Arrow Falcon Exporters Inc OH-58C N6264D accident near Brentwood, California

Probable cause

The failure of the main rotor mast due to fatigue cracking originating from corrosion pits.

NTSB analysis

During an aerial application flight, the helicopter's main rotor mast separated, and the helicopter fell from a height of about 30 feet and impacted terrain. Postaccident examination of the wreckage found that the mast had fractured below the point where it extended from the transmission and just above the mast seal retainer plate. Examination of the fracture surfaces revealed that an area of fatigue extended about 95 degrees around the mast’s circumference. The remaining portions of the fracture were identified as low cycle fatigue and overload. The fracture had multiple origins at corrosion pitting on the threads of the mast. The source of the corrosion could not be determined.

Maintenance records indicated that the most recent overhaul of the main rotor mast assembly was completed 1,486 hours before the accident. At that time, a magnaflux inspection was performed on the main rotor mast with no discrepancies noted. The records indicate that, during reassembly, a sealant was applied to the threads for corrosion protection.

When the mast is installed on the helicopter, the location where the corrosion and fracture occurred is hidden. To fully inspect the area, the mast assembly must be removed from the helicopter and completely disassembled.

A second mast failure on a helicopter of the same make and model series occurred on January 20, 2012, in Brawley, California (WPR12LA096). In February 2012, the helicopter manufacturer issued an alert service bulletin that reduced the main rotor mast overhaul interval from 2,400 hours to 1,200 hours. In July 2012, the Federal Aviation Administration issued Airworthiness Directive (AD) 2012-14-11, applicable to Arrow Falcon Exporters, Rotorcraft Development Corporation, and San Joaquin Helicopters model OH-58A, OH-58A+, and OH-58C helicopters. The AD required, within 30 days, overhaul of the main rotor mast assemblies of these helicopters.

Text from the NTSB. Open the NTSB docket for WPR11LA303 for the full record.