June 17, 2013
Credit: Jens Flottau
The last few attempts to display the Airbus Military A400M in Paris or at Farnborough have been thwarted by engine problems with the Europrop International (EPI) TP400 turboprop. But with just a few weeks to go before the official handover of the first example to the French air force, officials from Airbus Military and EPI are looking forward to finally being able to put the aircraft through its paces in front of a Paris crowd.
The TP400 is the most powerful Western turboprop engine yet developed. Assembled by MTU in Munich, the engine is a product of the combined efforts of ITP, MTU Aero Engines, Rolls-Royce and Snecma, with each company being responsible for major modules of the engine. When the engine goes into heavy maintenance, the individual modules are returned to their manufacturers for servicing. The development of such a complex engine has proved a major challenge and the program has faced several headaches along the way both before and after flight trials began.
Airbus Military successfully displayed the aircraft in 2010 at the Berlin ILA air show in June and then at Farnborough in July, but at Paris in 2011, one of the trials aircraft suffered a gearbox problem which relegated the aircraft to the static display.
Within weeks, EPI analysis found that fatigue cracks in the gear-tooth fillet radius were caused by resonance in the idler gear at cruise propeller speed. As a result, the idler gear was redesigned to shift the frequency at which resonance occurs, reducing fatigue levels. Engineers also developed an algorithm to help identify early signs of failure to avoid an inflight shutdown. The problem was fortunately not big enough to ground the fleet. The problem was a bearing in pre-production gearboxes and not an issue that would have affected production engines, and was resolved by changing the way the aircraft fitted with pre-production gearboxes were flown.
Then, just in time for the 2012 Farnborough air show, a fault occurred with a cover plate in the gearbox. This plate had cracked and threw off metallic chips into the oil system. The result was a re-design of the plate, which required a retrofit program of all the engines then built, an issue further complicated as it required disassembly of the gearbox.
This retrofit program forced a ramp up in engine production at MTU, pushing the line up to maximum long before it was deemed necessary, as it is dealing with both “new build” and engines for retrofit. The cover plate issue occurred halfway through the A400M’s critical EASA 300-hr. Function and Reliability (F&R) testing regime forcing the company to re-start the process. The aircraft finally completed the F&R trials in early December 2012.
The first three A400Ms will have to have their engines changed after 1,500 hours/two years because of temperature limitations experienced in the low-pressure turbine backend casing made from steel.
Work to replace the casing material has just gone through the preliminary design review and a critical review was due to take place this month. Initial work will aim to extend the time of replacement to four years, roughly around the time the first aircraft will have to enter their first major service, around 3,000 hours or four years. The issue was discovered when the aircraft entered flight trials; it has not experienced such high temperatures during ground trials.
TP400s have now completed more than 27,000 running hours, of which around 17,000 are flight hours.