That Airbus Military has to start a key A400M test campaign about one month late is disappointing. But of greater concern could be that problems with the aircraft's Europrop International (EPI) TP400D engine are again rearing their ugly head.

The uncertainty over the propulsion system comes as Airbus Military is in an intense development period for the airlifter and is trying to launch 300 hr. of so-called function and reliability (F&R) testing, in which the aircraft has to demonstrate to the European Aviation Safety Agency (EASA) certification authority that the airlifter is operationally ready. That trial period, using the development aircraft closest to the production standard (MSN6), was to have begun more than two weeks ago, but an inflight vibration problem is forcing the change of the No. 2 engine. This problem could delay the start of testing for another week or more.

Compounding the problem is that another development aircraft, MSN4, suffered an inflight engine shutdown recently. “The reliability of the engine is not as good as we would like,” says Ed Strongman, chief test pilot for Airbus Military.

But some industry officials say the situation may not be as severe as Airbus Military indicates. The vibration issue on MSN6 that led to removal of the powerplant was within established limits. Nevertheless, the build process is being tweaked to improve performance.

A backup engine, also in the production standard—a key aspect of F&R testing—is available to get F&R going, but it must be readied for installation before the test campaign can start.

What is not clear is exactly where the cause of the vibration problem lies; that has developers concerned. Under review are how the engine was built and how it was installed. Another concern is that if there was a problem with the TP400D during assembly, why was it not caught during “pass off” testing when the powerplant was put through its paces by EPI (the joint venture of Rolls-Royce, Snecma, MTU and ITP)?

To help mitigate impact on the schedule, Strongman says the test team has already revised plans and will undertake a more intense test regime earlier during F&R than is the norm. Flights out of the two A400M test centers, Toulouse and Seville, Spain, will initially be lengthened to make up some of the lost time. The full EASA certification—the restricted certificate was issued this month—will likely be slightly delayed. Critically for the program, however, Airbus Military still expects to maintain its contractual target of delivering the first production A400M to launch customer France no later than March 2013.

Progress toward those targets will depend, in part, on what engineers find in inspecting the other engine that recently caused headaches when MSN4 suffered a TP400D shutdown while inbound to Muscat, Oman. The engine, which was not new but far from the end of its useful life, suffered power fluctuation before shutting down. Europrop International is now trying to ascertain what the problem was.

A backup engine in Muscat will allow MSN4 to return to Europe. “We are focusing our efforts on identifying the root cause of the incident and ensuring that any subsequent changes minimize any disruption to the flight-test program,” EPI President Simon Henley says in a statement. “The purpose of a flight-test program is to identify potential issues and ensure that these are rectified prior to entry into service, and we are confident that we will achieve this,” he says, noting the engine has already received EASA type certification.

Recent events underline the fraught relationship between the aircraft and engine developers that has hung over the program for years, exacerbated by European governments' forcing of the engine choice on the aircraft program. The situation has brought the two sides to the verge of a legal battle.

Complicating matters is a very mixed picture for the TP400D. On the one hand, engine developers point out the powerplant is beating the specific fuel consumption target set for production-standard versions already with prototype engines. On the other hand, the development history of the TP400D reads like a litany of things that can go wrong. It includes gearbox failure, improperly documented software development for the full-authority digital engine control, startup problems, and numerous engines removed during flight trials.

For the A400M, the focus is on clearing up the engine situation and continuing other critical work, such as optimizing the flight controls for difficult operations such as air-to-air refueling. The A400M recently underwent refueling trials with an A330 tanker. Unprepared runway trials also are about to start.

The A400M flight-test program has logged more than 1,070 flights and 3,100 flight hours. MSN1, the first flight aircraft, leads the fleet with about 1,000 hr. and more than 300 flights.