Is the twin vs quad engine industry preference really challenged?

Question

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?

Answer 1

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.

Answer 2

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.