With fuel burn a critical performance discriminator for Airbus in its challenge to Boeing with the A350, the European manufacturer has some encouraging signs from engine supplier Rolls-Royce as it heads toward flight tests later this year.

Rolls says initial results from testing of the Trent XWB-84s delivered to Toulouse for installation on the first A350-900 show fuel-burn performance meets specification and is slightly improved over that seen on earlier flight-test units. Rolls Trent XWB program director Chris Young says, “We are well on track to meet our targets for entry into service. So it is looking good on fuel consumption.”

Rolls declines to give specific numbers, but Airbus projects the largest variant of the family, the A350-1000, will have up to a 600-nm-range improvement and 25% fuel-burn-per-passenger advantage over the current Boeing 777-300ER, and up to an 8% operating cost advantage for the other versions over the initial Boeing 787. The bulk of this advantage is expected to come from what Rolls describes as the “most efficient engine flying in the world today.”

However, despite encouraging results from ground runs and flight tests on the Airbus A380 flying testbed, the true acid test is approaching with the start of flights of MSN1, the first A350-900, around mid-year. Rolls delivered the first shipset of flight-compliant XWB-84s in December and has completed assembly of a further pair. In addition, Young says three more XWB-84s are in build and parts are “kitted” for an additional four. The baseline engine received certification from the European Aviation Safety Agency (EASA) on Feb. 7. Certification covers the XWB-75 and -79 variants for the A350-800 and -84 for the A350-900, effectively encompassing the 75,000-84,000-lb.-thrust range required for both versions.

“The focus now is much more on the test program so we are ready for entry into service in the second half of 2014,” adds Young. From an engine perspective, this means a shift from the test and certification effort that has so far involved 11 engines and 3,100 test hours, to amassing hours and cycles on endurance units and flight-test-compliance engines. “You'll see that start to accelerate,” says Young.

While build-up of the first XWB-84s takes place at United Technologies Aerospace Systems' facility in Toulouse, Rolls is preparing to begin a prototype version of the Trent XWB-97 for the Airbus A350-1000, the most powerful production turbofan it has developed. The engine will help prove out several of the design features of the higher-rated variant, the first development unit of which is due to make its initial run in mid-2014. The Trent XWB-97 is on track to power the first A350-1000 in mid-2016 and enter service a year later.

Rolls completed the preliminary design review (PDR) for the 97,000-lb.-thrust Trent XWB-97 in January. “This is a key milestone for us as it takes us into detailed design and the making of individual components of engines, the pouring of castings and the creating of forgings. It also enables us to get ready to begin machining,” says Young. Although the XWB-97 is sized for considerably greater thrust, Young says a success from the PDR was confirmation that 80% of the line-replaceable units will be common among the engine variants. The only exceptions will be fuel pumps and fuel-metering units, which will need larger capacity for the 97,000-lb.-thrust version.

Development of the XWB-97 follows the 2011 decision by Rolls and Airbus to increase the A350-1000 thrust requirement from 93,000 lb. To maximize commonality with the baseline XWB and avoid changing the outer mold line with an expensive increase in fan diameter, Rolls sought to meet the higher power need by scaling up the core and boosting the flow. The fan will spin 5% faster, and the annulus at the base of the fan blades has also been redesigned with an inflection to boost the flow capacity around the spinner.

“We will get more power out of the core by making it bigger and improve the turbine's capabilities to run the engine hotter,” says Young, who adds the focus is on putting new technology into the turbine. Changes for the XWB-97 include the use of next-generation CMS-X4 single-crystal materials and anti-oxidation coatings in the high-pressure turbine which, for the first time on a Trent, will be shroudless.

The turbine disc will also be forged from a dual microstructure disc that will provide greater stress capability toward the center of the hub while at the same time exhibiting better creep resistance toward the tips. The blades will also have an advanced tip-cooling treatment. Other improvements designed to help the XWB-97 maintain the same relative fuel-burn improvement as the basic XWBs, despite the thrust jump, include a more sophisticated adaptive bleed system that is designed to turn off cooling air bleed during cruise when not required.

The intermediate (IP) compressor features a “rising line” or inner annulus line that increases in radius, thereby increasing the tip speed of the aft stages. The high-pressure compressor is derived from the European New Aero Engine Core Concept program and is connected to the IP by a swan-neck duct. The engine will also include an impingement effusion combustor design that uses a combination of metallic tiles and film cooling to reduce the cooling air required, thereby boosting efficiency.

“We will be testing one of the 84,000-lb.-thrust (XWB-84) development engines and putting a selection of '97K' new turbine technology into the engine so we can run it as close as possible to XWB conditions,” says Young. The prototype XWB-97 engine is expected to start first runs around the middle of the year, he adds.