What happened
On December 16, 2012, at approximately 20:00 UTC, a Robinson R22 Beta helicopter, registration PR-IJU, was conducting a flight instruction mission. The aircraft departed from Campo de Marte Aerodrome (SBMT) in São Paulo, carrying an instructor and a student.
During the final stage of the flight, shortly after takeoff, the engine experienced a loss of power followed by a complete shutdown. The aircraft lost altitude and struck the ground near the Guarap-piranga Reservoir. The impact caused substantial damage to the main and tail rotors, the cabin, the engine, and the skids. Both occupants sustained light injuries.
The investigation
CENIPA investigators examined the engine and fuel systems to determine the cause of the power loss. Laboratory analysis of the fuel samples confirmed that the fuel met all required chemical and physical parameters. A technical inspection of the engine revealed no internal mechanical seizures, and the oil filter was found to be clean and unobstructed.
However, during the testing of the engine on a different aircraft, investigators discovered that the carburetor mixture control lever was loose relative to the control valve shaft. The retaining pin for the lever had fractured. Technical analysis at DCTA indicated that the fracture was likely caused by mechanical work between the clip and the valve shaft hole, potentially due to insufficient torque on the lever's fixing screw or a dimensional mismatch between the shaft and the lever.
Findings
- The loss of engine power was directly linked to the fracture of the carburetor mixture control valve retaining pin.
- This failure allowed the mixture valve to move independently of the pilot's cockpit controls, leading to an unintended reduction in fuel flow and subsequent engine shutdown.
- Maintenance factors, specifically regarding the securing of the mixture control assembly, contributed to the failure.