Boeing hasn’t yet worked out how to change between alternative 787 engines in the space of 24 hours. Designing an engine mount that enables engineering crews to swap alternative General Electric GEnx and Rolls-Royce Trent 1000 powerplants in such a short time has proved “challenging,” the U.S. manufacturer concedes.
“That’s why we set these kinds of targets–to challenge ourselves and our partners to be innovating in providing ‘solutions’ to our customers,” Boeing 787 propulsion director Ron Hinderberger told Aviation International News. “We haven’t yet hit it 100 percent, but are making progress,” he said.
Asked about the major consideration in developing interchangeability, Hinderberger claimed, “The only major challenge is making the decision early enough in the program to be able to implement it. Minor challenges pop up in ensuring that we are meeting this [interchangeability] goal, but nothing has gotten in the way or caused us to rethink.”
But given Boeing’s declared mission to build the aircraft that the market requires, are carriers seeking interchangeability or is the concept driven by other interested parties, such as leasing companies, finance houses or banks? “Banks are now customers,” pointed out Hinderberger. “They have been for a long time, but the 787 is the first airplane we’ve designed with this in mind.”
However, financiers had in fact told Boeing that they would prefer the manufacturer to pick one engine supplier, since they liked a standard asset. “Airlines wanted the flexibility of more than one engine provider. By creating an interchangeable design, we satisfied both,” said Hinderberger. He refuted the idea that lessors, who often acquire a selection of airliner types with alternative engines to satisfy a variety of airline preferences, had driven the requirement in an effort to reduce the numbers of models they held.
Instead, Hinderberger sees the greatest benefit of engine interchangeability as being to “most airlines,” although he acknowledged that a typical carrier “won’t see the benefit for some time.” He said the ability “to transition an asset more efficiently will be a benefit to all carriers, as will the financing benefits offered by creating a more flexible product.”
So what contribution will engine interchangeability make to the overall “value proposition,” since the facility does not contribute to direct operating costs? “A value proposition has to consider the full life cycle of an airplane,” according to Hinderberger. “It is not uncommon for an airplane to serve three or four carriers in its lifetime. Leased airplanes have even more operators; being able to easily transition into a new fleet is important.”
Boeing has confirmed that interchangeability principally influences an airliner’s residual value by contributing to its qualifying for a higher loan-to-value ratio when the owner wants to borrow money. In other words, as with, say, real estate, financiers would be happy to provide a higher level of collateral against the asset because of its perceived higher market appeal in the event of payment default on the loan.
Hinderberger believes that interchangeability needs to be a speedy process, even though leased aircraft might go in for maintenance between lease or sale contracts for a period longer than that needed to make the powerplant switch. “Anything that can be done to minimize the transaction complexity or time provides value,” he said.
And that value is perceived to be greater because the change also can be made more than once. “Any number of times,” said Hinderberger. “You can see how this adds flexibility for the product.”
The Boeing executive claimed that an absolute example of interchangeability in practice had been the sale of the original Boeing 777 airplane, which had been built with Pratt & Whitney engines but (following completion of flight-test and certification work) was sold with Rolls-Royce units. Hinderberger said that exercise provided lessons that helped Boeing in its approach to providing interchangeability on the 787, remarking, “It helped to illustrate the importance of the decision, but the design effort is pretty straightforward.”
This is not to say that 787 engines are hung differently from other powerplants. “The physical attachment is no different,” explained the Boeing official. “It is just a matter of deciding what the attachment interface is like, and driving it into the design for both engines.”
Typically, the attachment interface between engine and airframe is designed as part of the individual engine package, with the main considerations driving attachment design being systems, weight, performance and the design of the pylon or strut. In the case of the 787 attachment, the difference is that attachment commonality will be at the pylon/wing mounting rather than at the pylon/engine interface.
Hinderberger said the structural design of each engine is sufficiently different from the other to require a unique pylon mounting, thus preventing the desired commonality at that point. “Driving the engines to a common pylon/engine interface would have compromised performance adversely,” he maintained.
Boeing has also confirmed that the 787’s “more-electric” systems architecture has had no effect on interchangeability. But the engine mounting did enter into the equation when Boeing set the schedule for 787 development. “The systems architecture had to be defined to a certain degree earlier than it might have been [otherwise], but in terms of being interchangeable, it is largely a software issue,” said Hinderberger, who is not aware of any previous aircraft being designed from the outset for retrofit with alternative engines.
He said interchangeability neither requires nor prevents use of, for example, standard nacelles, fan sizes or gearbox locations. While these are different between the two engine designs, this is for other reasons. Thrust reversers, designed by Goodrich for both engines, also are slightly different from each other.
Although the Rolls-Royce Trent is the lead powerplant, Hinderberger explained that Boeing has been developing interchangeability for the alternative engines in parallel. “The lag time between the two [designs] entering the market is not great,” he said.
The Boeing official concluded that resolving the engine-interchangeability issue has been easier than some other elements of 787 development. “There have been far more difficult aspects of the program,” he remarked. “It was a great engineering challenge–and great engineers love that. This was the right solution for the 787. It wasn’t that difficult to achieve and there is real value for our customers.”