What happened
On 1 August 2019, a Hughes Helicopters Inc. 369D helicopter, registration SE-JVJ, was performing aerial inspections of power lines in the Follebu region of Norway. During the flight, the pilot detected abnormal vibrations that gradually intensified, accompanied by a loud metallic noise. Seeking to investigate the source of the disturbance, the pilot decided to abort the mission and perform a landing in a nearby grassy field.
As the aircraft reached a hovering altitude of approximately 2 to 3 meters, a sudden loud bang occurred, and the helicopter lost all power to the rotor system. The aircraft rotated sharply, causing the right skid to collapse and the tail and main rotor blades to strike the ground. Despite the significant impact and substantial damage to the aircraft, the two crew members escaped without any injuries.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the wreckage and conducted metallurgical analyses of the components. The investigation focused on the mechanical failure of the transmission system and the maintenance history of the aircraft. Investigators scrutinized the maintenance records provided by the operator, First European Aviation Company Sp. z.o.o, and evaluated the airworthiness management processes in place at the time of the accident.
Findings
Technical examinations revealed that the failure was caused by a fracture in the main gearbox input pinion due to fatigue. Evidence suggests that this fracture had likely been developing for several years. Investigators discovered that an oil leak related to this fracture had been improperly addressed using two wine corks as makeshift plugs, which effectively concealed the leak and prevented the detection of the developing crack.
Furthermore, the investigation identified critical failures in maintenance management. The main gearbox had been operated for approximately 749 hours beyond its required overhaul interval. The operator's records erroneously indicated that the gearbox had significant remaining flight time, when in fact it was overdue for service. Additionally, the investigation found that the operator could not provide adequate documentation regarding vital maintenance performed on the gearbox, and that the continuing airworthiness management system contained serious errors regarding component operating hours.