Accident Details
On July 28, 2010, about 0900 eastern daylight time, an experimental Garlick Helicopters OH-58A+, registration N48LA, operated by B & S Air Inc., was substantially damaged while standing in a field with the engine operating near Newport, Tennessee. The certificated commercial pilot incurred minor injuries. The aerial application flight was conducted under 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed, and no flight plan was filed for the planned flight to Jackson, Tennessee.
The pilot reported performing a normal preflight inspection, finding no problems, and completing a normal start procedure. After increasing rotor rpm to 100 percent flight idle, the pilot was about to increase collective when he felt a violent shake, heard a loud noise, and the helicopter began to come apart. He observed the transmission and rotor blades on the ground in front of the helicopter, then shut off the engine and exited.
Aircraft Information
The helicopter was manufactured in 1970 and delivered to the U.S. Army. In 1995, it was transferred to state law enforcement. Subsequently, it was equipped for agricultural application and issued an FAA experimental airworthiness certificate in 2005. According to maintenance records and the pilot, the most recent 100-hour inspection was completed on June 15, 2010, at which time the helicopter had accumulated approximately 6,704 total hours of operation. An additional 76 hours had been accumulated between that inspection and the accident.
Examination Findings
An FAA inspector examined the wreckage and found that an outboard 2-foot section of main rotor blade had separated. The transmission had also separated, resulting in substantial damage to the fuselage. The separated blade section was forwarded to the NTSB Materials Laboratory for metallurgical examination.
Metallurgical examination revealed that a fatigue crack originated from an inertia screw hole located at the bottom surface of the spar portion of the main rotor blade. The crack propagated more than 50 percent of the spar cross section before fracturing consistent with overstress.
Prior Maintenance Notices
According to a representative from the helicopter manufacturer, Bell Military Alert Bulletin USA-OH-58-87-1 was issued on June 12, 1987, requesting military operators to visually inspect the inertia screw hole locations on the blade spar with a 10X magnifying glass for cracks and corrosion. The inspection was to be performed every 8 flight hours or 32 cumulative flights. On April 12, 2007, Bell Operations Safety Notice OSN-GEN-07-38 was issued to all OH-58 operators, referencing the earlier bulletin.
According to the FAA inspector, the outboard section of main rotor blade was secured with eight inertia screws. Although the screw holes were supposed to receive a maintenance inspection every 8 flight hours, they were also a preflight inspection item for the pilot. Review of the helicopter's maintenance logbooks revealed an entry stating that the inertia screw holes had been inspected within the past 8 flight hours with no anomalies noted. However, the inspector observed corrosion present in the inertia screw hole that failed.