NTSB Factual Report

On October 22, 2001, about 1700 hours Pacific daylight time, a MDHI 600N helicopter, N959SD, operated by the Los Angeles County Sheriff's Department, was found to have a fractured tail boom attach bolt during a preflight examination by a pilot. A routine ground examination of the helicopter was being performed prior to evening ground standby duty from the Sheriff's Aerobureau in Long Beach, California. The pilot stated that as he pushed sideways on the tail boom it seemed loose. Subsequent examination by maintenance personnel found that the head half of the upper right 5/16-inch tail boom attach bolt was hanging loose by the safety wire. Examination of the bolt revealed that it had failed about three threads from the bolt shank. The remainder of the threaded portion of the bolt remained in the plate nut affixed to the fuselage. Fretting was observed on the fuselage and tail boom mating points in proximity to the fractured bolt. Areas of the bolt shank exhibited polishing.

Subsequent examination for collateral damage revealed cracks were present at the aluminum attach fitting and associated angles. The operator examined two of their other 600N helicopters and found those with cracks at the same location. Two other local area law enforcement operators also found cracks in the same location. According to the Federal Aviation Administration (FAA) Aircraft Certification Offices responsible for certifying the 600N, a service bulletin and a priority letter airworthiness directive are in process.

The tail boom is attached to the fuselage with four special OEM produced bolts. The helicopter tail boom is required to be removed every 300 flight hours for inspection, and, in addition, is occasionally removed for other servicing procedures. Examination of the maintenance records disclosed that the bolts may have been through about 10 torque cycles. The bolts have no life limit. The failed 5/16-inch bolt, p/n HS5482-5-18, is torqued to 150-inch/pounds during installation.

On November 2, 2001, MDHI issued service bulletin SB600N-036, which prescribes procedures for inspecting the upper right tail boom, attach bolt, and attach fitting assembly.

"The Part 1 inspection requirements of this bulletin shall be accomplished within the next five (5) hours of helicopter operation after the receipt of this bulletin or within thirty (30) days of the issue date of this bulletin, whichever occurs first."

"The Part 2 inspection and repair requirements of this bulletin shall be accomplished within the next twenty-five (25) hours of helicopter operation after the receipt of this bulletin or within ninety (90) days of the issue date of this bulletin, whichever occurs first."

On November 28, 2001, the FAA issued Priority Letter Airworthiness Directive (Emergency AD) 2001-24-51, mandating compliance with the service bulletin 600N-036.

NTSB Probable Cause

The fatigue failure of a 5/16-inch boom attach bolt was most probably caused by the angular misalignment between the tail boom bolt hole and the fuselage mounted nutplate.