International investigators are assessing what role an unexpected feature of the AW609's high-speed flight-control software may have played in a prototype's fatal October in-flight breakup in Italy.
Italy's national agency for aviation safety, which leads the probe of the Oct. 30 crash that killed two test pilots, said the civil tiltrotor's "behavior at high speed was not completely predicted by the manufacturer," said the agency (which uses the Italian abbreviation ANSV) in a June 23 preliminary report.
Leonardo predecessor AgustaWestland and Bell partnered in development of the civil tiltrotor. Bell played a major role in developing and refining the aircraft's flight-control software.
The ANSV report added that the simulator (SimRX) used to build and develop the AW609's flight-control laws "was not able to reproduce the phenomenon that happened in the accident flight."
"The only way to obtain a reliable representation of the accident flight," the preliminary report said, was "to input unrealistic geometric and aerodynamic parameters in the SimRX."
Supporting the ANSV's investigation are the U.S. FAA and National Transportation Safety Board, Italy's civil aviation authority (ENAC) and the European Aviation Safety Agency (EASA). Leonardo is cooperating with the investigations, as is Bell.
The flight crew was attempting a third dive to the AW609's maximum dive speed of 293 kt indicated airspeed (KIAS) when the accident occurred, said the ANSV. The airframe had a new rear fuselage and tail fin configuration on the flight, the agency said, adding that the configuration had not been dived beyond 285 KIAS before the accident flight. The ANSV said that achieving 293 KIAS was required for certification.
The preliminary report said the flight crew had dove twice to 293 KIAS during the accident flight. As they attempted that a third time, the ANSV said, analysis of the AW609's combined flight data and cockpit voice recorder and telemetry data gathered on the ground by Leonardo's flight test team showed that the pilot-in-command felt the onset of roll oscillations.
He tried to counteract the oscillations "by maneuvering the aircraft on the roll axis," the ANSV said, a response "that is assumed correct according to the normal flying technique."
But the AW609's current flight control laws are designed so that input in the roll axis also generates a control force in yaw to compensate for the expected aerodynamic effect of the flaperon's movement on yaw. This created a phenomenon described during the investigation as "like an augmented dutch roll," the ANSV said.
Within less than a half a second of the onset of roll oscillations, the report's data shows, the AW609's roll and yaw each was swinging beyond plus and minus 10 deg. During this third dive, the ANSV said, "the accident happened."
The flight ended when the No. 2 prototype disintegrated in flight over the small town of Santhià, about 30 nm (55 km) west-southwest of the aircraft's departure point, Leonardo Helicopters' base at Cascina Costa di Samarate.
The ANSV noted that data from previous flights with the No. 2 prototype and the No. 1 aircraft showed each had experienced "a slight positively damped lateral-directional augmented mode," or oscillations in roll and yaw, in the past. But neither had been dived to 293 KIAS in the latest configuration.
The ANSV's report shows that the prototype's debris was recovered over a ground track south of Santhià that runs 1.67 mi (2.68 km) under the direction of flight, from east to west. The debris field is bisected roughly by train tracks that run north-northeast to south-southwest from Santhià to the city of Tronzano Vercellese.
Debris covered farthest east included pieces of light foam and pieces of tips from the AW609's two three-bladed rotors. Moving the west, pieces of the tiltrotor's right and left wing skins and stringers were found alongside the train tracks.
This type of wreckage also was found farther west, straddling the north-south SP3 highway south of Santhià. Pieces of what the report described as light foam and blade-tip pieces also were found on either side of that highway.
Wing skin and stringers were found farther west in a mixed field of debris that included light foam and blade-tip pieces, the AW609's baggage compartment door and the aircraft's horizontal stabilizer and elevator. That mixed field laid between the SP3 and the A26 superhighway that runs northwest-to-southeast between Santhià and Tronzano.
About a quarter mile farther west from the A26, the last debris field, investigators found the AW609's right and left engine nacelles and fuselage and the bodies of test pilots Pietro Venanzi, the copilot on the flight, and Herb Moran, the pilot-in-command.
Photo courtesy of Leonardo Finmeccanica