Italian investigators believe that flight control laws governing the AgustaWestland AW609 were partly responsible for the fatal crash of the programme’s second flight-test prototype (N609AG) after they initiated an “augmented Dutch roll”.

The developmental tiltrotor came down in the north of Italy on 30 October 2015 during high-speed testing; both pilots on board were killed in the crash.

In an interim report, investigation agency ANSV says analysis of data retrieved from the AW609’s combined flight-data and cockpit voice recorder indicates that shortly before the accident the pilot in command had detected the “onset of oscillations on the roll axis of the aircraft”.

Although he attempted to counteract the motion by manoeuvring on the roll axis, the tiltrotor’s flight control laws caused it to behave unexpectedly “generating a control on the yaw axis” to compensate for expected aerodynamic effects.

This, says the report, formed “a phenomenon described during the investigation as ‘like an augmented Dutch roll.’”

Analysis of data from previous test flights indicated that both prototypes had experienced the oscillations before, although to a lesser degree.

However, this was the first flight with a new aft fuselage configuration and modified tail-fin, during which the AW609 was being put into high-speed dives of 293kt (542km/h) for certification purposes; earlier flights had only achieved 285kt.

“During the accident flight two dives to maximum dive speed of 293kt were executed; during the execution of the third one, the accident happened,” says the report.

ANSV says the tiltrotor’s aerodynamic behaviour at high speed was not accurately predicted by the manufacturer Leonardo Helicopters; during simulator tests it proved virtually impossible to replicate the accident sequence.

The agency recommends that as part of the certification process, which is being led by the US Federal Aviation Administration, that verification take place of the aircraft’s behaviour in high-speed conditions, to ensure an accurate picture of its flight characteristics.

Additionally, a review of the tiltrotor’s flight control laws should take place covering “the extreme flight conditions in which the aircraft could possibly fly”, it says.

“That verification should be addressed to ensure the effectiveness of flight control inputs given by the pilot avoiding the possibility of unexpected and uncommanded coupling effects.”

Prior to the release of the report, Leonardo Helicopters had indicated that it was still confident of meeting its 2018 certification target.

No flight-test activity has been conducted since the accident. A third flight-test prototype was in early May impounded by an investigating magistrate from Vercelli, the nearest town to the crash site.