Italian investigators have determined that repeated contacts by an AgustaWestland AW609’s proprotor blades with the leading edge of its wing precipitated its in-flight break-up and fire.

In its final report into the 30 October 2015 accident, Italy’s ANSV air accident safety body indicates that flawed flight-control laws triggered and exacerbated the event, which took place during high-speed tests in airplane mode.

In addition, it was the first time that recent modifications to the AW609’s tail assembly had been flown at such high speeds, says the ANSV.

Two experimental test pilots were killed in the crash in the north of Italy, which led to a self-imposed grounding of the AW609 by the manufacturer and subsequent certification delay.

ANSV says the accident happened during a third round of high-speed dives using the programme’s second flight-test prototype (N609AG), with two previous efforts that day having passed off without incident.

The pilots commenced the dive with a 180° left turn, aiming for a speed of 293kt (542km/h), but on roll-out observed the start of “slight lateral-direction oscillations”.

These were not initially countered as the crew believed they were “self-damping” but as they grew in amplitude and frequency, the pilot flying attempted to correct the roll with “counterphase input roll manoeuvres and then pedal inputs”.

“However these actions did not dampen the oscillations, which instead became divergent, bringing the sideslip angle to reach values above the maximum allowed in those speed conditions,” says ANSV.

Flawed flight-control logic meant that pilot inputs to counter the excessive roll, and then yaw, had the effect of simply amplifying them, it says.

This caused the AW609 to reach a sideslip angle of 10.5° - well above the 4° maximum – “so inducing contact of the right proprotor with the right wing due to excessive flapping of the proprotor blades”.

The investigators have “reasonable certainty” that the in-flight break-up was caused by “multiple contacts of the proprotors with the aircraft wings”, says ANSV. This, in turn, severed fuel and hydraulic lines in the leading edge of the wings, triggering a fire.

Onset of the issue was difficult to detect by either the pilots or ground technicians, says the ANSV, “until the roll and yaw magnitude reached excessive levels, which was only a few seconds before loss of control”.

However, it was not the first time an AW609 had displayed similar behaviour. ANSV has disclosed that during testing in July 2014, the same aircraft had sustained damage when its right-hand proprotor hit the wing leading edge during “a significant sideslip [which] developed due to lateral acceleration” after the wing stalled.

Tests carried out after the October crash found that AgustaWestland was unable to replicate the accident in its AW609 simulator. Only by modifying software parameters to an excessive degree was it able to induce the condition, says the report.

ANSV says insufficient flight and wind tunnel testing of the aircraft’s new tail configuration contributed to the lack of data on how it would perform at speed.

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