What happened
On August 31, 1997, a Robinson R-22 helicopter, registration PP-ELR, operated by the Rondônia State Transit Department, was performing a short-duration flight from the Aeroclube de Rondônia to the Military Police General Command. While leveling off at 200ft, the pilot observed that both the engine and rotor tachometers indicated an overspeed of 110% RPM.
In an attempt to control the RPM, the pilot reduced the manual throttle completely, disengaged the RPM governor, and increased the collective pitch. When these actions failed to stabilize the RPM, the pilot decided to perform an autorotation. During the landing attempt in an area of tall, irregular vegetation, the tail skid struck the ground first, leading to a loss of control. The aircraft subsequently struck the main skids and the nose, causing the main rotor mast to break. The two occupants were able to exit the aircraft uninjured, though the helicopter sustained serious damage.
The investigation
The CENIPA investigation examined the aircraft's maintenance history, the pilot's experience, and the operational procedures used during the emergency. The investigation noted that the aircraft had recently undergone a 100-hour inspection and was within weight and balance limits.
Investigators analyzed the pilot's decision-making regarding the flight altitude and the execution of the autorotation maneuver. The investigation also looked into the operator's training protocols and the specific mechanics of the R-22's low-inertia rotor system, which requires highly precise flare maneuvers. The investigation could not definitively determine why the overspeed persisted even after the governor was disengaged, as the specific cause of the RPM fluctuation was not fully established.
Findings
- Improper flare execution: The pilot performed the flare too low to the ground and with excessive cyclic input, failing to level the aircraft as required by the flight manual.
- Deficient judgment: The pilot chose to fly at 200ft, an altitude that complicates standard autorotation procedures, and opted for an autorotation when a manual throttle landing might have been possible.
- Lack of training oversight: The operator lacked a formal training program for its pilots, specifically regarding emergency procedures and autorotation recovery.
- Operational error: The pilot's failure to level the aircraft at the recommended 8ft height led to the tail skid collision.