What happened
While descending toward a forestry site to collect passengers, a Eurocopter AS 350BA helicopter, registration C-GATX, experienced a sudden loss of power. The pilot reported hearing two loud bangs immediately preceding the engine failure. On the ground, crew members witnessed the aircraft trailing smoke as it descended behind a treeline. The pilot successfully executed an autorotation, landing the aircraft in a swamp. While the helicopter sustained minor damage, there were no injuries.
The investigation
Investigators examined the Turbomeca Arriel 1B engine and discovered that all power turbine blades had detached from the hub. This failure caused significant damage to the exhaust duct, the power turbine nozzle assembly, and the vertical fins, with shrapnel also impacting the leading edges of the tail-rotor blades.
Technical analysis revealed that a crack had formed within the muff coupling, which connects the power turbine to the reduction gearbox. Evidence showed that this crack originated during the manufacturing process, prior to the final nitriding and magnetic particle inspection (MPI). The fatigue of this crack eventually led to the coupling's failure. Furthermore, because the couplings lacked individual serial numbers, investigators could not determine if other components from the same production batch were also defective.
Findings
- The engine lost power due to the failure of the muff coupling connecting the power turbine to the reduction gearbox.
- The failure was caused by fatigue progression of a crack that originated during the manufacturing stage.
- The existing inspection and manufacturing processes failed to detect the defect, as the subsequent nitriding layer obscured the crack during MPI.
- A lack of part serialization prevented the traceability of the component's service life and manufacturing origin.
Safety action
Following the investigation, the engine manufacturer, Turbomeca, implemented several changes to its manufacturing and inspection protocols. New procedures include performing MPI on bar stock before processing and conducting both MPI and eddy current inspections after heat treatment but before the nitriding process. Additionally, the manufacturer transitioned to serializing all couplings from the point of manufacture. Service bulletins were also issued to instruct repair shops to replace in-service couplings with new units during overhauls.