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:
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:
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.