Accident Overview
A helicopter crashed onto a rugged hardened lava flow after experiencing a loss of engine power during cruise. Witnesses, including pilots of other helicopters, heard the pilot transmit a mayday call with an audible "engine out" warning in the background. A post-crash fire consumed most of the wreckage.
Examination Findings
An airframe examination revealed no evidence of a preimpact malfunction in the remaining structure. An engine inspection and metallurgical examination identified the failure of the compressor-coupling adapter, a Parts Manufacturer Approval (PMA) part produced by a company other than Rolls-Royce. The adapter had fractured above the shear point due to fatigue cracking initiated by fretting on the pilot diameter. This failure disconnected the compressor from the turbine section.
The metallurgist noted that the damage was consistent with rotation of the aft piece of the fractured adapter within the stub shaft after separation. Fretting damage on the impeller corresponded to that on the adapter, indicating relative movement between the parts.
Adapter Failure Details
Rolls-Royce had records of 12 other compressor-coupling adapters that had fractured and failed, all in this specific new coupling design, which differed significantly from the previous design. The nature of the fretting and fractures suggested that the newly designed couplings experience a small amount of longitudinal movement between the outer diameter of the adapter and the inner diameter of the impeller—a factor absent in the previous design.
If the spur adapter gear, compressor-coupling adapter, and compressor impeller (all coaxial spline joints) were in alignment, no significant longitudinal movement of the pilot diameter would occur. However, any axial misalignment of these components during engine buildup could induce misalignment resulting in relative motion with each engine rotation. This misalignment would also introduce bending stresses into the adapter in addition to torsion stresses, potentially leading to fatigue. The combination of fretting damage and cyclic bending stresses was considered most likely the main factor in the failures of these couplings.
Maintenance History
One month prior to the accident, company maintenance personnel had replaced the engine gearbox due to a loose stud on the gearbox-to-turbine-section mounting. This replacement involved disturbing the coaxial spline joints between the spur adapter gear, compressor-coupling adapter, and compressor impeller.