What happened
On 1 August 2019, a Hughes Helicopters Inc. 369D helicopter, registration SE-JVJ, was conducting power line inspections for an electricity grid company in Follebu, Norway. Approximately 52 minutes into the flight, the commander detected abnormal vibrations in the aircraft. As the flight continued, the vibrations intensified and were accompanied by a loud metallic sound.
In an attempt to inspect the cause of the disturbance, the commander decided to abort the mission and locate a landing site. While hovering at an altitude of approximately 2 to 3 meters over a grassy field, a loud bang occurred, and the aircraft suffered a sudden loss of power to the rotor system. The helicopter rotated sharply, the right skid collapsed upon impact, and the main rotor blades struck the ground. The crew members were uninjured, though the aircraft sustained severe damage.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the wreckage and performed metallurgical analyses on the components. The investigation focused on the mechanical failure of the main gearbox and the maintenance history of the aircraft. Investigators scrutinized the continuing airworthiness management of the operator, First European Aviation Company Sp. z.o.o, and reviewed the maintenance logs and documentation regarding the gearbox's service life.
Findings
- The primary cause of the loss of rotor power was a fracture in the main gearbox input pinion due to fatigue.
- Metallurgical examination revealed that the fracture had likely been developing for several years.
- An oil leak related to the developing fracture had been improperly addressed using two wine corks as plugs, which effectively concealed the leak and prevented necessary troubleshooting.
- The main gearbox had exceeded its required overhaul interval by 749 hours due to errors in the operator's maintenance program and inaccurate tracking of operating hours.
- The operator's continuing airworthiness management and maintenance documentation were found to be inadequate, specifically regarding the ability to trace vital maintenance history for safety-critical components.