Any re-engining programme is about obtaining the maximum fuel efficiency benefit at the least cost while accepting certain compromises.
For the 777X, Boeing would introduce an all-new engine and scale up the composite wing technology from the 787. But that is where the cost an risk was supposed to end. The compromise of re-engining the 777X meant accepting the cross-section and cabin experience of the original 777, bypassing the 787’s advances in cabin pressurisation and other technologies.
That formula has changed slightly with Boeing’s announcement at the Farnborough air show that the 777X cabin will “replicate” the maximum, 6,000ft pressure altitude of the 787 cabin, a significant cabin improvement from the 8,000ft maximum pressure altitude of the 777.
As a result, Boeing accepts slightly more risk and cost that it takes to strengthen the 777X pressure bulkheads and protect the airframe from being corroded faster by slightly more humid air.
“We’ve been getting input from customers all along that they wanted to replicate the 787 experience,” says Scott Fancher, senior vice-president and general manager of airplane development at Boeng Commercial Airplanes.
One of those potential customers was Air Lease founder Steven Udvar-Hazy. ALC has not yet announced orders for the 777X, but has an orderbook that includes 15 777-300ERs and 45 787s.
“On the pressurisation, they made a big deal out of that on the 787,” says Udvar-Hazy. “So how can you have an airplane that comes out after the 787 in terms of time and then the pressurisation is the same as the original 777 that came out in the mid-‘90s? So our position was, look, you’ve got to make some improvement on the pressurisation.”
Although the appeal seemed obvious, it took Boeing several months after the programme launch last November to be convinced that the 777X could replicate the 787 pressure altitude for an acceptable cost, Fancher says.
It was only two months ago that Boeing said the 777X would offer a 787-like cabin experience, but declined to comment specifically on whether it would offer the same pressurisation.
Boeing introduced the 6,000 pressure-altitude on the 787 as an upgrade enabled by the higher strength and corrosion resistance of the type’s pioneering composite fuselage. The metallic fuselage structure of the 777 seemed to present an obstacle to managing the same level of pressurisation, but Boeing found a way to provide the same strength with “local reinforcements” to structure, Fancher says.
Air Lease president John Plueger says the modifications were not quite so simple, however. Higher cabin-pressurisation risks lowering the fatigue life of the structure. But Boeing discovered that the original airframe was certificated with more take-off and landing cycles than necessary, so the 777X could enter service with a lower fatigue cycle rating with no impact on customers.
Udvar-Hazy adds that Boeing’s modifications will have the 777X fall short of truly replicating the 6,000ft pressure altitude of the 787, but still offer a significant advantage compared with the 777-300ERs.
Boeing’s Fancher, however, disputes that the 777X pressurisation will be any higher than 6,000ft.
“We’re replicating the 787 cabin altitude,” Fancher says, “and we’ll have to get with Steve and show him the latest data.”
The 777X cabin will also feature larger windows, similar to a 787 cabin that offers passengers a view of the horizon from any seat. There will still be some technical differences between the two aircraft, unless Boeing announces more changes before the type enters service in 2020. The 787, for example, introduced electro-chromic windows, replacing window shades with devices that can adjust the opacity of the transparencies.
“We’ll look at what customers tell us about the windows,” Fancher says. “We might make some design changes there, but today it’s mechanical shades.”
Source: Cirium Dashboard