Summary
On September 20, 2021, at about 13:18 Japan Standard Time, a Kaman K-1200 helicopter, registration JA6200, operated by Akagi Helicopter Co., Ltd., crashed in Okuwa-mura, Kiso-gun, Nagano Prefecture, Japan. The helicopter was engaged in helicopter logging and had just transitioned to a hover at an altitude of about 40 m over a loading site. The engine shut down, and the helicopter subsequently crashed into a mountainside. The captain, the sole occupant, sustained a minor injury. The helicopter was destroyed; no fire occurred.
Flight History
According to statements from the captain and a worker on the ground, the helicopter took off from the Tonooku Operation Site at about 13:01 JST. After several transport runs, on the sixth run, the helicopter flew to a loading site approximately 2 km east of the operation site and began hovering eastward at 40 m altitude. One worker was directly beneath the helicopter preparing to bundle timber. The captain reported feeling a momentary vibration in the collective pitch lever accompanied by a "rasping" sound, followed by a "whoosh" sound as the engine shut down. The captain transmitted "Run-away" over the radio and maneuvered the cyclic stick to the right forward direction to ensure the worker's safety. The helicopter descended with its nose facing downward, contacting trees and crashing about 20 m south of the hover position.
Damage
The helicopter was destroyed. All four rotor blades were fractured. The vertical fin (rudder) fractured at its root, the vertical stabilizer was damaged, and the fuselage and tail boom were damaged. The windshield was broken. Twelve trees were broken off at the crash site.
The engine, a Lycoming T5317A-1 turboshaft engine (serial number LE81009), was examined. External appearance showed no fire damage or casing penetration. Light scratch marks were found on compressor blade tips. The gas producer turbine had no damage. The exhaust diffuser had a crack on a structural support strut and scratch marks. The power turbine (PT) rotor showed significant damage: the PT1 rotor had damage to trailing edges and shrouds; the PT2 nozzle was deformed with severe trailing edge damage; all 62 PT2 rotor blades were broken, with 43 fractured at the root and 19 at intermediate positions.
Detailed Examination of Blade 44
A detailed examination of PT2 rotor blade 44 revealed a smooth fracture surface on approximately one-third of the leading edge side and a rough surface on the trailing edge side. Optical and scanning electron microscopy showed the smooth region exhibited fatigue characteristics, with a fatigue origin at a subsurface faceted area near the leading edge. Shell-like beach marks indicative of high cycle fatigue (HCF) propagation were observed.
Probable Cause
According to the engine manufacturer (Company B), the high cycle fatigue of the PT2 rotor blade was caused by high-frequency vibration loads generated by flutter in the PT rotor blades, which occurred due to an excessive shroud gap between blade pairs.