What happened
On a visual flight rules flight from La Tuque to Val-d'Or, Quebec, the pilot of a Bell 206L-3, registration C-GDTM, noticed the engine oil pressure gauge fluctuating. Following this observation, the pilot performed a precautionary landing in a marsh. After a brief inspection and engine shutdown, the pilot restarted the engine with the intention of moving the aircraft to a more accessible road located one kilometre away.
During this second attempt, the pilot observed bluish smoke exiting the exhaust. Despite the smoke, the pilot proceeded toward the road. As the helicopter reached an altitude of approximately 50 feet, the engine oil pressure and torque indicators began to fluctuate again. Immediately following these fluctuations, an engine explosion occurred, causing the engine to fail. The pilot performed an autorotation, but the aircraft struck the ground with the rear skids first, causing the nose to tip forward. The impact caused the main rotor to strike the tail boom, resulting in heavy damage to the aircraft. The pilot, who was alone on board, escaped without injury.
The investigation
Investigators examined the engine and found that the area surrounding bearings 6 and 7 had reached temperatures exceeding 900°C. At such extreme heat, engine oil evaporates and burns rapidly. This loss of oil caused the oil level to drop, leading to pump cavitation and the observed pressure fluctuations. While the pilot noted the pressure fluctuations, the oil temperature remained relatively stable, which led to the incorrect conclusion that the gauge itself was malfunctioning.
Engine analysis also revealed that while bearing 8 was damaged, there was no evidence of the thermal distress that would suggest it had operated without lubrication. The investigation also noted that the pilot's decision to move the aircraft was influenced by the difficult conditions in the marsh, including heavy mosquito activity and the lack of level ground.
Findings
- The extreme heat near bearings 6 and 7 caused the engine oil to dissipate, leading to a loss of lubrication.
- The decision to continue the flight toward the road while the engine showed signs of malfunction contributed to the failure of the bearings.
- The helicopter was not level during the autorotation, which resulted in a hard landing and subsequent damage to the tail boom.