Incident Overview
While returning from a spraying operation, the pilot of a helicopter observed that the main transmission oil pressure gauge indicated zero. The pilot reported that the engine master caution did not illuminate and that the oil temperature gauge was normal. Believing the oil pressure gauge to be faulty, the pilot decided to continue the flight.
Sequence of Events
Approximately five minutes later, the transmission oil temperature gauge began to rise rapidly. The helicopter then began to shake violently, and the main rotor RPM started to decay. Due to the low rotor RPM, the pilot was unable to perform an autorotation. Instead, the pilot executed a power-on landing. Upon impact, the helicopter rolled over.
Post-Accident Investigation
Post-accident examination disclosed that the oil cooler housing gasket was damaged. The transmission oil reservoir did not contain any oil. The transmission outer housing and planetary gears exhibited signs of high temperature distress.
Conclusion
The investigation did not assign a probable cause. The findings indicate the helicopter experienced a loss of transmission oil resulting from a damaged oil cooler housing gasket, leading to transmission failure and the subsequent forced landing.