Dec. 15, 2015, © Leeham Co: LNC’s Bjorn Fehrm started a firestorm of discussion last Friday with his Corner about twins-vs-quads column. His focus was on the Very
Large Aircraft sector. Overlooked in all of the discussion was a piece of information LNC wrote April 6 from an interview with Alan Epstein, VP of technology and environment of Pratt & Whitney, in which he mused that quads could make a comeback—on smaller airplanes.
The original article was behind our Paywall, but the Summary with this reference was in the freewall portion. We’ve now opened the article to full freewall and it may be found here.
“Getting more efficiency from engines means the use of more advance materials, evolving and new technology but larger fans have traditionally been a key to gains. The 81 inch fan on PW’s Geared Turbo Fan on the A320neo is pushing the engine size on the single-aisle airplane in traditional configuration, Epstein says,” we wrote at the time.
“’It looks like you can get one more iteration at…taxi weight limits and longer landing gear. You could go for four engines,’ Epstein says—an idea that is counter-intuitive to trends since the introduction of the Boeing 757 for high-performance, 3,500nm-4,000nm aircraft, intended to replace the three-engine, mid-range Boeing 727-200A,” LNC wrote.
Maintenance costs of large engines on aircraft the size of the Boeing 777-300ER and the forthcoming 777-8/9 are huge and work against the secondary market of these aircraft. The smaller Airbus A350 and A330 have lower maintenance costs but the MRO costs of the 787s, and particularly their composite, intricately-designed nacelles, are more expensive than early customers figured into their cost equations, according to Market Intelligence.
“The idea of a four-engine single-aisle aircraft raises immediate questions about fuel burn and maintenance costs of four vs two. While our own analysis, with respect to the Airbus A380, concludes the four-engine maintenance cost perception is overblown, Epstein points out that if an airplane needs 80,000 lbs of thrust (for example), this may be achieved with two 40k or four 20k engines. Maintenance costs for four engines can be achieved for the price of two, he says,” LNC wrote in April.
Based on LNC’s discussions with engine experts, there is no likelihood of a four-engine single-aisle airplane in the next 10-15 years. But looking beyond 2030 could be another matter. The choice between conventional jet engines and the prospect of Open Rotors is not dead, either, despite an apparent cooling of the latter idea in recent times.
While many readers overlooked the fine nuances of Fehrm’s analysis last week—notably the per-seat application of costs and the capital costs of used airplanes—the future is likely to bring some designs that defy today’s conventional wisdom.