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Severe latero-directional oscillations during a high-speed dive caused the fatal crash of a Leonardo AW609 during flight-testing in 2015, Italian investigators have found.
The aircraft’s excessive yaw angle forced its proprotors to hit its wings multiple times, damaging the hydraulic and fuel lines, and causing an in-flight breakup and fire. The resulting crash fatally injured test pilots Herb Moran and Pietro Venanzi.
The findings are detailed in the final report on the accident from Italy’s national agency for flight safety (Agenzia Nazionale per la Sicurezza del Volo, or ANSV), in which it also points to two other causes: the AW609’s flight control system (FCS) controls laws, and a project simulator (SIMRX) that “did not foresee the event in any way.”
The ANSV also noted that the accident flight was the first in which such speeds had been reached in the new configuration of a streamlined fuselage in the tail and a reduced tail fin surface.
The aircraft’s wreckage was found in three main parts near the city of Tronzano Vercellese in Italy. The ANSV said the distribution of the debris was coherent with a structural breakup in flight, which then caused an explosion and ballistic trajectory towards the point of impact on the ground.
The accident took place as the aircraft was performing the third high-speed dive of a test flight on Oct. 30, 2015. The pilots commenced the dive with a left 180-degree turn, targeting 293 knots for the maneuver (though the aircraft reached a maximum airspeed of 306 knots as the crew attempted to resolve the ensuing controllability issues). According to the report, the oscillation started on the roll axis following the exit from the turn, about four seconds into the maneuver. Another oscillation, this time in yaw, was added to the initial slight oscillation in roll shortly afterwards.
“The crew did not initially react using inputs to counteract them,” the ANSV states, noting that the oscillating phenomenon had been noticed in previous test flights, but it was considered to be slight and not dangerous, and was believed to be self-damping.
As the crew felt the oscillations increase in magnitude, about 23 seconds into the maneuver, Moran tried to counter them with by “roll tracking” — maneuvering the aircraft on the roll axis — the standard pilot procedure for this type of condition. Noticing a pronounced yaw condition, he then attempted to counter this using his rudder pedals.
Around this time, an amber “QBALTH” message appeared on the EPDU, indicating a problem with the torque balancing ratio.
The ANSV explained that the aircraft’s control laws worked against Moran’s countering maneuvers. “A roll command [in the AW609] is transferred by the control laws into different commands that are sent to the control surfaces that act on the roll (for example: flaperons) and to the differential collective pitch control, that, in this aircraft, regulates yaw,” the ANSV stated in the report. This coupling is to compensate for the expected aerodynamic effect of flaperon control surface motion.
So, despite Moran performing the standard compensating procedure, it served to increase the oscillations.
A few seconds later, the first proprotor came into contact with the leading edge of the right wing “and the aircraft started to become irredeemably uncontrollable.”
The ANSV said the excessive flapping of the proprotors was caused primarily by the sideslip angle reached by the aircraft, that exceeded, by nearly two and a half times, the maximum flight envelope value at the speed of 293 knots (10.5 degrees as opposed to the four degrees maximum allowed).
A similar phenomenon had been found during a flight test on July 17, 2014, when angle of attack, angle of bank, mach number, rate of decent and number of “g” caused an accelerated stall of the aircraft right wing, and a significant sideslip developed due to lateral acceleration.
The situation caused excessive flapping on the right proprotor to the extent that it made light contact with the leading edge of the right wing, but in that instance, the crew was able to maintain control of the aircraft and perform an emergency landing.
Following the 2014 incident, Leonardo established new procedures and limitations in the flight envelope. A new parameter (QBALTH) was added to be continuously monitored; between 0.7 and 1, an amber message appeared on the EPDU, with no crew action required. Above 1, and the message appeared in red, and the test would have to be interrupted and the aircraft smoothly leveled.
During its investigation, the ANSV said that it visited AgustaWestland Philadelphia Corporation to use the aircraft’s flight simulator, but noted that it was “not possible” to reproduce the conditions that occurred during the accident.
“As evidenced by the tests carried out by the ANSV, the simulator demonstrated not being able to faithfully reproduce the dynamics occurred during test flight T664 [the accident flight], reasonably due to the non-representativeness of the aerodynamic set, for the unique and extreme conditions encountered, obtainable in the wind tunnel for the new updated configuration including the tapered rear fuselage and the modified tail fin,” the report states.
“Therefore, the [simulator] was not really able to properly carry out the role of test bench for the control laws and risk reduction.”
In its safety recommendations, the ANSV said the AW609’s control laws should be reviewed in the management of the extreme flight conditions in which the aircraft could possibly fly. “That verification should be addressed to ensure the effectiveness of the flight controls inputs given by the pilot avoiding the possibility of unexpected and un-commanded coupling effects.”
It also called for the mandatory requirement of flight data recorders in experimental aircraft — those on the AW609 were in place solely because Leonardo had chosen to do so, but were central to providing the information the agency needed to piece together the accident.
When reached for comment, Leonardo said it would issue a statement “following complete analysis and review of the report.”