What happened
On 1 August 2019, a Hughes Helicopters Inc. 369D helicopter, registration SE-JVJ, was conducting aerial inspections of power lines in the Gausdal municipality of Norway. During the flight, the pilot noticed abnormal vibrations that gradually intensified, accompanied by a metallic noise. Seeking to inspect the aircraft, the pilot initiated an approach to a nearby grassy field.
As the helicopter reached an altitude of approximately 2 to 3 meters, a loud bang occurred, and the aircraft suddenly lost all power to the rotor system. The helicopter rotated sharply, the right skid collapsed upon impact, and the tail and main rotor blades struck the ground. Despite the severe damage to the aircraft, the two crew members escaped without any injuries.
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 transmission system and the maintenance history of the aircraft. Investigators also reviewed the operator's continuing airworthiness management records and the documentation provided for the aircraft's maintenance history.
Findings
- The primary cause of the loss of rotor power was a fracture in the main gearbox input pinion resulting from fatigue.
- Metallurgical examination suggested the fracture had likely been developing for several years.
- An oil leak had been improperly addressed using two wine corks as plugs, which likely concealed the developing crack 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, with a lack of verifiable records regarding recent major maintenance on the gearbox.