IN FOCUS: Rolls-Royce readies Trent XWB for A350's first flight

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The recent grounding of the latest widebody twinjet to enter airline service - Boeing's 787 - means the Airbus A350 XWB will come under unprecedented public scrutiny when customer deliveries get under way in 2014.

Although the 787's current woes are not powerplant-related, Rolls-Royce is well aware that as sole engine supplier to the A350 it has a critical role in ensuring the European-built widebody enjoys a relatively trouble-free commercial debut.

Rolls-Royce is understandably endeavouring to leave no stone unturned as it seeks to demonstrate the maturity and service-ready credentials of the Trent XWB, its most advanced three-spool large turbofan, which has notched up more than 1,200 sales before the A350 even gets airborne.

An illustration of this determination is the fact that although all flight-test work required for certification of the Trent XWB has been completed, Airbus and Rolls-Royce have together decided to extend the campaign using the airframer's A380 flying testbed ahead of the A350's maiden sortie.

"We both consider that it's the right thing to do for powerplant maturity and we're both very supportive of making sure the powerplant is as mature as it possibly can be so as to not give [the A350] any form of problem at entry into service," says Trent XWB programme director Chris Young. "So we're prepared to carry on investing in the product between us," he adds.

The extra flying using the A380 - which first got airborne with the Trent XWB installed in February 2012 - facilitated additional systems tests and provided an opportunity to take the powerplant through its paces under extremely cold conditions.

The cold weather flying tests were performed in Iqaluit in Nunavut, Canada, where temperatures were -23˚C (-9˚F).

"This is all about proving the long-term service maturity of the engine and just carrying on getting experience in representative environments," says Young.

The engine installed on the A380 is the same one used for the most recent phase of certification testing, and is "very close" to the final configuration that will power the A350 on its first flight later this year.

"There are a few minor changes as always around things like pipe routings and some of the last minute external changes that we've found but overall it's very representative of the bill of material, which is why it makes sense to carry on flying and getting the evidence and data from it," says Young.

TRENT XWB FOR A350-1000 TAKES SHAPE

While the baseline A350-900 is powered by the 84,000lb (374kN)-thrust Trent XWB-84, the -800 shrink will be equipped with the de-rated XWB-75, or XWB-79 for hot-and-high operations.

The stretched A350-1000, however, requires the 97,000lb Trent XWB-97 growth variant to preserve the type's transpacific range capability for customers such as Cathay Pacific.

The XWB-97 will retain the same fan diameter, mounting points and nacelle aerolines as its less powerful siblings, and derive much of its extra thrust via increased fan flow. A larger core will be required to power the fan, and this will feature turbine blade tip clearance control, upgraded materials and advanced cooling technology. The result, Rolls-Royce hopes, is the extra thrust capability can be delivered with no impact on specific fuel consumption or on-wing life.

With development work for the baseline XWB-84 essentially complete, Rolls-Royce is ramping up activity on the XWB-97, which has entered the component-level design phase ahead of the start of assembly of the first test engines. The preliminary design review milestone was passed in early January.

"Now it's all-systems-go, to do the very detailed individual component design and manufacture and to start pouring the castings and cutting the metal as we go through this year in order to get the first parts in store for the first prototype engine and go towards that first engine run around the middle of next year," says Trent XWB programme director Chris Young.

TESTBED TALKS

The first A350-1000 is scheduled to fly in mid-2016, a year ahead of entry into service.

"Both ourselves and Airbus think that for the overall maturity of the product, doing a flying testbed [for the XWB-97] is a beneficial thing to do," says Young. "We are in discussion with Airbus about exactly what the approach is that we take for the 97k engine."

Flight tests of the XWB-97 engine should "most likely" start in the second half of 2015, he says. The first engine run is scheduled for mid-2014.

Options include redeploying the A380 as the testbed, or fitting a Trent XWB-97 to an ­A350-900 test aircraft. Using a twin-engined A350, however, would involve more stringent regulatory requirements and the engine would have to be "more mature" before flying could begin, says Young. A decision between the A380 and A350 is expected "towards the middle of this year".

Some key advances being introduced in the XWB-97 are in the turbine and combustion sections, and include shroudless turbine blades, tip-clearance systems and advanced materials and coatings. ­Rolls-Royce claims to have achieved 80% commonality with the baseline XWB-97 engine in terms of line-replaceable units.

The third build standard for Rolls-Royce's Environmentally Friendly Engine programme is being installed on the testbed in Bristol, UK and will demonstrate technologies aimed at extending turbine life. There will also be a cyclic endurance test.

"Apart from just proving it in a representative environment as we did with builds one and two, it's now about starting to prove the full-life capability as an advanced de-risk of the 97k engine," says Young.

Certification of the Trent XWB was awarded by the European Aviation Safety Agency on 7 February following successful completion of the critical full engine blade-off test, conducted using 58 Bed at Rolls-Royce's factory in Derby, UK. This was preceded by a blade-off test using only a fan module at the company's Dahlewitz site in eastern Germany on 2 November, which provided the "data and confidence" to move to the full engine test in the UK on 29 November.

"It's lots of millions of dollars to do [the full engine test], and we like to make sure that we're completely confident in the successful outcome of that test before we go into it," says Young, adding that the fan module test is conducted to identify any "last-minute design tweaks" that may be required.

Conducting the full engine blade-off test inside 58 Bed threw up a host of technical challenges, as the Trent XWB has the biggest fan built by Rolls-Royce, as well as the biggest individual blades. The high energy levels involved meant the company's engineers had to be sure 58 Bed - the newest and most modern test facility on the Derby campus - was structurally capable of hosting the demonstration.

"That was the first time we've done a big fan indoors - it had always been an outdoor test before," says Young. The move inside was necessitated by the UK company's decision to decommission its outdoor test facilities at Hucknall in Nottinghamshire, UK, which had been encroached by housing estates in recent years.

"It means that we can do everything that we want to 24 hours a day and be more effective in our operations rather than just do it a few hours a day when we're not annoying the neighbours too much," says Young.

ABSOLUTE MAXIM

In the event all went well as the blade was released at the root and the engine ran down, and was then shut down, in the presence of representatives from Airbus and EASA.

The blade was released at the "absolute maximum, red-line N1 speed, and then we actually add a little bit of margin to make sure that we're comfortable that we have some growth capability on the engine if we ever need it in the future", says Young. "It's a very arduous test that really proves the robustness and overall capability of the engine. We effectively do it at a higher speed than would ever occur in a service environment. The good news is it went very well and delivered all of the evidence that we needed for certification," he adds.

The blade-off test involved the baseline 84,000lb-thrust Trent XWB-84 for the ­A350-900. The more powerful, 97,000lb-rated XWB-97 in development for the stretched A350-1000 will require another test as it features stronger, heavier fan blades which rotate at a higher speed.

"At the minimum, we'll do a full-fan rig test," says Young.

Among the final certification tests performed for the XWB-84 was a second 150h endurance test to clear modifications designed to provide higher turbine temperature margins to extend service life. "The low-pressure turbine rotor dynamics was one last test that we had to do, again successfully completed," says Young.

The first engine (SN21002) for the A350 flight-test campaign has been delivered to the assembly line in Toulouse for podding, following pass-off testing and the installation of flight-test instrumentation in Derby.

The second engine (SN21003) has also completed pass-off testing and was due to be shipped to Toulouse imminently. Follow-on engines will arrive in Toulouse "at a reasonably fast rate", says Young. Engine four entered pass-off testing in January.

"We're starting to see a drumbeat of flight-compliant engines coming very quickly through the process and fully supporting the needs of the Airbus programme," says Young. "We're really starting to get our industrial system proven out very well. The supply chain is operating well, the assembly processes, and the new production facility that we've put in place is proving that it's building the engines to the right quality and repeatable.

"The engines we are passing off are flight compliant and even better on performance, as we expected them to be," he adds.

A total of 11 Trent XWBs have participated in the test programme to date, accumulating more than 3,100h in ground tests and aboard the A380 testbed.

Still under way are some final tests needed to secure FAA cross-certification, and then the focus will shift to securing early extended-range twin-engined operations approval. The target is to eventually certificate the A350 to fly up to 350min from the nearest suitable diversion airfield at single-engined flying speed.

The cold start capability of the Trent XWB has been extended down to -26˚C after winter demonstrations using the company's testbed in Manitoba, northern Canada, and this is expected to be further lowered to -40˚C.

"We are now able to do far more full envelope testing, which is great for the reliability and maturity of the product," says Young.

Between 16 and 18 engines will have been assembled in the pre-production facility at Derby prior to the start of series production, in an effort to understand the optimum way of assembling the engine and to determine appropriate work-station content.

"Assembly hours are coming down nicely, as we learn how to build the engine," says Young. Fully-fledged flow-line assembly should begin by mid-2014, in time for the planned ramp-up in A350 production.

"We're very close to finalising our footprint and layout for the full flow-line facility," says Young.

A couple of spare engines are being shipped to Toulouse to support the A350 flight-test programme in case an installed engine is damaged, for example by foreign object ingestion. Rolls-Royce engineers will fly on many A350 test flights to monitor engine performance.

"The engines are ready and capable to go to their limits from day one, should Airbus choose to do so," says Young. "The programme - which we're fully supporting with our initial flight-compliant engines - has a very clear aim to make sure that first flight is prior to the Paris air show."