If one of today’s market fashions becomes permanent, the Boeing 787 could be the last commercial widebody aircraft that offers buyers a choice of engines from competing suppliers – in this case the GE Aviation GEnx-1B or the Rolls-Royce Trent 1000.
This increasingly rare engine competition has delivered two propulsion systems with reliability levels well above the average at the aircraft level.
At the same time, it has so far failed to produce a turbofan engine designed by either competitor that meets Boeing’s original promise of a 10% reduction in specific fuel consumption.
Additionally, competitive pressures have not provided airline customers with immunity from brief operational crises with both engines, in one case an operational restriction that still continues.
Both engines boast despatch reliability levels above 99%, the benchmark Boeing is still seeking to claim for the aircraft as a whole.
“The engines are operating flawlessly,” says Zemene Nega, vice-president of maintenance, repair and overhaul for Ethiopian Airlines, a GEnx-1B customer.
It has not always been so. In July 2012, All Nippon Airways, a Trent 1000 customer, grounded five 787-8s after Boeing informed it of a potential problem in the gearbox. Crown gears had corroded faster than expected in endurance tests on the ground, causing damage to the engine. R-R traced the problem to a manufacturing process change by gearbox supplier Hamilton Sundstrand. It was corrected within weeks.
The GEnx-1B became the focus of the next engine crisis. A decision by GE Aviation to adopt a new lead-free coating on the fan mid-shaft backfired with explosive results. The coating caused the component to corrode faster in humid climates. In late July 2012, a GEnx-1B on board a newly assembled Air India 787-8 sustained a contained failure. GE reverted to a previous lead-based coating, and the problem disappeared.
A longer-term problem for GE Aviation is a relatively new phenomenon called ice-crystal icing. Liquid water is not present above about 22,000ft, so airframe icing is never a concern at cruise altitudes for a turbofan-powered widebody aircraft.
However, meteorologists have recently discovered the presence of ice crystals at even higher altitudes, especially in tropic latitudes. In massive storm concentrations stretching 100km (62mi) across, convection forces can carry ice crystals the size of a grain of flour to cruising altitudes above 30,000ft. The crystals bounce off an aircraft’s skin, but can be ingested into an engine. It is believed that crystals land on a warm blade and begin to melt, which attracts other crystals to stick to the blade. Eventually, enough ice develops on the blade to cause damage downstream when it sheds.
The phenomenon is particularly acute on the GEnx engine. On its predecessor, the CF6, the ice build-up would most often shed as the aircraft descended. The GEnx experiences the ice shedding problem at cruise altitude, leading to in-flight engine shutdowns. As a result, the US Federal Aviation Administration issued an airworthiness directive last year requiring airlines to steer 787s at least 50mi wide of major storm concentrations.
For some airlines, the restriction is an annoyance but not a network issue. Japan Air Lines, however, has pulled the 787 off three routes originating in Tokyo: Bangkok, Delhi and Singapore.
By contrast, the Trent 1000 engine faces no such operational restriction, says R-R project director Gary Moore. Fortuitously, the three-spool architecture of the Trent engine family happens to be less prone to ice crystal build-up inside the core. The intermediate compressor section, which is absent in the GEnx design, rotates at a higher speed, making it more difficult for dangerous quantities of ice to build up on the blades.
“We don’t have this problem,” Moore says. “It is just a very clear difference in the two engines.”
Another clear difference between the engines is the order split. So far, 787 customers have chosen the GEnx-1B over the Trent 1000 by a nearly two-to-one margin, with 17% of the order backlog still unspecified.
R-R places a couple of caveats on the GEnx-1B’s strong start. First, not all airline decisions have been the result of a competition. When given the chance to compete, the Trent 1000 has claimed nearly half of the orders, Moore says. Moreover, the Trent 1000 is starting to gain some momentum. In the last 19 engine selections, the Trent 1000 has won orders 11 times, he says.
R-R’s next move is to deliver the Trent 1000-TEN upgrade in mid-2016. GE has acknowledged that the GEnx-1B misses, by 1-2%, Boeing’s original specification for reducing specific fuel consumption. The Trent 1000-TEN – packed with technological improvements inherited from the Trent XWB – is still aimed at achieving the 787’s original fuel-burn target.
“We’re targeting the original spec that was put upon the airplane,” Moore says. “You don’t spend this level of investment to think we’re not going to get there. We’re going to get there.”
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Source: Cirium Dashboard