Hiller UH-12E N99090 accident near Wenatchee, Washington

Probable cause

Fatigue failure of the six bolts that secured the bevel gear to the first stage planetary carrier, which caused the transmission to seize while the helicopter and main rotor were still operating.

NTSB analysis

The pilot reported that, after takeoff, when he was 3 miles from the airport, he heard a noise above and behind him. The pilot immediately turned the helicopter back toward the airport; when about 1 mile away, he heard a second noise. The pilot continued to the airport and was hovering the helicopter about 4 feet above the ground when the main rotor head and blades separated from the helicopter, flew up into the air, and impacted the ground about 300 feet away. The helicopter impacted the ground hard, and the tailboom separated about midspan.

Postaccident examination revealed that the main transmission housing exhibited multiple fractures on its second stage planetary gear housing; the fractures were primarily orientated vertically and were through the thin-walled section of the housing. Analysis of these fracture surfaces revealed that the cracks were consistent with overstress from radial stresses. The transmission was disassembled and substantial damage was observed to the first stage planetary gear teeth and the input gear teeth. A high quantity of metallic chips and flakes were discovered at the bottom of the lower housing and throughout the first stage planetary assembly. All six bolts securing the bevel gear to the first stage planetary carrier were fractured. Only one of the six separated bolt heads remained secured with safety wiring. The six bolts were removed each bolt exhibited fatigue fracture features consistent with overstress, such as crack arrest and beach marks, that surrounded a center region. The fatigue cracks initiated along the thread roots of the bolts; these cracks were generally positioned about 180 degrees from each other, and were consistent with failure from reverse bending fatigue. After the bolts fractured, the bevel gear was unconstrained, leading to overstress fracture of the adjacent components. The fracture surface of the main rotor shaft was analyzed; the fracture surface was orientated 45 degrees from the longitudinal direction of the shaft, and the shaft was deformed to one side. All features were consistent with overstress failure in tension.

A complete set of maintenance records were not available for review, thus, it could not be determined if the bolts met installation requirements or if regular inspection of bolt tightness was met.

Text from the NTSB. Open the NTSB docket for WPR13FA243 for the full record.