What happened
On October 30, 2015, an experimental AgustaWestland AW609 tiltrotor, registration N609AG, was conducting a scheduled flight test program. The aircraft departed from Cascina Costa (VA) with the intention of transferring to a reserved airspace near Santhià (VC).
During the execution of a high-speed descent (test point T664), the aircraft entered an uncontrolled flight state. The aircraft experienced severe lateral-directional oscillations that led to its destruction in flight, followed by an in-flight fire and impact with the ground. The two experienced test pilots on board were killed in the accident.
The investigation
The ANSV investigation focused on the flight dynamics and the aircraft's behavior during the extreme flight conditions of the dive. Investigators analyzed flight data from the Flight Data Recorder (FDR) and Flight Test Instrumentation (FTI), as well as cockpit voice recordings.
The investigation examined the interaction between the pilot's control inputs and the aircraft's Flight Control System (FCS) control laws. Specifically, the inquiry looked into why the aircraft reached a sideslip angle that exceeded the maximum flight envelope by nearly two and a half times. The investigation also reviewed the effectiveness of the SIMRX project simulator in predicting such extreme dynamic conditions.
Findings
- The primary cause of the accident was the development of unintentional aircraft-pilot coupling (also known as Pilot-Induced Oscillations).
- During the high-speed dive at 293 knots, the pilot's attempts to control the lateral-directional oscillations resulted in a low-frequency (0.1Hz) diverging oscillation.
- The FCS control laws created an unexpected in-phase amplification of sideslip in response to the pilot's control inputs.
- This oscillation induced excessive lateral flapping of the proprotor blades. The resulting flapping angle caused the blades to strike the aircraft wing, leading to the loss of control and subsequent structural failure.
- The extreme flight conditions—specifically the high speed and negative angle of attack—placed the aircraft at the boundaries of its design envelope.