A400M Engine Flying Test Bed/
13 August 2014
Marshall Aerospace and Defence Group de-risked Airbus’ A400M airlifter programme by designing and developing a Flying Test Bed for the TP400-D6 engine.
Developing the Engine
Airbus’ most recent venture into the military transport market emerged as the A400M Atlas – a high-wing, four-engined, large capacity airlifter. During the development phase, an increasing emphasis on the aircraft’s tactical role led to the decision to use turboprop engines, rather than jet or turbofan as initially planned.
With no suitable turboprop available “off the shelf”, Europrop International developed a new 11,000 shp engine: the eightbladed TP400-D6. Needing the aircraft to be certified to civil standards and following the advice of Rolls-Royce, Airbus decided to reduce the risk of the programme by testing the TP400-D6 on an engine Flying Test Bed (FTB), and contracted Marshall to develop and operate the FTB.
Modifying the Airframe
Marshall already had experience of developing an FTB, having previously converted a C-130K Hercules to test the upgraded Allison/Dowty-propeller C-130J engine in the No2 (port inboard) position. With over 40 years’ experience on this airframe, Marshall decided to once again use a C-130 as the base for the FTB and selected XV208, a meteorological research aircraft originally converted by Marshall in 1973.
Testing the TP400-D6 would be a similar undertaking as for the C-130J engine, but on a much larger scale. Systems integration was a challenge due to the fact that the C-130 was an analogue airframe and the TP400-D6 a digitally-controlled engine. An Avionics Full-Duplex Switched Ethernet (AFDX) converter was produced to allow the engine to “talk” to the airframe. During the course of the modification, Marshall also needed to integrate the engine with a number of the aircraft systems, including fire detection, fire extinguishers, hydraulics, bleed air and engine instrument displays.
Considerable structural work was required to accommodate for the size, weight and power output of the test engine. In addition to the structural modifications, two Lynx hydraulic dampers were adapted to join the engine mountings to the upper and lower fuselage, to reduce the severe load imposed on them by the eight Ratier-Figeac blades rotating at 655-842 rpm.
The test propeller would also rotate in the opposite direction to XV208’s own engines, introducing possible difficulties with the airflow around the elevator and rudder of the aircraft. Little wind tunnel data was available to indicate what these would be, and any problems would most likely only manifest for the first time during fast taxiing.
Understanding the Risks
In order to remove a great deal of the “unknown” from the programme, Marshall built a simulator with a fully-representative flight deck. This retained the analogue flight instruments, but basic test engine parameters and power from all engines were shown on a purpose-built digital display mounted above the glare shield, to allow immediate comparison of the test engine output.
This proved invaluable in establishing the correct throttle settings to balance the engines’ power for take-off, as the test engine produced more thrust at flight idle than the standard engines at full power. Tests with the real aircraft proved close to expectations.
The simulator allowed the flight test crew to establish protocols and emergency procedures and thoroughly investigate the ramifications of engine failure immediately before or after take off. They could familiarise themselves with the atypical control setup: the test engine was controlled by an extended power lever to the left of the throttle quadrant, so that P1 in the left seat controlled the test engine and yaw, and P2 controlled the standard throttles, pitch and roll, allowing P1 to concentrate on the test engine behaviour.
Marshall also designed and built a “mock-up” engine, in order to identify and resolve any problems that might arise during the installation of the test engine. This allowed refinement of the cowling design so once the real test engine arrived, the installation encountered only very minor issues.
Performing the Tests
Airbus had specified over 700 parameters that would need to be measured during the testing. Marshall had added 200 more, including strain gauging of the No1 propeller for comparison with the adjacent test unit. Altogether during testing, some 900 data channels were constantly being monitored, using consoles designed and integrated by Marshall. During the installation of the test recording and data retrieval equipment, over 30km of wiring were changed or added.
The TP400-D6 ran for the first time on the FTB in June 2008, and the design appeared from the outset to be sound. Only the vibration levels caused concern, especially in the cabin floor, testing the crew to the limit. The fuselage was carefully inspected for any signs of fatigue cracking.
The 18 taxiing tests were completed by 10th December, and at 10.30am on 17th December, XV208 took to the skies again for a flight lasting 75 minutes, reaching 8000ft. The hours practising coordination of throttle movements in the simulator proved their worth as both take off and landing went smoothly. During the test flights, the TP400-D6 was advanced to take-off power, the maximum thrust the engine would produce, and shut down and restarted using an airframe bleed supply. 18 flights were conducted, reassuring Airbus that the behaviour of the FTB in most circumstances was well understood.
Throughout the whole FTB project, the team at Marshall adopted a flexible and open-minded approach, taking problems in their stride and keeping pace with changes to the maturing A400M design. Thanks to the hard work and dedication of the Marshall team, the FTB achieved precisely what it set out to do: improve the understanding of the TP400-D6 and as a result, greatly de-risk the A400M programme.