What happened
On April 24, 2021, at approximately 11:30 UTC, a Robinson R44 II helicopter, registration PR-DDM, was conducting an agricultural reconnaissance flight over a sugarcane plantation at Fazenda Sorrento, in São Manuel, São Paulo. The mission was to survey the area for the upcoming application of pesticides.
During the flight, the pilot observed an object in the field, suspected to be a silo bag, and initiated a 180-degree reversal maneuver (a turning pattern known as a 'balloon' turn) to investigate. The maneuver began at an estimated speed of 55 knots. During the descending phase of the turn, the aircraft experienced an unexpected increase in the rate of descent, accompanied by heavy vibrations and a low-rotor RPM warning alarm. The pilot attempted to correct the sink by increasing collective pitch, but the aircraft lost control and struck the ground. The impact caused the aircraft to tilt to the left, resulting in the main and tail rotors striking the terrain. The aircraft sustained substantial damage, though the pilot remained uninjured.
The investigation
CENIPA investigators examined the aircraft, the pilot's credentials, and the environmental conditions at the time of the accident. The investigation confirmed that the pilot was properly licensed, qualified, and held a valid medical certificate. The PR-DDM was found to be airworthy, with up-to-date maintenance logs and within weight and balance limits. No mechanical failures or malfunctions of the aircraft systems were identified as contributing factors.
Meteorological data indicated favorable flying conditions, with visibility and weather well above the required minimums. The investigation focused on the aerodynamic state of the rotor system during the reversal maneuver. The sequence of events—specifically the sudden increase in sink rate and vibration during a descending turn at low airspeed—was consistent with the phenomenon known as Vortex Ring State (VRS), also referred to as 'settling with power.'
Findings
- The pilot initiated a reversal maneuver at a speed that, combined with the descending flight path, placed the rotor system in a state susceptible to aerodynamic instability.
- The aircraft entered Vortex Ring State (VRS), where the helicopter becomes trapped in its own turbulent downwash, leading to a rapid, uncontrolled increase in the rate of descent.
- The pilot's attempt to recover by increasing collective pitch likely exacerbated the condition by increasing the downward airflow and the area of the rotor affected by the vortices.
- The pilot's focus on identifying the object in the field may have contributed to a loss of situational awareness regarding the aircraft's airspeed and descent rate during the maneuver.