What happened
On October 17, 2014, a Robinson R22 helicopter, registration PT-LNA, was performing a ferry flight from Carlos Prates to Divinópolis, Minas Gerais, carrying a pilot and one passenger. Approximately twenty minutes into the flight, the aircraft began experiencing intense vibrations. In response to the increasing instability, the pilot initiated an autorotation maneuver. During the descent, the vibration amplitude increased suddenly, making effective aircraft control impossible. The helicopter struck medium-height vegetation near Juatuba, MG, with the tail cone making initial contact. The aircraft sustained substantial damage to the fuselage, engine, main gearbox (MGB), main rotor, tail cone, and tail rotor. The pilot escaped uninjured, but the passenger sustained serious injuries.
The investigation
The investigation by CENIPA focused on the mechanical condition of the main gearbox and the maintenance history of the aircraft. It was established that the aircraft had been imported from the United States and underwent assembly and initial maintenance in Belo Horizonte. Records showed that the aircraft had previously triggered the main gearbox chip light indicator, signaling the presence of metal particles in the transmission.
Technical analysis of the MGB, conducted by the manufacturer, revealed a significant amount of metal shavings on the chip detector and substantial wear on the transmission gear teeth. Small damages to the bearings were also identified. The investigation also reviewed the maintenance logs, noting that the maintenance procedures performed during consecutive annual inspections (IAM) did not align with the manufacturer's requirements regarding the interval between chip light occurrences.
Findings
- Improper maintenance of the main gearbox: The maintenance performed did not follow the manufacturer's manual, which requires the gearbox to be returned to the manufacturer for repair if the chip light illuminates within 100 flight hours of a previous occurrence.
- Mechanical failure: Significant wear on the transmission gear teeth and the presence of metal particles in the oil and chip detector created severe vibrations and the risk of a catastrophic failure.
- Pilot decision-making: The pilot's decision to continue the flight despite experiencing abnormal vibrations during a prior test flight, and the decision to execute an autorotation instead of a safe landing in an available nearby clearing, contributed to the outcome.
- Pilot experience: The pilot had limited experience, with only 48 hours of flight time in this specific model.
- Inadequate information transfer: The second maintenance facility was not informed of the previous history of chip light warnings.