Mark Huber is an aviation industry veteran who has worked in the flight training, aircraft sales, aircraft component, and airport development businesses. He has flight time in more than 50 different and diverse aircraft, including helicopters, aerobatic aircraft, vintage warbirds, jet fighters, business jets and turboprops, and transport-category aircraft. He has jumped out of several of these—on purpose—with a parachute. His defense and aviation journalism has been published in a variety of national and international periodicals.

Contributor

Mark Huber

The fatal crash of the second Leonardo Helicopters (formerly AgustaWestland) civil tiltrotor prototype (AC2), N609AG, on Oct. 30, 2015 at Tronzano Vercellese, Italy, is ascribable basically to the “combination of three factors”: the development of latero-directional oscillations; the inability of the fly-by-wire flight control system (FCS) control laws to maintain controlled flight; and the failure of the engineering flight simulator (SIMRX) to “foresee the event in any way,” according to the final report from Italy's National Agency for Flight Safety (ANSV—Agenzia Nazionale per la Sicurrezza del Volo). The accident aircraft had accumulated 567 hours since first flying in 2006. It took off from the company's production facility at Cascina Costa and crashed at 10:42 a.m. local time while executing a third planned high-speed descent as part of test flight T664. During the descent the aircraft entered uncontrolled flight in a series of lateral-directional oscillations, broke up and caught fire in flight before striking the ground, killing both test pilots.


<h2>Difficulty of Recovery</h2>


The ANSV said that a combination of ground debris mapping and telemetry data led it to “hypothesize with reasonable certainty” that the aircraft broke up in flight as a result of multiple prop-rotor strikes from excessive blade flapping on the wings as a consequence of excessive yaw angles reached during the fatal dive. This damaged the hydraulic and fuel lines that are positioned along the wing leading edges, precipitating the in-flight fire. The aircraft was equipped with flapping stops, but they were not designed to “contain the effects of the extreme aerodynamic forces generated during the event.” Because of the aerodynamic characteristics of the aircraft and the specific conditions created by the dive, the flying pilot's attempt to counteract the oscillations with a roll-tracking maneuver to level the wings was ineffective, partly because the FCS was designed to “couple” on more axes than the command inputs given on the single axis by the pilot.


Specifically, “Total lateral control resulting from the summation of pilot input and automatic FCS input has an effect on the yaw axis through aerodynamic coupling and feedforward and feedback turn coordination automatically provided by the FCS. Consequently, giving a command in counterphase on the roll axis to dampen the relative oscillations creates an effect on the yaw axis that can be in phase with the yaw oscillations. This occurred during the accident: the correction of the roll oscillation induced, by the control laws of the FCS, a manuever in phase with the oscillations of the yaw axis, generating a divergence of the oscillations.” The ANSV said that the “low frequency and low amplitude nature of the oscillations” made them difficult for the pilots or ground crew to perceive until the roll and yaw “reached excessive levels only a few seconds before loss of control.”


The pilot flying also made rudder-pedal inputs. As explained above, the inputs exacerbated the situation, taking sideslip to maximum values. The tiltrotor entered a dive at the 293-knot design dive speed. AC2 was fitted with a new tapered rear fuselage and redesigned vertical fin with less surface area. During the dive, the aircraft reached 306 knots.


Investigators attempted to recreate the accident flight in the AW609 SIMRX in Philadelphia using the same software and flight conditions, but could not; they came close by inserting algorithms that changed the aerodynamic configuration of the aircraft, but even then the lateral-directional oscillations developed were in a different phase. They did, however, use the exercise to verify the “great difficulty” of recovery to controlled flight under the conditions. The ANSV found the inability to replicate the accident flight in the simulator unremarkable, given “the lack of experimental data obtained previously in the wind tunnel and in-flight evaluations with those speed conditions and relating to the recent modified geometry of the tail fin; this last change was considered conservatively by entering a reduction in the tail fin area into the database and then implementing the computational fluid dynamics.”


The ANSV made several safety recommendations after the accident: more high-speed and complex-flight-condition modeling, verification and wind tunnel testing as part of the AW609 certification process; and verification of the flight control laws in extreme flight conditions, in particular reviewing their effectiveness with regard to pilot inputs and uncommanded coupling effects.&nbsp;&nbsp;


AW609 flight-testing resumed in August last year. AC3 is flying from the company's Philadelphia facility and recently completed testing for flight into known icing. AC4 is under assembly in Philadelphia and is expected to fly next year. AC1 is in Italy undergoing modifications before return to the flight-test program. Leonardo Helicopters expects FAA certification next year.

News about significant aircraft accidents and information from accident reports.

AW609 Final Accident Report Issued 

The pilot's control inputs exacerbated extreme aerodynamic forces.

2024 Corporate Aviation Leadership Summit

Amy Wilder

News and issues relating to business, corporate and private aviation, primarily regarding turbine-engine-powered airplanes and helicopters. Subjects include aircraft, engines, maintenance, operations, personnel, acquisitions, accidents, safety, security, and training.

Pilot died of cardiac arrest during the flight

NTSB: Pax Safely Lands King Air after Pilot Collapsed

Vehicle also struck parked ground-support tractor; there were no injuries

Teen Driver Breaches Fence, Hits PlaneSense PC-12 at KPSM

News and information on safety procedures and concerns.

Six fatal accidents resulted in 22 fatalities in the quarter

NBAA Report Flags Powerplant, Collision Risks in Q2 Data

Curt Epstein

News and issues relating to international air transport and cargo carriers, national airlines and regional airlines, including aircraft, engines, personnel, acquisitions, accidents, safety, security and training.

Twenty years ago, a business jet and commerical airliner collided; the smaller airplane survived

Remembering a Solemn Anniversary: the Amazon Midair 

David Jack Kenny

Preliminary and final accident reports, September 2026

Business Aircraft Accident Reports: September 2026

News and issues regarding all manner of civil and military rotorcraft and their powerplants, including helicopters, tilt rotors and unmanned air vehicles.

More In Accidents

NTSB: Pax Safely Lands King Air after Pilot Collapsed

Pilot died of cardiac arrest during the flight

Teen Driver Breaches Fence, Hits PlaneSense PC-12 at KPSM

Vehicle also struck parked ground-support tractor; there were no injuries

NBAA Report Flags Powerplant, Collision Risks in Q2 Data

Six fatal accidents resulted in 22 fatalities in the quarter

Remembering a Solemn Anniversary: the Amazon Midair

Twenty years ago, a business jet and commerical airliner collided; the smaller airplane survived

Business Aircraft Accident Reports: September 2026

Preliminary and final accident reports, September 2026

NTSB Cites Heliport Misidentification in Aug. 2025 Crash

Captain opted out of instrument approach over autopilot concerns

Fire of Unknown Origin Led to 2024 Citation X Accident

NTSB: Ignition source could not be conclusively identified

NTSB: Bell 206L Damage Caused by Aeroelastic Interaction

Helicopter experienced a vertical hop for which the pilot didn’t know the recovery procedure

AIN Products

SUBSCRIPTIONS

RESOURCES

ABOUT AIN