TOULOUSE — The Airbus Military A400M has cleared an important certification milestone even as developers are exploring vibration believed to be linked to the transport’s engine.

The European Aviation Safety Agency issued the restricted type certificate for the military airlifter. Airbus is pursuing both civil and military certification of the four-engine turboprop, which is due to be delivered to its launch operator, the French air force, next year.

Full-type certification is due by midyear, Airbus Military says, and still requires completion of 300 hr. of so-called function and reliability flight tests. The next milestone is for Initial Operating Clearance before year’s end.

Even as flight hours increase, Airbus Military has signaled continued frustration with problems on the TP400D turboprop, noting that the five-aircraft fleet has now logged more than 3,100 hr. “despite continued engine challenges.”

The exact nature of the problem, or problems, is not fully understood, with root-cause analysis still under way. However, the vibration experienced has led to at least one engine shutdown.

Airbus officials have previously expressed frustration over development issues with the Europrop International TP400D involving a range of hardware and software issues.