What happened
On January 10, 2013, a Hughes 369D helicopter, registration OH-HNP, was engaged in a tree-trimming mission for power line clearance near Teisko, Tampere. During the flight, the engine lost power and likely shut down. The pilot, attempting to manage the emergency, performed a forced landing in a field.
During the descent, the pilot attempted to release the heavy tree-saw equipment via the mechanical release handle but was unsuccessful. The saw likely became entangled in a tree, causing the attachment boom to break. The helicopter sustained significant damage upon impact, including a broken tail boom and damaged main rotor blades. The pilot sustained minor injuries.
The investigation
The investigation focused on the technical cause of the engine failure and the maintenance history of the aircraft. Investigators examined the engine's air control system and found that a nut (B nut) on a pipe fitting following the PC filter had completely unscrewed. This caused the pipe to disconnect, leading to a loss of compressor discharge pressure in the fuel control unit and the subsequent loss of engine power.
Technical analysis revealed significant variations in the torque required to open various fittings on the aircraft. Specifically, the nut in question was found to have been tightened with a much higher torque than its counterpart, potentially creating residual stress that contributed to the fitting loosening. The investigation also looked into maintenance practices, noting that the company's maintenance documentation did not always separate complex installation tasks into individual steps, which increased the risk of human error.
Findings
- The immediate cause of the accident was the loosening of a pipe fitting nut in the engine's air control system, which led to the loss of compressor discharge pressure.
- The underlying causes were an imprecise pre-flight inspection and either an incorrect maintenance method or human error during previous maintenance work.
- Variations in tightening torques between different fittings may have induced stress on the connection.
- The heavy weight and configuration of the tree-saw equipment made emergency procedures, such as releasing the load, particularly challenging during a power loss.