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This question relates to this article at Leeham, essentially this article challenges the concept that twin engined airliners are more efficient than quad engined. However the article fails to show the data and prove the arguments. My question is asked to those who are able to confirm the arguments of the article. I will describe my argumentation and formulate the specific answer at the end.

Essentially, in the industry it is preferred to use twin engines vs quad engines based on the following arguments:

  • Twin engines have less maintenance cost (2 vs 4).
  • Less aerodynamic interference and weight.
  • Less cost (4 enginers are more expensive including systems for the same power)
  • However less bypass ratio for twin engines and overdimensioning due to compliance with the take-off certification at engine failure.

Netly, twin engines win against quad engines except in the past for ETOPS certification.

Ok, previous statement is my understanding of the trade-off between twin and quad. The article argues (using always seat mile costs) the following:

  • Little efficiency between B777-200ER and A340-300 / A380-800.
  • 4 CFMs engines or A380-800 4 engines have less maintenance costs than B777-200ER 2 engines. Fleet of CFM is higher as well.
  • Argues that B777-200ER requires more powerfull engines. This argument does not contribute, as finally is an efficiency driver and the important is the first argument.
  • Previous comparisons are not valid to previous low bypass engines.
  • Article is not introducing airplane cost in the comparison (important for capital costs)

So, my explicit questions are the following:

  • Can somebody demonstrate with evidences that the efficiency of B777-300ER is lower or higher than A340-300 or A380-800?
  • Can somebody demonstrate with evidences that the maintenance cost of the engines of B777-300ER is lower or higher than A340-300 or A380-800?
  • Can somebody support further my arguments or support the article?
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    $\begingroup$ Kinda curious why the author compares the A340 to the 777 instead of the A330 which is essentially the same aircraft except for the number of engines. They were developed side by side and released within months of each other. If he wants to avoid apples vs oranges I couldn't think of a better choice $\endgroup$ – TomMcW Jan 17 '16 at 0:47
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    $\begingroup$ Yeah, and the A330/340 AIUI are more comparable to the 767, not the 777 $\endgroup$ – UnrecognizedFallingObject Jan 17 '16 at 0:49
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    $\begingroup$ Larger engine is more efficient in theory. Four engines reduce wing weight. There are many factors to account for but only two four engine planes for sale. I don't know of a historical example of head to head same generation planes with same generation engines. I believe the example in the article is for planes with some age difference. A moot point for airlines as they will buy twins unless they buy the A380. $\endgroup$ – Brinn Belyea Jan 17 '16 at 1:02
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    $\begingroup$ This seems more of a request to confirm your arguments, rather than a question about aviation per se. $\endgroup$ – David Richerby Jan 17 '16 at 3:22
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    $\begingroup$ @DavidRicherby no, in fact I want to know if the arguments of the article are really true. $\endgroup$ – Trebia Project. Jan 17 '16 at 9:35
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The main problem with the article as far as I can see is that they offer little in terms of evidence. The best we could do in the absence of their 'propriety model' is to use real world examples.

  • The best one is to use the Airbus experience in developing A330/340 which had little difference than twin/quad engines. While A330 is still in production, A340 has been stopped, indicating customer indifference. It is unlikely that the customers would've stopped operating A340 based on some myths.

  • Another thing to note is the present operation- Even in case of long-haul routes, where quads like A380 must be more efficient (in short routes, the more powerful engines of twins should help them reach cruise quicker), more twin engines are used compared to quads. In fact, the 777 LR is the predominant aircraft operated in ultra-long haul routes, followed by A380.

  • Also, look at the latest iteration of the 747, 747-8. Boeing is having trouble having takers for the jet (similar story in A380). In airline industry, where every single cent saved counts, almost all airlines are preferring twins over quads. As 747-8 has most of the aerodynamic improvements of current generation aircraft, maintenance should be an important factor in airline's decision in not selecting it.

  • Another important point- no quad is under development right now. With more and more aircraft development being done in collaboration with airlines, this is significant. in fact, since ETOPS came into force, quads are being slowly phased out (like trijets before).

  • The article never makes any point to support its assertion that the maintenance cost of twins and quads are the same. At the simplest level, assuming that they both have the same high-efficient engines, quads should take twice as the twins in man-hours for maintaining. For airlines, where engine maintenance takes a significant chunk of total maintenance, this is important. Also, more engines means more points of failure, which practically increases the inventory.

  • While one could argue that the quads can have better designs compared to twins (by virtue of improves wing bending relief, for example), this is questionable and the associated systems (fuel, fire suppression) would add more complexity and weight, compared to the twins.

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Why would Airbus go to the effort of developing the A330 when they could offer the A340? Both have the same airframe and differ mainly in the number of engines. With ETOPS requirements at the time of development (when they were known as TA9 and TA11) being just 2 hours (ETOPS 180 operations were only begun in 1989), the only reason for developing the 4-engined variant was its capability of flying long overwater routes.

The time it takes to check an engine is the same regardless of size, so having only two of them saves time and money. In addition, the higher power reserves of a two-engined aircraft allow for higher climb speeds and enable it to reach the most efficient cruise altitude faster.

Today, most airliner engines are no longer owned by the airlines, but leased from the engine manufacturer. They offer a simple thrust per hour model where the airline knows exactly how much the engine will cost them over time, maintenance included. The only open issue is how long the airliner is grounded due to unforeseen maintenance, and with four engines per aircraft the number of unforeseen snafus is twice that of a twin.

At the same time, the airline is in the best position to judge how expensive a seat-mile in any of their aircraft is. Engine cost can be calculated very precisely, so every airline should have a very clear picture of how expensive their twins and four-engined aircraft are.

There is a small chance that the geared turbofans which are now being introduced will give the A340 an edge over a twin variant. The higher thrust class of large twins makes a geared fan currently impossible, so a four-engined aircraft will have an edge because it can use a more advanced engine.

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  • $\begingroup$ Anyhow, the author argues that the maintenance cost is not highly different. The theory for the maintenance cost is clear, anyhow the article puts on the table (without showing data) that the factual situation does not show that difference. $\endgroup$ – Trebia Project. Jan 17 '16 at 9:37
  • $\begingroup$ @TrebiaProject.: You pointed it already out in the question, and I fully agree: Nowhere does the article show any trace of proof. It just claims it is so. Sounds like religion to me, not sound proof. $\endgroup$ – Peter Kämpf Jan 17 '16 at 10:58
  • $\begingroup$ What I am looking for is somebody that, based on evidences in operation, that can support or challenge the article. For the moment I agree with you, that article is based on faith. $\endgroup$ – Trebia Project. Jan 17 '16 at 12:54
  • $\begingroup$ The article is based upon a small sample size. The price asked by an engine manufacturer is what the market will bear, not what it costs them to maintain the engines. With such a small sample size there is no easy way to determine whether the prices are inherent or due to a markup. Airbus and Rolls Royce offered a program to cut A340-500 and -600 engine costs to the level of the 777. airinsight.com/2013/12/05/… $\endgroup$ – Brinn Belyea Jan 17 '16 at 16:20
  • $\begingroup$ I've been thinking about the cost of maintenance and one explanation I can think for the assertion of the number of engines not making a difference in practice would be if the cost of maintenance per plane is dominated by the need to make sure enough planes are available even if some minor repairs are needed. The time needed for routine inspections and maintenance really might make no practical difference in that case, since it would be small in proportion to the time reserved in case issues exist. It would still reduce time available for other things, but the cost of that is not per plane. $\endgroup$ – Ville Niemi Jan 17 '16 at 16:53

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