Flawed flight-control logic triggered AW609’s in-flight break-up: report
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”. […]
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.
Source:
FlightGlobal.com
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