What happened
On August 31, 2016, an AS 350 B2 helicopter, registration PP-EJK, operated by the Minas Gerais Military Police, was conducting an annual firefighting training flight near Nova Lima, Brazil. The mission involved using a Bambi Bucket to practice water collection and dispersal.
During the fourth refueling attempt at Lagoa dos Ingleses, the aircraft was hovering out of ground effect to fill the external bucket. While maintaining the hover, the helicopter entered an abrupt and uncontrolled descent, estimated by the crew to be approximately 2/3 of a flight level (20 meters). During this descent, the tail rotor struck the surface of the lagoon. Despite the impact causing substantial damage to the gearbox, rear camshaft, and tail rotor assembly, the pilot managed to regain control and successfully landed the aircraft at the BH Helicenter.
The investigation
CENIPA's investigation focused on the aerodynamic conditions during the descent and the pilot's recovery efforts. The investigation established that the aircraft entered a Vortex Ring State (VRS), characterized by a vertical descent rate exceeding 500 ft/min with little to no forward airspeed.
Investigators analyzed the pilot's corrective actions, noting that upon recognizing the loss of altitude, the pilot applied maximum collective power while simultaneously pushing the cyclic forward. The investigation determined that this specific combination of inputs was counterproductive; increasing power via the collective actually expanded the stalled area in the inner section of the rotor disk, further increasing the descent rate rather than arresting it.
Findings
- Vortex Ring State (VRS): The aircraft entered a state of aerodynamic instability where the downward airflow recirculated through the rotor disk.
- Improper Recovery Technique: The pilot's decision to apply maximum collective power while moving the cyclic forward contributed to the increased rate of descent.
- Perception and Decision-Making: The crew faced difficulties in recognizing the early warning signs of VRS and choosing the appropriate recovery maneuvers.
- Training and Management: There were indications of inadequate training regarding the identification of VRS signals and potential deficiencies in organizational risk management and training frequency.