The last-minute problem with the gearbox of the TP400D-6 turboprop engine, which led to the decision to cancel the spectacular Airbus A400M flying display at the Paris Air Show, has still not been resolved, Europrop International (EPI) said.

The malfunction occurred on Thursday last week on an A400M flight-test aircraft, though not the one that flew into Le Bourget to be on display.

“We’re continuing to analyze what happened,” explained EPI president Simon Henley. “We still don’t know the root cause or even the sequence of events that took place.” The A400 flight test program is unaffected, however. “It’s not a safety issue,” he said. “This is a benign failure.”

The engine was shut down automatically during a test flight after sensors detected the engine and propeller had become “decoupled.” Manufacturer Avio is investigating the gearbox, but the engine’s development program continues, following the TP400’s civil certification on May 6.

“We’re extremely pleased with the performance of the TP400 to date and are hitting the milestones regularly,” said Henley. More than 13,000 hours of running time have been accumulated, 8,600 of which were on-wing and 6,400 hours in flight.

So far EPI has delivered 24 development engines, with a further four due for delivery this year. The first production engine will be assembled in the third quarter of 2011, for delivery in early 2012. Under the new contract with Airbus Military, production will ramp-up slower than originally planned, with “less than 20 engines” to be produced in 2012, said Henley.

Flight testing of the TP400 is 95 percent complete and has taken the 11,000-shp turboshaft through high-power, in-flight shutdowns and engine-out takeoffs. The flight envelope has been extended to 42,000-foot altitude (2,000 feet higher than the A400M specification) and has included high angle-of-attack and sideslip performance checks. The A400M has flown at a maximum speed of Mach 0.72.

“Performance is better than expected,” said Henley. “And we’re about one percent better on fuel burn.”

Delivery of the first A400M is due to the French air force in late 2012 or early 2013. In April agreement was reached on production of 170 aircraft for Belgium, France, Germany, Luxembourg, Spain, Turkey and the A400’s only export customer, Malaysia.