History of Flight
On November 20, 2009, at 10:20 Hawaiian standard time, a McDonnell Douglas 369E helicopter, registration N551CR, experienced a left rear landing skid strut collapse while stationary with its engine running on a movable landing platform at the Turtle Bay Resort Heliport in Kahuku, Hawaii. The helicopter was operated by K & S Helicopters, Inc., doing business as Paradise Helicopters, under Part 135 regulations. Four passengers had boarded for a sightseeing tour of Oahu. The pilot increased engine power to flight idle in preparation for takeoff when the left rear landing gear gave way. The helicopter tilted left rearward, causing the tail stinger and vertical stabilizer to contact the ground, and the tail rotor blades struck the ground. The pilot shut down the engine and applied the rotor brake. The left rear landing gear strut fractured at the bolt hole where it attached to the fuselage drag brace. The tail stinger, vertical stabilizer, and tail rotor blades sustained damage.
Personnel Information
The pilot, aged 36, held a commercial pilot certificate with a rotorcraft-helicopter rating issued on January 4, 2006, and a second-class medical certificate issued June 26, 2009, with no limitations. He reported 6,200 total flight hours in helicopters.
Aircraft Information
The helicopter, a five-seat single-engine model, serial number 0534E, was manufactured in 1997. It was equipped with a Rolls Royce 250-C20B engine producing 450 horsepower. At the time of the incident, the airframe had accumulated 11,393 hours, and the engine 15,997 hours. The most recent engine inspection was performed 30 flight hours prior. The last 100-hour airframe inspection was conducted on November 7, 2009, at 11,361.8 hours total time (30.2 hours before the incident). During that inspection, the landing gear was examined per the manufacturer's maintenance program.
Examination Findings
The left and right rear landing gear struts were sent to the NTSB Materials Laboratory for analysis. The left strut was fractured into two pieces at the bolt hole attachment point for the drag brace. Microscopic examination revealed a planar, thumbnail-like area near one bolt hole with striations consistent with fatigue crack growth. Similar fatigue striations were found on the opposite side of the same bolt hole and extending downward. The remaining fracture surface showed a shear lip indicative of an overstress fracture.
The exterior of the fractured strut showed grinding marks around the bolt holes, consistent with abrasive paint removal. The intact right strut also exhibited grinding marks. Measurements of the right strut's bolt holes showed the smaller hole had an average diameter of 0.5057 inch (maximum allowed: 0.500 inch), and the larger hole averaged 0.5336 inch (maximum allowed: 0.5313 inch). The smaller hole also had evidence of cold working on its interior surface from bushing movement. A visible crack approximately 2 mm long was observed on the exterior of the intact strut adjacent to the cold-worked area.
A functional installation check was performed at MD Helicopters in Mesa, Arizona, using an MD 369E airframe. The right strut from the accident helicopter was installed on the aft right side, and a new strut on the left. Per the maintenance manual, the inspection involves jacking the helicopter until dampers are fully extended and checking for play. The right strut exhibited fore/aft and vertical play, while the new strut was completely rigid. The manual specifies maximum torque for the brace-to-strut hardware at 5 inch-pounds, with a caution not to exceed finger-tight. The actual torque on the accident helicopter's hardware was not measured.