NTSB Identification: SEA06LA162.|
14 CFR Part 133: Rotorcraft Ext. Load
Accident occurred Friday, August 18, 2006 in Metaline Falls, WA
Probable Cause Approval Date: 8/30/2007
Aircraft: Kaman HH-43F, registration: N559D
Injuries: 1 Fatal.
The pilot was about to release a load of logs onto a log pile from an altitude between 150 to 200 feet when ground crewmen heard an explosion, then observed the helicopter descend in a nose down attitude before impacting the log deck. An examination of the accident site by an FAA inspector revealed that all major portions of the helicopter were accounted for. A subsequent examination of the wreckage revealed a failure of the transmission's left-hand cross-shaft internal spline, which resulted in the loss of gearbox timing and subsequent in-flight blade contact and separation. Maintenance records revealed that the helicopter's transmission was overhauled on May 1, 2006; the time between the overhaul and the accident was 409.4 hours. An examination of the fractured transmission center housing was caused by two fatigue cracks, which were probably caused by severe cyclic stresses that resulted from repeated loading and unloading of logs.
The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
The failure of the left-hand cross-shaft input internal spline while the helicopter was in a hover, which resulted in the loss of gear box timing, in-flight blade contact and separation, and the subsequent loss of control and impact with a log deck.