The success of Rolls-Royce’s flight test program for the Boeing 787’s Trent 1000 engine has led it to consider retaining the Boeing 747 it acquired for the tests and using it as a flying test bed for future programs. These programs include the new RB212 turbofan Rolls-Royce is developing for the Dassault Falcon super mid-size business jet.

The last time Rolls-Royce operated its own flying test bed was in the early 1970s, when it used a VC-10 for RB211 trials. Now Boeing and other customers are trying to push risk down the supply chain, corporate flight test specialist Andy Roberts told a recent meeting of the Royal Aeronautical Society. The objective of the latest program, accordingly, was “to demonstrate the new engine’s stability, operability and maturity, and to ensure a safe, efficient and unrestricted start to the Boeing 787 flight test campaign.”

The test airplane, a B747-200 operated previously by Cathay Pacific and Air Atlanta, was modified and crewed by L-3 Communications Integrated Systems at Waco, Texas. Project development engineer Steve Bowen, who was engineering lead on the flight test program, said modifications required to integrate the engine with the aircraft included a new throttle, an air data computer with digital output and data bus interfaces.

The airplane also had to be equipped to dissipate the 500 kW of electrical energy generated by the engine; on the 787 it will be used instead of bleed air to power aircraft systems. Operated under an experimental category special airworthiness certificate, it was instrumented to measure 800 parameters.

The first phase of testing started in June 2007 and involved 109 flight hours in 35 flights. Some of the most dynamic tests were to certify compatibility of airframe and nacelle and confirm that the intake aerodynamics and distortion levels did not cause unacceptable fan blade vibration or stress build-up. They involved power-on and power-off stalls, wind-up turns to the aircraft’s g limits, steady heading sideslips to a maximum of approximately 18-degree sideslip and simulated high rotation takeoff maneuvers.

A key objective, Roberts said, was to confirm that the results of experiments–conducted in the altitude test facility at the U.S. Air Force Arnold Engineering Development Center (AEDC), Arnold Air Force Base, Tennessee–read across to the installed aircraft and flight. “We also look at quick windmill relights which consider the ability of the engine to recover from being briefly switched off and on again, which has happened on some fairly large twins in flight with chop times of between five and 30 seconds,” he said.

Another of the more dynamic tests, Roberts said, is the fuel suction feed assessment, “an aspect that’s getting a reasonable amount of attention at the moment as a result of various incidents around the world.” These tests are conducted with Jet C-1 fuel, which had to be specially made for the assessment and is meant to be representative of the most volatile fuel that has been cleared in service.

After a normal takeoff, he explained, the aircraft boost pumps are switched off and with max continuous power selected the aircraft climbs at approximately 250 knots, achieving a rate of about 4,000 fpm. Roberts avoided quoting specific altitudes, but said the climb continues “until approximately 30,000 feet, where the engine begins to struggle and after some fairly large thrust oscillations it eventually surges and shuts down.”

The crew then initiates a descent at 250 knots and 1,500 fpm, still with the boost pumps off, until the engine self-recovers with a windmill relight. “And when it stabilizes at idle we conduct a slam acceleration back to max continuous to prove satisfactory acceleration characteristics,” explained Roberts.

The relight is achieved at around 20,000 feet, he said. The boost pumps are then switched back on and the aircraft is flown to an altitude above where the original instability was observed– “somewhere in the mid-30,000s of feet. Then, with the engine stabilized again at max continuous, we turn all the boost pumps off and the engine actually flames out at this point. We stay in that condition with the engine off, cold-soaking it for approximately 15 minutes at 250 knots, and then switch the boost pumps on and attempt to relight again to prove that the self-priming capability of the fuel system is satisfactory.”

The engine achieved airworthiness certification in August 2007, just 18 months after the first run. But the flight tests continued to rack up outstanding performance and operability work with the latest full authority engine control (FADEC) design. The FADEC system is subcontracted to Goodrich. “Some of the innovative technology in the box is designed and developed in-house at Rolls-Royce, but a lot of the software is outsourced,” said Roberts.

The second phase of 10 flights and 50 flight hours using engine No. 12 ended in March. It included the final confirmation from Boeing that it is happy with the engine for the first flight. “This takes the form of a fairly wide ranging pilot assessment flown using hardware and software pretty representative of those that will be used for the first 787 flight,” said Roberts.

Lessons learned from the flight test program included an education in the additional skills that are required to run a flying test bed program compared with supporting a customer. “It also reinforced the benefits of proper preparation and test planning and the need for removing any ambiguity in test requirements from specialists and customers at an early stage,” concluded Roberts.

Bowen said the B747 would not be used for the BR725 that Rolls-Royce Deutschland is developing for the Gulfstream’s new flagship G650. The first engine is due for delivery to Gulfstream before the end of the year, with the first flight of a G650 following in the second half of 2009 ahead of service entry in early 2012. He added that he hopes Airbus will use its own A380 for the Trent XWB flight tests.

Rolls-Royce is also sending a service leader engine to launch customer ANA in Japan. “They will run the engine for 40,000 cycles so that they are happy and content that they have the fleet leader engine and will see any issues on the ground before they see it with their customers,” Bowen added.