normal use. A twin therefore has better take-off and climb performance than a quad.

The second relevant comparison twin versus quad we have done is the 777-300ER versus the A380. Here the engines are more of the same vintage and type. GE updated the GE90 in a major way for the EIS of the 777-300ER in 2004. The GE90-115 is at least equal in efficiency to the Rolls Royce Trent 900 or Engine Alliance GP7270 that EIS with the A380-800 in 2006.

Once again we found things that did not fit with the quad myth:

  • There is very little, if any difference in the efficiency of the 777-300ER versus the A380-800. The -300ER is then configured with our normalized three class cabin, as is the A380. In fact, the A380 is giving the -300ER a freebee; it equals the efficiency despite offering its passengers a higher comfort standard. On the other hand, the -300ER is more capable for cargo, so it evens things out.
  • Once again, the maintenance costs for the four engines on the A380 is lower than the two engines on the 777-300ER. So much for four engines being more expensive to run than two.

As we said before, these aircraft are different size and we do all comparisons per seat and per mile (as we should).


After several analyses of twins versus quads, we think that the myths of the quads inefficiencies and expense are just that. These were probably formed during the hey days of the Queen of the Sky’s later variant, the 747-400. This extraordinary aircraft stayed relevant for over 40 years, all the time being compared to the latest twin game in town. And it did exceptionally well during all this years.

Those that accused it of being inefficient and expensive to maintain did not think hard enough. One can’t expect a 1960s design with engines from the 1980s to match new twins from the 1990s or 2000.

151 Comments on “Bjorn’s Corner: Twins or quads?

  1. ” After the no-show of the Superfan, the A340 adapted the CFM56 short haul engine to a long range engine with pressure ratios of around 30. The A340 use the quads’ installed thrust advantage to compensate for this disadvantage and give the 777-200ER an even match for efficiency, if not for capability.”

    I used to do a lot of work in Asia from 2002 ~ 2008. I was flying with THAI Airlines mainly.
    THAI replaced their old MD-11 planes with the A340-600. Nice plane initially, but, after a couple of years, THAI started to use the B777-200ER on the BKK ~ MEL route.
    The B777 is a much nicer plane to fly in, compare to the A340-600. They were about 30 minutes faster on that route. Another thing in the B777-200ER’s favour, was the headroom. I never bashed my head on the overhead bins on a B777.

    • I don’t think Björn implied that there aren’t other differentiators between planes – such as comfort, etc. His point was that the number of engines doesn’t implicitly mean much for a type’s efficiency.
      As for the A346 vs 777: They’re both great airplanes, but given a choice I’d go for the A340, as it has a much quieter cabin, and, while being less roomy, it also feels less cramped. But that’s just my taste, and you don’t get much choice in these matters these days anyway. Hard to find routes flown by A346s these days.

    • Peter – a very late response while back-tracking through previous LNA ‘quads’ articles (after today’s final Sunset wrap-up): ‘I never bashed my head on the overhead bins on a B777…’
      I’m afraid that, being a full 6ft tall, that is something that I do – certainly when occupying my favored aisle seat in the middle block. So often when boarding, I would put my bag on the seat, open the locker, retrieve anything I needed from my bag, and then – as I lifted the bag to place it in the overhead – ‘bash my head’ on the open locker… I’m sorry, but that is/was the reality.

  2. This is kind of unbelievable. Can the whole industry be so wrong? Why not to compare 777-300ER to A340-600. These are closer in size and vintage. We know which one went extinct. I agree with you on maintenance cost. Having 4 smaller, easier to handle engines can be cheaper to maintain than 2 big giants.

    • I have not include the A340-600 in the analysis as it is not very comparable to the 777-300ER for two major reasons:

      – it is a typical example of a base design stretched to far. The cross section (8 abreast) stems from the A300 which had a fuselage fineness ratio of 9.5 (length/diameter, efficient ratios are between 9 and 12). This was stretched to 11.2 for A340-300 and 13 for A340-600. This is a fuselage to long and slender, you pay with weight. The A340-600 is at least 10t heavier than a 777-300ER (fineness ratio 11.8).

      – the engines for the A340-600 are ~10% less efficient than the GE90-115. A bit of that is size effects but not much, most of it the Trent 500 being half a generation older than the GE90-115. The Trent 500 was derived from the Trent 700 and 800, both a generation older than the GE90-115 which was a total renewal of the GE90 first generation.

      So what we are comparing is not quad vs twin but an optimal airframe with a totally renewed engine to a sub-optimal airframe with a somewhat older engine.

      • Hm – so what about the A343 vs the A333?
        As the A333 became more capable, it basically made the A343 redundant – and the main difference between the two is, well, the number of engines…

        • Hmm.
          sfc++, FBW twiddling for load alleviation to enable increased MTOW.
          2 Trent 700 @ 4.7t weigh less than 4 CFM56-5c @ 3.99t
          no 5th gear is another advantage enabled via the previously mentioned items.
          At one point there is rapid “selfcatalyzed” popularity transfer from one type to the other. ( like the mutually reinforcing “no sales” :: “no PIPs” combo )

        • Re-Hmmm : the A340-600 was overbuilt to cope with its overheavy MTOW … compare (Élodie Roux) OWE for A346 = 175 t vs same for A333 = 109 t so delta OWE = some 86 tonnes. Makes all the difference !

          • Hi FT,

            you or Elodie are mixing OWE and MEW. OWE for A340-600 is 175t and 128t for 242t A330. The A330 MWE (sans cabin) was 109t.

        • Hi,

          the A330 does not cover the range of the A343, if it would it would be heavier with larger and heavier engines, close to a Trent 800. And it would need the center gear. The A330 use the fact that very few sectors are beyond 10 hours, it skims the market and the 777-200ER and -300ER take care of the rest. They are both more capable then the A330 (and A343).

      • The length/diameter ration looks to me like there should be a A340-100neo (same length as the A300-600) and maybe a A340-200neo – of course with lighter/smaller wings to fit Cat. D gates.

        It would fit nicely in between A321XLR and A330-800neo. As engines, Airbus could re-use 4 engines of the A320-family for these new A340 aircrafts ~ around 200t MTOW.

        Sadly, the market dried up totally, so such an airplane would never be realized.

  3. How do the 777-300ER and A340-600 compare on cargo over a typical Pacific flight e.g. BKK-LAX?

    • Keesje, Bjorn, Uwe – a late reaction after re-reading this article (stimulated by Bjorn’s Quad Sunset wrap-up), what were the characteristics – presumably of only marginal value – of the A340-400? Was it overtaken by the Trent 500 and/or other developments?

  4. I’ve heard engines maintenance costs are related to overall thrust which is lower on quad.

    Could higher engine reliability required by EROPS for twin be a factor too?


    • I would be very careful about using this article for any meaningful conclusions. It was written three years prior to 777-330ER EIS.

      The 777-300ER data shown in no represents actual delivered performance.

  5. However it came into being this myth has been supported by carefully crafted messaging.

    My observation is that the variance in verbiage for people stating this myth is rather small. This would indicate that they all work from the same information pool.

    There are a couple of other potentially mythical statements “true facts” that show the same small source footprint.

    Asstroturfing is still around and bigger than ever.

    • Airbus/Virgin Atlantic didn’t exactly help this, either, when they put the “4 Engines 4 Long Haul” slogan on Virgin’s A346s.

      • PR rides on tackiness.
        making fun of 4e4lh was integral part of overstating 777 twinness PR activity.

        That was mostly the “War of the PR Agencies” related.

        • Sure – problem here was that the A346 was indeed less efficient than the 773ER, and this kind of attached itself to the idea of four engines in general.

          • “and this kind of attached itself to the idea of four engines in general.”

            IMHO carefully planned and executed meme creation.

  6. Interesting findings. The inescapable fact however is that manufacturers fit all their new aircraft with two engines, the oversized A380 being the sole exception. The main factor I suspect is that ETOPS is no longer a major barrier.

    Having said that, I wonder if manufacturers may move back to multiple fans due to every expanding fan diameters getting to be impractical. The proposed Airbus e-Thrust hints at this

    • The inescapable fact however is that manufacturers fit all their new aircraft with two engines, the oversized A380 being the sole exception. The main factor I suspect is that ETOPS is no longer a major barrier.

      That, and engines became powerful and reliable enough to achieve thrust levels that allowed just strapping two engines under a plane the size of a 777.

  7. When Airbus were promoting the TA11 (to become later the A340), focus was put upon the double commonality of crew training plus engine maintenance vs A320 Series, the sales pitch favouring larger engine pools and crew pools for economies of scale. I believe there is some truth in those arguments ? This raises the question of Fleet Management vs Aircraft type management in isolation. Fleet planners take their decisions based on cross-fleet criteria …

  8. Björn – one obvious question I had after reading this intriguing post ist this:
    What about the A340-500 and -600?
    They are of a similar vintage as the 777-200LR/-300ER with similar generation engines – but got a sound thrashing by their Boeing counterparts.

  9. Bjorn,

    Interesting! Perhaps, it is not the engine maintenance costs that are the determining factor but the inventory costs of spare engines. Worth looking into.

    Another point is that there must be some overwhelming reason (not just myth) for twins’ domination. Granted quads turn out to be too huge and hence difficult to fill routinely and the resulting load factors may be an issue. But overall, the market has reinforced the myth and so there might be some truth to it.

    A more acceptable comparison would be between B777-300ER and the brand new B747-8I. If quads were equally good, why is the newer version of the “Queen of the Skies”, which by any measure is a beautiful design, an endangered species? What does your analysis say about that?

    Clearly twins capable of 7 hour ETOPS is one reason for the extinction of the quads. No?

    • “A more acceptable comparison would be between B777-300ER and the brand new B747-8I. If quads were equally good, why is the newer version of the “Queen of the Skies”, which by any measure is a beautiful design, an endangered species? ”

      He addresed that in the article, the engines are new but the airframe is old and draggy.

      • It is not that simple! B737 fuselage dates back to the 1960’s B707 design. Yet, it is selling extremely well. B747-8 has redesigned wings and latest GEnx engines and I am quite sure its specific burn is reasonable. It has ample cargo space. Overall a truly elegant machine. Yet it will be extinct soon. Why? IMHO, it is not enough to say “the airframe is old and draggy.”

        • The shape says it all, its simple tubes on one hand and a hump backed 1.5 decker on the other. I allways thought the highly swept 747 wings designed for a higher cruise speeds in the days of cheap fuel went against the 747 design today.

        • wings more important than fuselage though a lot of junk on it (like all the antennae’s they are putting on these days) is a drag.

          Shorter routes not as critical on that as longer.

    • I am not denying there are factors that favor a twin over a quad (larger engines easier to get effective and lighter than smaller, fewer moving parts, …) but there are also positive factors for a quad (bending relief, non ETOPS which is a strain on the airline, better balanced field length situation, easier airframe integration when BPR sky-rises, …).

      It is just that the equation is not as clear as the myths make out, at least we could not find the patterns in those 1000 parameters. It is still a matter of building a good airrframe/engine combination, it can well be a quad going forward for very good reasons. But you don’t double the engine complexity for no good reason.

      What we can see in our data is that the knee-jerk “quads are duds…” is slander and not science.

      • Can you clarify what drives maintenance costs, since naively one would expect it to be proportional to the number of engines? How did the 3 engine planes (DC10, L1011 etc) compare? Lastly as a thought exercise, why not a 6 engine plane?

        • MD11 is still viable as a freighter though none in regular pax use (other than charter) I think

        • Well – I would imagine a higher thrust engine needing more maintenance, for starters, and parts to be more expenise.
          In the case of the A343 vs 772ER (which Björn says favour the A343 in terms of engine maintenance costs), I would also think the CFM engine is a lot cheaper to buy and maintain because it’s a variant of such a common engine with some 10,000 planes worldwide using it.

  10. “It shall be observed that we talk about a statute mile (1,151m) and not a nautical mile (1,852m) for all seat mile comparisons.”

    I thought a statute mile is 1,609.344 meters! !!!

  11. Bjorn,

    The 777 has higher passenger preference and lower operating costs partly due to shorter flight times than the A340, which never sold well and is no longer in production, while the 777 continues to be in high demand. Other advantages of a twin vs quad are higher engine reliability and lower engine maintenance labor cost.

    And a statute mile is 1,609 meters and not 1,151.

    • The reason for ETOPs indicates that the reliability measure is not sound, when considering the loss of 50% of your thrust.

  12. Thank you Bjorn! Good distillation of a trend that has been emerging from your articles. This is definitely a myth that needs to die. Surprising that so many airline CEO’s are believers.

    I was wondering if you could comment on a few specific quad or twin aspects, a few of which you’ve alluded to:

    -You mention that some of the A346’s SFC disadvantage came from size effects. Do you have an estimate of how big this effect was for A346? Am I right in thinking that tip clearance becomes less important the bigger an engine gets?

    -I remember reading somewhere that quads have an advantage in cruise due to less excess thrust, which means they cruise at higher OPR with lower SFC. Is this correct? Would be interesting to see where the crossover between twin climb, versus quad cruise, efficiency happens.

    -There’s also the question of greater quad wing bending relief. Lower overall thrust should mean a smaller vertical fin, depending on how far out on the wing are the quad’s engines.

    -But then 4 engines interfere with wing lift more than two.

    It seems to me that quads are definitely better for bigger planes, regardless of whether a twin is feasible, because (1) wing bending relief is more important and (2) engine scale effects are less important. Also if the quad is a stubbier double-decker the MLG can be shorter and lighter.

    But this all raises the obvious question: Why are 787 and A350 twins? Are Boeing and Airbus just missing the point? It seems like a quad would be complex and expensive to build than a twin. So OEM’s could prefer the myth to reality. 😉

    • Hi Eric,

      – yes, there are size effects for engines. The Trent 500 is a 60klbf engine versus the GE90-115 115klbf in TO thrust. Its due to tip clearances getting harder to manage as the blades gets smaller. I don’t have a specific formula or ratio but we are not talking 10% SFC, a couple of % at most. Weight also don’t scale down linearly with size, the smaller engine weighs a bit more, something like 2.5% extra.

      – the -300ER cruise at 12.7% of TO thrust on average, the Trent 500 at 13.9%. It is not enough of a difference to really influence the cruise SFC, the engine OEM can adapt the SFC “bucket” accordingly.

      – yes, the quad lose 25% of TO thrust but worst case it can sit at almost twice the distance from the aircraft centerline (10.7m inner engine which is the one engine for a A330 vs 18.7m for the outer engine on the A350-600) so the fin and rudder does not get much smaller. The wing bending relief from the outer engines is a bonus for wing weight.

      • The 13.9% and 12.7% figures are relative to SLS static thrust though. As a percentage of max thrust at altitude and 500knots, it’s much higher. I’ve read from one poster on that the 777-9, for example, has about 25% more thrust at top-of-climb than required at start-of-cruise. 😉

        If we reconfigure your A346/77W figures and assume that 77W also had 25% excess over cruise, and that A346 and 77W had equal cruise drag (they don’t, just for thought), then the A346 would have only 14% excess thrust over cruise. Thus A346 would cruise at ~88% of max thrust, 10% higher than 77W. And it seems that 10% higher thrust intensity works out to 10% higher OPR for a given engine.

        Now I recognize that the “cruise bucket” is pretty flat for a given engine. But wouldn’t that reflect a tradeoff of propulsive efficiency versus thermodynamic efficiency *for that engine*? If one anticipates cruising at higher % of thrust and therefore higher OPR, couldn’t the OEM reoptimize the engine so that the ratio of compressor RPM to fan RPM is higher? This would be harder on a two-shaft design but easier on a Trent. Of course some efficiency would be given back by the reoptimization but still it seems we would be more efficient cruising at 88% of max thrust rather than 80%.

  13. Brilliant analysis. Hopefully this ends the argument.
    My personal experience as a passenger was on take-off from Johannesburg, our BA 747 lost an engine. We continued onwards and the engine was seen to at the next airport.
    In another situation with a 777, the engine also failed, we then flew around in circles for an hour before returning back to Johannesburg for an overnight stay and new flight on another day.
    Give me a quad anyday.

  14. Not sure what to take away from this other than Bjorn is partial to Airbus (hard to conceal). I can’t imagine that it was only passenger preferences that lead to the 777 decimating the A340.

    • Bjorn is the single best and most honest technical expert I have had the pleasure to get information from.

      There is no bias. He stated why clearly.

    • I don’t understand the bias claim towards Airbus.

      The story has 2 parts:
      The first part is 777-200(ER)/A340-(2)300/ MD-11
      The second part is 777-300ER / A340-600

      The sales of 777-200/A340-300 show that they were on par. McDonald Douglas and Boeing were established in this market, while Airbus was the new entrant. From that perspective it did well. The 777-200 series sold better than the A340-300 but it was not a whitewash so it shows that airlines did consider them more or less on par. The MD11 was a dud and fizzled.
      The 777-300ER wiped out the A340-600 by a huge margin. And this article as well as Bjorn’s comments explain why.

      Why so upset?

  15. I would just love to see how the A340-300 numbers would look like with four PW1000G engines. For once it would surely be the most silent long haul aircraft.
    Max thrust of the PW1000G is higher than the CFM 56-5C. They would be lighter too, as the 5C weighs close to 4.000 kg, the PW’s less than 3.000 kg each. (GE90 around 8.000 kg). So that would give us additional 4tons of payload. But maybe a longhaul version of the GTF would be more heavy?
    Both fuel consumption and maintenance cost should go down by roughly 15%.
    Even for the capital cost, I would think 4 of these should be cheaper than two of the giant twin engines.

  16. I have to believe that part of the reason this is a “myth” is that engine manufacturers perform the same sort of calculations that Bjorn has done – and they need to ensure their offering is competitive in the marketplace. They are willing to lose money on new engines because there’s plenty of margin in parts and maintenance that they can play around with.

    I specifically recall a few years ago RR deciding to reduce what it charged for maintenance on the Trent engines for the A340-600 to keep the cost equivalent to the cost of maintaining two GE engines on the 777.

  17. One factor that I don’t beleive has been addressed is dispatch reliability; given equally reliable engines you have twice the chance on a four engine plane to have a fault delay the flight than a twin engine plane. How large, if any, of a factor is this?

  18. Historically you had better payload-range and more direct route with a four Engine Aircraft. As ETOPS made Twins more competetive they kept on increasing in size, range and thrust. The only 4 Engine Aircraft with competetive Engine maintenance cost is the A340-300 that stay on wing for 32 000-40 000hrs. Just putting 4 Trents thru shop will cost you north of $6M/each. Hence 4 engine Aircrafts must fly routes with payload-range way exceeding Twins capacity. Sad to say no new 4 Engine Aircraft is that good today compared to the 777-300ER. Maybe the A380neo will be if they get 10-14% better fuel burn compared to todays A380-800 and min 5% better than 777-9 or A350-1000 per seat and fly 500-1500nm longer.

  19. What if Airbus decided to do a A340neo?

    Airbus could take the A330-900 and put four P&W GTF (the same engine as the A320neo) on it.

    How would such an aircraft compare with the twin-engine version? Along the dimensions; performance, fuel-efficiency, cost of purchase, operational cost including engine maintenance cost etc.

    Any thoughts?

    • As Bjorn already stated above: the A330 would need an additional center leg for the MLG, and the fuselage diameter is still the one of the good old A300. Basically, the only gain would be an increased range compared to the A330, which is a very limited market, but already addressed by 777-200ER and LR and the 788. So you’d have an old-generation fuselage competing against a current-generation aircraft on super-long distances… now guess who would win?

  20. Hmmm…

    unfortunately IMHO, have to strongly disagree with your conclusions. However due to many reasons, I cannot argue publicly in detail about the subject.

    You may consider to contact airlines who have been operating both the A340-200 and A330-200 and opted to phase out their A340.

    It is true that in real life it is difficult if not impossible to compare apples-to-apples. You seem to the forcing this comparison for some reason…

    However, from a very simplistic perspective if one imagines the same aircraft with two larger or four smaller downscaled engines (same Technology, same design, etc.) one would easily conclude that:
    – the cost of manufacturing four smaller engines is higher;
    – the cost of maintenance of four smaller engines is higher;
    (the cost differential increases together with the manhour rates)
    – as the installed thrust required for the four engine aicraft is slightly smaller (to compensate for the one engine out condition), fuel consumption will be slighly smaller.

    This “simple calculus” has driven Airbus to develop the A340 for the very long range routes (>11.hrs.). I doubt that Airbus at that time was driven by a myth.

    So, at least imho, the idea in discussion is no myth, it is just simple engineering and calculus.

    • Ferenc
      I am not so sure about your comment on engine maintenance.
      If you look at historical data smaller, lower thrust engines are less expensive to maintain. I don’t know if they are half the cost (the engine companies manipulate these numbers a lot) but the will have longer time on the wing and are less expensive to referb.
      The point of this discussion that I find amazing is that the engine companies have been able to build very large, very reliable, and very efficient engines. These are now well beyond the size that anyone was imagining 20 years ago.
      My personal bias is to fly upstairs in 747. I have also been on a takeoff where we lost an engine. didn’t change climb and headed out over the ocean for 6 hours without a worry.

      • I would hope that if you loose and engine on takeoff you go back and get it fixed.

        Sort of wrecks the idea of redundancy gone

        there is a pretty famous one of British Air taking a 747 off from LA I believe it was, loosing an engine over Vegas, flying onto East Coast and over to Ireland where they declared an emergency due to low fuel.

        US wanted to prosecute the pilots, Britain contended it was all fine. Not by me but then…….

        • I remember reading about that. I think it stated that the engine was lost during climb pretty much right from the start. It declared an emergency and landed in Manchester, UK.

          • I think they landed in Ireland, could not even make England

      • Ed,
        Try to imagine exactly the same engine (which you can then scale up or down a bit).
        In such case the amount of assembly/disassembly work is almost the same, right?
        This is why the “per pound of thrust” specific manufacturing and maintenance (overhaul) costs go down with the increase in thrust.

        • Ferenc,
          I get what you say and you are correct, I don’t dispute that. But I have in my analysis put the actual prices that the Engine OEM’s MRO organization charge for the engine overhaul, reduced to price per seat mile for the 777-200ER vs A340-300 and A380 vs 777-300ER. And the twins prices are higher per seat mile. It can very well be that Rolls Royce gain more for the Trent 800 and GE for the GE90-115 (things don’t change much for GE90 or PW4k for the -200ER or GP72 for the A380), but the prices that are quoted in the industry calculated per seat mile are higher.

          BTW, I’m not alone in finding this, Ascend did a study of the 777-200ER vs A340-300 and they came to the same conclusion.

          • Björn,
            The actual numbers of “engine catalog price” and “engine overhaul price” are influenced by various market factors such as the competitive environment.
            A more competitive environment would drive down not only the engine sales price but also the maintenance costs.
            I believe your models do not really account for this aspect of “Real World” and may easily lead to incorrect conclusions in this complex space.

    • Agreed. I think Bjorn is trying too hard to create this “myth”. The A340 has been unsuccessful and it has directly to do with its maintenance costs. I’m sure even Virgin Atlantic would admit it even though they were such vocal supporters of the A340 in its short-lived heyday “4 engines 4 long haul”.

      • You seem to limit the A340 to the A340NG (i.e. -500/-600) series. Otherwise, your statement doesn’t hold.
        Björn did address the A346 separately in the comments; he also never said four engines are better that two.

        He summarised his findings thus:
        It is just that the equation is not as clear as the myths make out, at least we could not find the patterns in those 1000 parameters. It is still a matter of building a good airrframe/engine combination, it can well be a quad going forward for very good reasons. But you don’t double the engine complexity for no good reason.

        What we can see in our data is that the knee-jerk “quads are duds…” is slander and not science.

  21. Boeing could have solved the engine clearance problem on the 737 with four 60″ fan high BPR engines. Morph a 737-900 into a 720MAX.

  22. The fewer people that share “my” airplane ride, the better I like it. That statement is intentionally leaving out the obvious and relevant factors of efficiency and ticket co$ts.
    Why? The crowd/lines factors. From traffic at the originating airport, to TSA, gate crowds, de planing times, luggage locating hassles, and then again, airport traffic jams.
    I like the A380. The look, the concept, the efficiency.
    Maybe in another 25 years it will come into its own when all the major airports will likely be capable, and more efficient, at handling that large number of people all at the same time. Until then, I’m happy to find other flights when possible.
    And that likely will be riding on 2 engines

  23. In 25 years there will be no more A380!

    Bjorn: thank you, I did not know 4 smaller engines were lower cost maint than two big ones.

    I think what is missing here is the ETOPS allowance (or lack of)

    If we go back to the original concept (pistons) you have to have 4 engines on the aircraft to ensure you had two working at trips end (or allow a turn around). Lots of moving parts and the last super pistons were a nightmare of complexity with turbo super charging and even less reliable than previous.

    Jet engine initially were horrible reliability and they had power issues (more power, less reliable until they mastered the technology )

    I remember the 747 with the P&W engines that kept failing. They had a 747 with a spare engine mount to fly new engines to where yours was broken down (same engine mount?). Idea was they ran it while flying to get it broken in and hopefully not fails on the way or shortly after install.

    MD11 and L1011 were response to having to have more than two engines for long overwater flights.

    The A300 came along and could not sell in the US for those routes but Asia did not have those limitations and they bought them (I flew on one and none to happy at the time as there is a comfort level with 3 or 4)

    So we have old thinking involved as well and the fact that people equate more as better at play here.

    It does not always work that way.

    like the DC10 I think it was that left Miami, mechanic put in the oil plugs wrong on two engines and it limped back on 1.

    Fuel starvation on a 747 going into Japan and the last engine quit at touchdown (fuel mismanagement, they could not find the full tank)

    Or when I was a kid and they ditched a DC6 (I think it was) off Biorka Island (its in the crash logs) with 3 good engines! Why? they had a runaway engine and they had one disappeared on the way to Hawaii if I remember it right and the assumption was runway engine tore off the aircraft damaging the wing and system enough to take it down (never found)

    so, the directive was for a runaway engine to ditch (they did, all did well as we had a rare and wonderfully calm day for them and one injury caused by Coast Guard insisting that they had to transfer to that ship vs the FAA ship they were on rather than just escort them in.

    so its an interesting thing, maybe Boeing next large one will be 4 engines!

    Someday though we will have a big twin loose both engines and then will see what happens (we see at least two a year engine losses over the Pacific that divert to Anchorage or Shimya, mid winter, grim survival chances. )

    • Transworld:”In 25 years there will be no more A380″

      Apart from 4 engines many forget the A380 has twice as much passenegers decks too. In a single slot.

      Now is that really important / relevant to this discussion? Hell Ye$!

      I’m always amazed people see capacity just as one of many variables. In reality it is the most important variable in selecting / ordering 🙂

      • Airbus has a decision to make on the A380 in the next 3 to 5 years.

        That’s going to be a hard one for them.

        Boeing has the4 same decision on 747 (sans orders) sooner. Not so hard but still painful.

        • The decision for Airbus will be what type of engines the A380NEO will get and how large the stretch will be.

          • Difference of opinion obviously

            Not enough bills as no book lately

          • It seems like ~30+ airframes/year is the magic number for Airbus to make a per-unit profit on the A380. The backlog is somewhere around 120 (official number is higher, but the official number has Amadeo, Virgin, and Transaero as firm orders and given Airbus’ eternal optimism about its order book, I slightly deflate the number). They start cutting metal ~3 years in advance so Airbus is perhaps within one, but not more than two years, from starting to cut metal to build the last airplane in its current order book.

            There’s not much time to firm up a redesign, test it, certify it, and drive in new (non-Emirates) orders to keep the line alive.

            The question is whether Airbus has the will to slow down the line to 20/year or less to find out if slot constrained airports drive new orders in five years.

            The only thing going for the 747 is their ability to build at a snails pace at a profit.

          • “The only thing going for the 747 is their ability to build at a snails pace at a profit.”

            Afaik the 747 product line is in forward loss and will stay that way. The 747-8 upgrade was significantly more expensive than planned due to delays but also significant errors made.

            I think that the current rate is determined by “least losses for production of sold frames and the opportunity to maybe sell some more including the projected AirForce1 frames.

            In this context. From the way Boeing does bookkeeping I’d assume that forward loss statement applies to the whole program block over all 747 types ? Or is the 747-8 a “new” standlone project ?

  24. Bjorn,

    I still think B777-300ER and B747-8I comparison with your “normalization” approach would be useful.

  25. There is one modern Twin vs Quad issue that is harder to address and may be relevant.

    Modern commercial transports have evolved to use higher aspect ratio wings. The result is outboard wing chords that are relatively small.

    Integration of outboard modern high bypass ratio/high pressure ratio engines on these smaller outboard chords may prove to be a challenge both aerodynamically and structurally due to their increased size and weight, creating a Quad disadvantage.

    • Hi Bob,

      I agree there are issues with quads (as there are with twins), in no way am I proposing that future aircraft shall be quads (if there is no really good reason). I just wanted that the thoughtless mantra of “quads are always a bad thing” should stop, the reality is more nuanced and in the two cases I mentioned in the article the costs picture bucks common wisdom.

      • Bjorn:

        I had not realized how nuanced so its a really good one to bring up

        • But to counter that, one airline that operated both A340 and 777 was Air France. They bought reasonable numbers of A340 but most have been sold off , some after only a short period, however their 777 fleet has increased in size and all continue in service.
          Surely they had their own data to make decisions on and it doesnt seem to back up the quads are Ok theory.
          There wouldnt be any airlines that discontinued 777s for more 340s but the reverse has happened – eg Swiss which had MD11 then A340 and no is getting 777!

      • No problem, a reasoned engineering discussion is always welcome and your analyses are well thought out.

        Airplane design is often wrapped up in the details though and it’s good to lay all of them on the table.

        Don’t let anyone get you down, no matter what forum is hosting the discussion.

      • Of course it’s nuanced!

        Everything in aerospace is a trade-off – performance, efficiency, weight, cost, safety margins, manufacturability, maintenance concerns, regulations…

        The whole premise of the myth is silly – if there was such a natural law on fewer engines being better despite all of the above then why stop at two engines when one could do the job…?!

  26. Bloody good article Bjorn.

    My fondness for quads is born out of bitter experiences on quads, primarily 747’s of various marques.

    Two flame-outs, one catastrophic turbine failure, innumerable lightning strikes always seemingly occurring over water & at the point of no return, have instinctively taught me wherever possible to fly only quads. If this involves extending flying time & obscure routing, so be it…..

    Best to arrive than not at all.

    • Still the same old problem of a short range engine on a very long range airliner as Bjorn has mentioned with the CFM56.
      It seems the pressure rise of around 30 in the GTF is not too different to the CFM56 while still well short of the GE90s over 40.
      In fact a possible A340 neo would be better off with 4 x GE leap 1a as it does have a pressure ratio around 40

  27. Reading these excellent analysis (and others) I am wondering why a quad like a potential A380 neo isn’t built with two big, superefficient engine likt XWB-97 combined with two smaller ones like Trent 7xxx. Then use the two big ones for cruising and all four for special situations like take-off (high temp) and if one fails. Is this not possible, was it ever done or are there reasons why not doing it?
    I think a non-expert like me is allowed to ask such a question (there are no dumb questions only dumb answers 🙂 )

    • It is a good question. Navy does a variation turning off engines when on patrol with the P3s (harder with a P8!)

      C123 had pod jets that closed up for flight.

      • Of course, the drag of a feathered turboprop is far less than the drag of a windmilling turbofan.

        With RR working on a variable pitch fan, it’ll be interesting to see if they can move the blades enough to “feather” the fan. That doesn’t achieve a big drag saving without also feathering the fan’s stators, which sounds like a major engineering challenge…

        On that topic, does anyone know if RR’s variable pitch fan will also feature variable pitch stators? If they don’t then one wonders how they’re planning on dealing with the torque they’ll get when the fan pitch doesn’t match the stator pitch…

        • Variable pitch is said to potentially obviate dedicated thrust reversers.

          Make of that what you want 🙂

      • Hmm, I’ve taken a look at pictures of engines carried on the 5th pylon on a 747. The fan is removed but of course the stators are still there. A lot of air will pass through and create a torque.

        It clearly wasn’t a problem on the 747, so perhaps it’s just not something anyone needs to worry about.

        • Youll notice the fan was removed, but importantly a fairing was fitted front and rear to block off the airflow through the core, air would pass through bypass area. No compression would occur for bypass air, so I think you are wrong on that.

          • From what I’ve seen the stators are generally not straight through, they are twisted too. Makes sense – something has to counteract the fan torque otherwise the engine would not be torque neutral overall.

            Air passing through the stators will apply a torque to the engine. With the fan missing there is no fan torque for the stators to counteract, so there is a net torque on the whole engine.

  28. After careful consideration of this interesting article and comments, I am ready to formulate *** for free ***, my recommendations to Boeing regarding their upcoming Middle-0f-the-Market aircraft:

    — 3 PW1100G engines, possibly in a novel pylon configuration, so as to avoid the risks associated with uncontained failures inside the tail section.

    — 3-2-3 economy seating in a fuselage just narrow enough so that there will be no margin for too-clever carriers to stuff one more seat in there.

    • I’m pretty sure any aircraft that could do 3-2-3 would find more customer satisfaction with 2-4-2. So much so that I am unaware of any plane ever being fitted 3-2-3 (if they have, I’d enjoy pictures!).

      I am a fan of the 2-3-2 cross-section myself, but I think we’ll only see that in long-haul premium economy sections once the venerable 767s all retire and end the standard econ 2-3-2 era.

      • Right. Only after posting my comment did I realize that I should have written 2-4-2, if only because of easier service by flight attendants.

  29. Re Transworld’s comment on 747 spare carriage.
    Spare engine carriage was conducted on revenue flights. There were no connections for fuel. controls or anything. Therefore no running in as postulated. In fact the fan had to be removed and a special fairing installed inside the engine inlet in front of the core. Google it (747 spare engine) for lots of pictures.

    • So much for my memory, do I get half credit?

      Still was relevant that engines of that era not nearly as reliable as current not to mention the PW issues specifically.

  30. The airfreighter conundrum in the perspective of modal shift from Triple E to AGA-liners calls for gigantism (MTOW > 875 t for payloads > 350 t) and Prandtl geometries, entailing a need for thrusts beyond the 4 x 115 klbf class. If compressor blade and fantip shock aerodynamics could be better mastered, the UHBPR turbofans of tomorrow could involve trans- or supersonic nano-calculus where the RdM (Résistence des Matériaux) equations are solved reverting to fan diameters in the 45″ to 65″ range. In that perspective – of maybe preferably 6 x 75 klbf ? – Björn’s findings herewithin will come quite handy …

  31. I don’t think the worldwide airline industry’s clear preference for twins is due to belief in a “myth”. They live or die by evaluating and understanding their costs, and they clearly disagree with Bjorn’s cost analysis.

    • The airliners can only choose from the aircraft offered by manufacturers.
      Bjorn doesn’t say the market has a good quad option but that it potentially can be done and anyone who says otherwise base their argument on an apples-oranges comparison.

      • Manufacturers base their offerings on what airlines will buy. The best apples to apples comparison in recent history is between the A330 and A340. It would be hard to imagine a better real world comparison. The A330 has been very successful while the A340 bombed. Both of the other current quads are also in deep trouble. Bjorn’s got his proprietary models, but airlines have real data based on hundreds of thousands of hours of actual operating experience. A manufacturer would have to have a death wish to invest in another quad today.

        • Very airline actually operate both , so their information comes from other sources.
          Lufthansa had A340 (200, 300, 600) but not 777 but did have A330
          Air France had 777-200/300 and 340-300.
          Its interesting that all AF 777 are still in service but half its fleet 340s are in storage ( but not the 330s)
          So here is an airline that had both 340 and 777 but is retiring its quads !

  32. The A330 FAL was (and in theory still is) actually a dual A330/A340 FAL and could at minimal cost be revived for such duality. I doubt the A320 Series pylons and nacelles for the NEO engines (either Leap or Purepower) can be transposed “as is” for the A340 NEO to be revived as well but we may play with the idea. Under the foregoing set of assumptions, the come-back of factory-new A340 in the NEO dressing is with minimal risk. A test of Björn’s proposition would be if the Airline community responded massively placing A340 NEO orders alongside with the success of A330 NEO ?

    • You realise that there was an A340NEO, right?
      It was called the A340-500 and -600, and it didn’t do so well.

      Also – it seems to be very hard to understand what Björn actually said in his article. Which was not “quads are better”, but “quads are not necessarily in all cases inherently worse than twins”.
      But people’s unability/unwillingess to understand/accept this may go a long way in explaining what’s keeping the “twins are better than quads” myth alive.

      • -500 and -600 are more an NG style upgrade.
        new wing, new bigger engines overstretched fuselage.
        Independent of Twin or Quad a dud at EIS.
        Boosting MTOW as a fix has limited utility.

        Still wonder what went wrong at Airbus in that timeframe.

        • I think Airbus saw an opportunity to corner a large part of the long range market. The A340-600 is more efficient than the 747-400.

          Unfortunately for their strategy, the engine OEM’s (GE & RR) were willing to develop engines to make the 777-300ER possible.

          • The exclusive engine on the 777-300ER ( and its cousins 777-200LR and 777-300F) is the GE90. I dont know if RR offered an engine but missed out or it was stitched up with only GE

          • RR offered an engine for the 777-300ER but GE made a better business deal.

            Ironically, only engine OEM was selected based on the limited market size.

            The market was a lot less limited than originally thought.

  33. Bjorn

    Will it be possible to issue a compare table of the wings including the centerbox of the 7773ER and the A340600 ( weight / surface/tank volumes/ etc……)?

  34. In the past I thought Boeing would replace the 747 / 380-500 seat long haul segment with a brand new design, fitting nicely under the A380. No engines were/are planned for a twin 500 seater. So I thought a 2.5 engine (ATPU) would be most efficient. No huge engine in the tail but something strong enough to generate the ~160klbs combined to clear the fence after an enginefailure just before V1.

    Through the year similar patents where filled by Boeing and Airbus, even last year money was offered to have the Ecoliner IP! It seems at this stage the market segment has become to small to invest the $20bill to do a “moonshot” design.

    • Keesje:

      With all due respect, that is why efforts of people to design aircraft for Boeing and Airbus are seriously silly.

      Boeing has its own take as does Airbus as to what is feasible.

      Big difference in throwing designs out as opposed to being responsible for a company future health (like the distance from here to Pluto)

      I say that as I have been there on more than one occasion and shot down, it just did not make sense in regards to the whole picture for those who make the decisions and what money goes where.

      Boeing does not feel there is a large market for a sub A380 or even A380 sized aircraft and tried to limp the 747 along on the cheap. So far Boeing is right.

      they failed, cost a lot more than they intended (screw ups) and there is not enough market for it.

      It will be gone in a few years.

      No one is talking about a sub A380 quad as the 777-9 fills that position very well and its not nearly as costly as all new let alone overcoming the quad myth.

      On the other hand with all its early issues, the 787 looks to be a serious winner (including financial eventually) with its orders, options and intake of new orders as time goes by.

      A350 is doing well but not as good as the 787 so time will tell.

      Its obviously going to be in the 767/A330 sales class but may not reach the 787 class which looks like it may stand alone in that regards.

      • Transworld Quote: “A350 is doing well but not as good as the 787 so time will tell.”

        Well…seeing that the A350 is financially at least $30 Billion Dollars ahead of the 787, I would say it’s doing comparatively well: spectacularly, in fact.

        • “Well…seeing that the A350 is financially at least $30 Billion Dollars ahead of the 787, I would say it’s doing comparatively well: spectacularly, in fact.”

          Really? So the whole A350 program was done with just about 5 billion dollars? Is that is the case, the A350 program would be the best run program in the history of aviation (with its multiple iterations, delays and everything in between).

          • Karl:

            Your math is…astounding – to say the least. Perhaps you should cool down, and then compare Development and Production Costs for each program and see what ya’ get.

      • Whether or not the A380 will be gone in a few years (as you say, Transworld) is not important. The crux of the matter is to exist, thrive, mature, advance, be resilient through the spiral windings of time in aerospace History. The OEM’s core value is constituted in essence of thousands of highly competent design Engineers and to keep this pool of distillated know-how busy creating new and ever better aero-contraptions is what matters. The MAX is an example of an obsolete-in-the-egg creation putting an OEM’s values at risk. The programme management model of the 787 was yet another blunder, thinning out your core values. Perhaps Boeing will recover. The A380 will eventually pass the threshhold of cycle recovery, allowing the carroussel to keep turning around for another human generation, leading onto the next upswing of the learning/inventing/building aerospace adventure. It will be remembered as a formidable feat. The speculations about the dissappearance of the A380 are somewhat premature or greatly exaggerated ?

        • Quote: “The speculations about the dissappearance of the A380 are somewhat premature or greatly exaggerated ?”

          Perhaps. The A380 might be a nice plane, but it’s not the “World Beater” that Airbus envisioned and its potential to make money for Airbus is…extremely limited, let us say.

          On the good-news front, the A380 is no longer a financial drag on Airbus. Also, if it comes to program cancellation – then “adios amigos”, the A380 is out the door and Airbus will be happier for having done it.

    • How to optimize the size of a double decker and shrink it for less weight than the A380? Width of 223″ at armrest main deck, 183″ at armrest upperdeck, and build a new smaller size container for the freight deck.

  35. Should Boeing also stop creating and publishing similar concepts? Google Boeing patents images..

    Boeing believes there’s a VLA market, thats why they invested. When competition won they adjusted their (public)vision.

  36. Over 10 years ago, despite of the existence of A340, Aibus proposed a change in ETOPS rules explaining that the most important safety point was that the airport used for diversion should be able to handle a large group of passenger and make sure repair or landing and taking off of another aircraft was possible …

    Just imagine a diverted WB end up in a remote airport in the middle of Siberia in winter !!!

    • ETOPS/LROPS is a bag of competing/parallel rule making.
      going by the distinct differences in certification ( ETOPS-FAA, ETOPS-ICAO, ETOPS-EASA ) and the FAA discriminating between Their Frames and Other Frames … 🙂

      Provisions for medical emergencies also come into play.
      On long flights you lose the “Golden Hour” in mitigating the effects of (human 🙂 ) circulatory system upsets ( stroke, trombosis, .. ).
      fire supression ….

      just being able to “limp on” for a full workday ( 6+hours) is definitely not enough.

  37. Paxliner model ______ 788 __ A333 __ A359 __ 773 __ 779X __ 748i __ A388
    Typical nº of passengers 219 ___ 295 ___ 314 __ 344 ___ 394 __ 405 __ 525
    Nº of LD3 in lower deck 28 ___ 32 ____ 36 ___ 44 ____48 ___40 ___ 36
    LD3 available for freight* 16 ___ 16 ____ 19 ___ 26 ____ 27 ___ 18 ___ 8

    The above table extracted from Leeham shows the underperformance of Airbus WB or VLA paxliners – specially the A388 – in terms of the number of LD3 available for freight … the venue of A343 NEO or an A346 come-back would restore A SOLUTION (absent today) into the WB offering palette of Airbus for operators with Long Haul freight-rich markets : eg the A346 has 42 LD3 whereof 24 LD3 available for freight, closely matching 77W in numbers of LD3 but outperforming same aircraft in terms of freight density thanks to the quad TO-perfo and a generous MTOW …
    * after CIL = checked-in luggage

    • Quote: “The above table extracted from Leeham shows the underperformance of Airbus WB or VLA paxliners –”

      No it doesn’t.

    • – I saw somewhere long haul belly’s are filled on average 37%. Is that correct?

      • Hi keesje : I understand you are quoting tonnage cargo load-factors ? Freight trend indexes are set to green, pointing at a near-term market recovery. Belly-hold load-factors are currently thwarted from four factors : persisting market depression + backward effect of costly/cumbersome restowages from TEU to whatever ULD (mainly LD3) are applicable + paxliner airport turn-around considerations + volumetric restrictions mainly from cargo density restrictions, with 3500 lbs/159 cuft – or IS 0.352 – being the theoretical LD3 density limit, whereas ISPF – itemised, small, packaged freight – comes in much lower densities. A come-back of WB quadris presupposes a revision to the hold structure and better LD3 loadability, for natural modal shift from maritime —> airfreight to sprout and explode : masses of heavier freight is available out there waiting for the tooling (eg AGA, reefer LD3 etc … ?) but also operator freight category billing practices to evolve …

    • You’d have to also show that carriers can actually utilize those LD3 positions to make your argument worthwhile.

      type LD3 LD HD
      A358 36 27t/32t 57t/63t
      B789 36 27t/32t 57t/63t
      ( weights as net / tare )
      both types have ~~~55 t max payload!

          • The A333 and A359 are doing well, overserving 788 markets, matching 789 markets and undercutting the 777/77W. The problem is with A388 vs 77W and the subsequent 779 (the 748i is on its way out, never mind). Airbus doesn’t offer a way to service correctly the thinner long haul freight-rich markets, having killed the A343/A346. An intelligent revival is warranted, or Airbus needs to supplement its product portofolio with an UltraFeighter ? To leave Modal Shift in the long haul freight segment for to Boeing to pick up singlehandedly equates to gross negligence ?

  38. In fact, Boeing could come up with a distinct WINNER if Björn’s theory holds : the 77W quadri NEO with a MTOW of 375 metric tonnes or more ? Or a trijet version of same for the TO-perfo, the third engine going idle in cruise for minimal drag/maximal cruise perfo ?

    • Quote: “In fact, Boeing could come up with a distinct WINNER if Björn’s theory holds…”

      I think you need to carefully re-read what Bjorn has written so that you will understand.

  39. Hi Jimmy : Björn has axiomised “neutrality” of quads vs twins in the cost picture. consequently for long haul freight-rich routes, the yield equation gives the economic advantage to the quad, due to revenue enhancement from additional freight capability. I’ll leave it for you to chew upon, I know it is hard to grasp ?

  40. I think that if you compared a flight from e.g. HKG to LAX, the A380 can carry more cargo than a 777-300ER. Even on top of 525 spacey seats.
    Because you need so much fuel, available belly space becomes irrelevant, payload-range (weight) takes over, from a limited hot HKG runway, with one engine gone boom.

  41. Did some quick, available numbers for this specific city pair.

    ** HKG-LAX including 15% reserves, headwinds, ETOPS = 7200NM.

    ** Cargo opportunity load for 777-300ER (45t) -350 passengers (35t) & fuel = 10t.

    ** Cargo opportunity load for A380 (75t) -525 passengers (53t) & fuel = 22t.

    ** If LD3s weight 1250kg on average (max 1500kg), a 773ER could carry 8 (available space : 26?) , an A380 17 (available 8) on HKG-LAX.

    ** Excluded:
    – hot /short runway restrictions (Twins!)
    – relative economy cabin comfort on 13 hrs flights (both 10 abreast)
    – the 175 extra revenue dudes in the A380.

    *** Everybody is welcome to keep thinking / telling the 777-300ER has superior cargo capacity over the Pacific. They seem pretty even to me. (“cargo load per seat” is nonsense, most goes in an old cold 747 via Narita/ Anchorage anyway)

    • For 12 plus hour flights, is the volume for freight suboptimal in aircraft design for single deck aircraft? If the freight deck is half empty, that is a lot of frontal area and skin area creating drag for no reason. Possibly the double decker is a better solution for adding passenger capacity at 70m and up. Plus, smaller containers. Just because the A300, 747, 787 were designed by drawing a circle around two LD-3s, doesn’t mean that creating large cargo bays is cost effective aircraft design for increasingly long distance passenger routes.

  42. Brillant demo. keesje, Thank You ! It puts the picture on the wall, conspicuously, so Jimmy & Co can let the message sink in … take a look, folks ! Conclusion : the potential for yield enhancement is definitely there, to collect the inherent value increment of Quadris vs Twins you need to accept higher freight densities (in the case of A388) or simply revive A346 and/or A343, provided Björn’s axiom is meaningful (personally, I adhere !).

      • Not quite so, Uwe : assume a 13 frames stretch to A389. Typically, it would give room to 10 additional rows Y-class 11 abreast @ 32″ pitch on main deck, plus 6 rows of Premium seats 6 abreast @ 54″ pitch on mezzanine, total pax addition 110 + 36 = 146, requiring 146/19 = (rounded up) 8 additional LD3 containers for CIL, whereas the number of LD3 available was incremented by 5 x 2 = 10 … the net gain is only two more LD3 made available for payfreight, up from A388’s initial eight ?

        • But you’ve also reduced available payload mass via more pax+bags and more fuel for a heavier plane.

          • Correct, so for retrieval of maximum cargo utility, raise MTOW ? They say the current wing structure can lift 650 t or more, so we have a comfortable weight margin up our sleeve … the volumetric limitation to payfreight remains, though ?

  43. Bjorn,
    I wonder if an A380 derivative could be an effective carbon reducer in the global battle to curb global warming. I’m thinking ~4000 nm range, .8 mach, 750-800 passengers in comfortable all-economy for high density routes — including to and from the USA. It would have a new smaller composite wing with folding wingtips and possibly some laminar flow, four smaller engines (new or existing updated) or possibly two larger > 105 K thrust twin engines, weight reduction (lighter landing gear, new wing box, tail changes etc)

    It might be the world’s champion low carbon emissions per seat aircraft — in service between 2022 and 2025. How about it?

    Let’s hear more. There may be room for a similar 600 passenger 777-9 derivative

  44. There is an error made in the pressure ratio of the CFM engine

    “After the no-show of the Superfan, the A340 adapted the CFM56 short haul engine to a long range engine with pressure ratios of around 30.”

    This is incorrect, data from CFM shows the CFM56-5C series have a pressure ratio just above 38 at ‘max climb’
    This is a considerable improvement on the ‘around 30’ claimed. maybe thats what applies for short range versions .

    • I don’t think it is an error, what I write about is the typical pressure ratio at cruise setting as this is what determines the fuel consumption of the aircraft. The “Max Climb” setting of the engines I compare with are closer to 50 than 40, with around 40 being the typical pressure ratio for these engines when in cruise. All top of climb values are specified a bit differently so its a bit iffy to compare them (some at “Max climb” top of climb OPR, others at Max Continuous, different flight levels etc).

      The most reliable comparison of engine data at apples to apples conditions is the EASA Engine Emission Databank compilations. It only handles engine data around an airfield but conditions are clearly specified and like for all. There the TO SSL OPR for the long range engines I write about are around 10 units higher than the CFM56-5C. Most of this difference is then to find again at cruise.

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