Sikorsky S-58HT N1078T accident near Belmont, West Virginia

Probable cause

The fatigue failure of a tail rotor blade during an external load lift.

NTSB analysis

The helicopter was performing external load operations, and was removing a tower from the top of a 620-foot tall smoke stack. After lifting the load clear of the smoke stack, the pilot observed an "over torque" condition on the torque indicator. Several seconds later, the pilot felt a high frequency vibration through the helicopter, and shortly thereafter the tail rotor assembly separated from the helicopter. While the helicopter yawed through two 360-degree rotations, the pilot released the external load, reduced the collective pitch, and performed an autorotation. Postaccident examination of the separated tail rotor assembly revealed that one of the four blades had separated in-flight. Detailed examination of the separated blade revealed that its skins had cracked due to fatigue, and that the blade then separated due to overstress. The adjacent and overlaying doublers also cracked, likely due to fatigue, but no fatigue striations were found on those members. Some ductile dimples were also found in the damaged regions, indicating that at least part of the propagation was by unstable crack growth. The skin fatigue cracks were completely hidden by the overlying doublers, making detection impossible unless the doublers either cracked or were removed. Cracks in the doublers would have been visible on the inboard side of the blade, but not on the outboard side, where the inspection/repair tag covered most of the doubler. The bonds within the blade appeared to be in good condition, and no corrosion was found, suggesting that the blade failure was the result of a fatigue life issue. The helicopter’s maintenance manual specified that the tail rotor blades had no life limit, provided that certain flight restrictions were observed, but the investigation was unable to determine whether the helicopter ever exceeded any of the specified flight restrictions. None of the tail rotor blades were in compliance with an existing airworthiness directive, which required the installation of an abrasion strip along the entire length of the blade leading edge; however, it was not clear if this non-compliance affected the failure.

Text from the NTSB. Open the NTSB docket for NYC08LA129 for the full record.