What happened
On April 24, 2019, an AS 350 B3 helicopter, registration PP-MMG, operated by the Military Police of the Minas Gerais State, was performing a traffic circuit for on-board shooting training at the Military Police Instruction Center in Ribeirão das Neves, Brazil. During the sixth training circuit, the aircraft was executing a sharp left turn at a low airspeed of approximately 20 knots and a height of 150 feet AGL.
During this maneuver, the aircraft experienced an uncommanded yaw. The helicopter entered an uncontrolled spin, losing altitude until it struck treetops and subsequently collided with the ground. The impact caused the aircraft to break apart, and a post-impact fire consumed more than 75% of the structure, destroying the aircraft. Of the five occupants, the pilot and second-in-command sustained minor injuries, one passenger was unharmed, and two passengers sustained serious injuries.
The investigation
CENIPA's investigation focused on the aerodynamic conditions present during the maneuver. Investigators analyzed the flight profile and determined that the aircraft was operating within a high-risk flight envelope, specifically near the "Dead Man's Curve," where low speed and low altitude limit the safety margins for engine failure recovery.
Technical examination of the tail rotor assembly showed marks indicating a command to the right, suggesting the crew attempted to correct the yaw. The investigation examined the influence of wind conditions, noting that a tailwind and relative winds from the right sector could contribute to tail rotor vortex ring state or increased pilot workload. Furthermore, the investigation looked into the crew's management of the aircraft, noting that the pilot in command had transferred control of the aircraft in an unstable condition and had only reassumed control after the yaw had already begun.
Findings
- Unintended yaw occurred during a low-speed left turn, leading to a loss of control.
- The aircraft was operating at a speed below 30 knots, which increased susceptibility to uncommanded yaw.
- Inadequate Crew Resource Management (CRM) was evidenced by successive, uncoordinated transfers of command between the pilots.
- The organization lacked established, standardized procedures for conducting on-board shooting training.
- The crew struggled to recognize and react to the initial aerodynamic signs of the unintended yaw.
- The aircraft's airframe and engine logbook records were found to be outdated.