What happened
On September 20, 2021, at approximately 13:18 JST, a Kaman K-1200 helicopter, registration JA6200, operated by Akagi Helicopter Co., Ltd., crashed in the mountains of Okuwa-mura, Nagano Prefecture. The aircraft was performing helicopter logging operations, transitioning into a hover at an altitude of roughly 40 meters to allow ground personnel to secure timber.
During this hover, the pilot experienced a sudden vibration in the collective pitch lever accompanied by a loud "whoosh" sound, followed by an immediate engine shutdown. To protect the worker positioned directly beneath the aircraft, the pilot maneuvered the cyclic stick toward the valley side. The helicopter subsequently struck trees and crashed into the terrain at a steep nose-down angle. The pilot sustained a minor injury, and while the aircraft was destroyed, no fire occurred.
The investigation
The investigation focused on the failure of the Lycoming T53-17A-1 engine. Disassembly of the engine revealed extensive damage to the power turbine (PT2) rotor, with all 62 blades broken. Detailed microscopic analysis of a specific component, Blade 44, identified a fatigue region originating from a sub-surface area near the leading edge. This fatigue was characterized by beach marks indicative of High Cycle Fatigue (HCF).
Investigators examined the maintenance history of the engine, specifically the second overhaul conducted in 2017. The investigation scrutinized the measurement of the PT2 shroud gap—the space between the rotor blades and the outer shroud. It was noted that the maintenance facility used self-fabricated shims and followed an inspection procedure that used ambiguous terminology, such as "light drag," which could lead to inconsistent measurement techniques between different inspectors.