What happened
On September 26, 2017, an MDHI 369D helicopter, registration SE-JVI, experienced a critical loss of power shortly after departing from Älvsbyn/Högheden Airport (ESUV). The aircraft, powered by a Rolls-Royce Corporation RR 250-C20 engine, suffered a sudden reduction in altitude and airspeed. This loss of power prevented the engine from providing sufficient energy to the rotor system, leading the aircraft to collide with the ground near a forest edge and a building. The accident resulted in one fatality and one serious injury.
The investigation
The investigation focused on the failure of the engine' and the integrity of the fuel control unit connections. Investigators examined the use of B-nuts in the engine's design and evaluated whether additional safety measures, beyond torque specifications and inspections, were necessary. The inquiry identified that a B-nut had become loose, leading to the engine power loss.
Crucially, the investigation found that the hardware involved in the failure was not representative of the manufacturer's original design. The fitting on the fuel control unit did not match the engineering drawings, featuring a different thread count and a rounded thread profile. Furthermore, the investigation looked into the maintenance history, noting that a change to the gas generator governor had been performed 129 hours prior to the accident.
Findings
- The primary cause of the engine power loss was a loose B-nut on the fuel control unit.
- Maintenance technicians likely failed to follow prescribed Rolls-Royce maintenance procedures for tube assembly installation during recent engine work.
- There was evidence that torque paint installation and inspection protocols were not properly followed prior to the flight.
- The hardware used in the accident was non-original equipment manufacturer (non-OEM) hardware, which did not meet the original design specifications for thread count or profile.