On January 20, 2012, about 1900 Pacific standard time, an Arrow Falcon Exporters, Inc., OH-58A helicopter, N902SF, sustained substantial damage when its main rotor mast fractured during takeoff near Brawley, California. The commercial pilot, the sole occupant, was not injured. Farm Aviation, Inc., was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 137. Night visual meteorological conditions prevailed for the aerial application flight, and no flight plan was filed. The helicopter was departing for a local flight when the accident occurred.
The owner of the company stated that the aircraft was performing outlying operations and was operating off the back of a truck. The helicopter had just been refueled and loaded with applicant. When the pilot pulled up on the collective to begin the takeoff, the main rotor mast fractured near the point where the mast extends from the transmission, and the pilot aborted the takeoff.
Both pieces of the fractured mast were examined at the National Transportation Safety Board's Materials Laboratory. The fracture was located near the point where the mast extends from the transmission below the threads for the mast nut and above the bearing journal for the upper mast bearing. Multiple fatigue cracks were found that initiated at the outer diameter of the mast. The overall extent of fatigue was estimated to be through about 50 percent of the mast's cross section. The fatigue initiation areas and the through-the-wall portion of the fatigue were heavily corroded with extensive pitting and dark deposits visible. A dark oily material was found on the mast's outer surface in the vicinity of the fracture, and the mast's surface was severely corroded on both sides of the fracture. Fourier-Transform Infrared spectroscopy analysis of samples of the dark oily material suggested that the material was a mixture of lubricating oil and water.
When the mast is installed on the helicopter, the location where the corrosion and fracture occured is hidden. To fully inspect the area, the mast assembly must be removed from the helicopter and completely disassembled.
The helicopter was initially manufactured by Bell Helicopter as a military aircraft. On December 5, 2003, it was certificated as a civil aircraft in the restricted category with Arrow Falcon Exporters listed as the aircraft manufacturer.
Review of the helicopter's maintenance records by a Federal Aviation Administration (FAA) inspector indicated that the mast's most recent 2,400-hour inspection, which included a magnaflux inspection, was completed on February 27, 2007. The mast was installed in another helicopter (N6251G) from March 1, 2007, to October 26, 2011, and accumulated 2,235.4 flight hours during this time. On October 26, 2011, the mast was installed in the accident helicopter, and, at the time of the accident, the mast had accumulated a total of 2,499 flight hours since the overhaul in February 2007. When the mast fractured, it had been operated for 99 hours beyond the manufacturer's recommended overhaul time of 2,400 hours.
A similar mast failure on an Arrow Falcon Exporters OH-58C occurred on July 3, 2011, near Brentwood, California (WPR11LA303). In that case, the most recent overhaul of the main rotor mast was completed 1,486 hours before the accident. On February 20, 2012, Arrow Falcon issed Alert Service Bulletin 2012-58-01, which reduced the overhaul interval for the main rotor mast assemblies on its OH-58s from 2,400 to 1,200 hours.
On July 23, 2012, the FAA 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. The AD also required performance of specific inspections for a crack, pitting, or corrosion in the threaded area of the mast and associated parts.