Kahuku, HI, USA
N551CR
MCDONNELL DOUGLAS HELI CO 369E
The helicopter was positioned on a landing platform. Four passengers had boarded the helicopter for a sightseeing tour. The pilot brought the engine up to flight idle, preparing for takeoff, when the left rear landing skid strut collapsed. The helicopter tilted to the left rear, and the tail skid and vertical stabilizer struck the ground and sustained damage. The tail rotor blades also struck the ground. The pilot shut down the engine and stopped the rotor using the rotor brake. A postaccident examination of the left rear landing gear strut revealed that the strut fractured at the bolt hole where the strut was attached to the drag brace. The struts had been inspected 13 days (30.2 flight hours) prior to the accident as part of a 100-hour inspection in accordance with the manufacturer’s maintenance inspection program. Examination of the broken strut (left rear) revealed that it fractured due to a fatigue crack that initiated from the circumference of one of the bolt holes used to secure the strut to the drag brace. The intact strut (right rear) was also examined and found to have fatigue cracks growing outwards from the circumference of the two bolt holes used to secure the strut to the drag brace. Additionally, the diameters of the bolt holes were larger than the maximum allowed as outlined in the maintenance manual. The right strut was installed on an exemplar airframe in order to determine if any play could be identified in the strut. The 100-hour landing gear inspection was performed and the strut exhibited play, which according to the manufacturer’s maintenance manual requires removal and additional inspections.
HISTORY OF FLIGHT On November 20, 2009, at 1020 Hawaiian standard time, a McDonnell Douglas 369E helicopter, N551CR, sustained substantial damage when the left rear landing skid strut collapsed while the helicopter was standing with its engine operating at the Turtle Bay Resort Heliport, Kahuku, Hawaii. K & S Helicopters, Inc., d.b.a. Paradise Helicopters, operated the helicopter under the provisions of Title 14 Code of Federal Regulations Part 135. The commercial pilot and four passengers were not injured. Visual meteorological conditions prevailed, and a company flight plan had been filed. The helicopter was positioned on a movable landing platform. Four passengers had boarded the helicopter for a sightseeing tour of Oahu. The pilot brought the engine up to flight idle, preparing for takeoff, when the left rear landing gear collapsed. The helicopter tilted to the left rear and the tail stinger and vertical stabilizer contacted the ground. The tail rotor blades also struck the ground. The pilot shut down the engine and stopped the rotor using the rotor brake. Examination of the left rear landing gear strut revealed that the strut fractured at the bolt hole where the strut was attached to the fuselage drag brace. The tail stinger and the vertical stabilizer were bent, and the tail rotor blades were damaged. PERSONNEL INFORMATION The pilot, age 36, held a commercial pilot certificate with a rating for rotorcraft-helicopter issued on January 4, 2006, and a second-class medical certificate issued June 26, 2009, with no limitations. The pilot reported that he had 6,200 flight hours in helicopters. AIRCRAFT INFORMATION The five seat single engine helicopter, serial number (S/N) 0534E, was manufactured in 1997. It was powered by a Rolls Royce 250-C20B, 450-hp engine. The operator reported that the airframe had 11,393 hours on it at the time of the accident, and the engine had 15,997 hours. The last engine inspection was performed 30 hours prior to the accident. The most recent airframe inspection was a 100-hour inspection performed on November 7, 2009, at a helicopter total time of 11,361.8 hours, 30.2 hours prior to the accident. As part of the 100-hour inspection, the landing gear was inspected in accordance with the manufacturer’s maintenance inspection program. TESTS & RESEARCH The rear left and right landing gear struts were sent to the National Transportation Safety Board’s Material Laboratory for examination. Of the two struts submitted for examination, one was fractured and consisted of two pieces (left strut). The fractured skid strut tube had broken in the area of the bolt holes used to attach the strut to the drag brace. A section containing one end of the fracture was cut off the smaller portion of the strut to facilitate examination under the stereo microscope and in the scanning electron microscope (SEM). Examination of the fracture surface under the stereo microscope revealed an area adjacent to one of the bolt holes which had a planar, thumbnail like appearance. This area was further examined with the SEM, which revealed a series of striations consistent with the growth of a crack due to fatigue. Additional areas exhibiting striations consistent with a fatigue crack were also observed on the opposite side of the same bolt hole and spreading downwards along the fracture surface. The remainder of the fracture surface had a distinct shear lip, consistent with an over stress fracture. On the exterior surface of the fractured strut, in the areas surrounding the bolt holes, evidence was noted of grinding consistent with an abrasive process to remove paint. On the exterior surface of the intact skid strut tube there was also evidence noted of grinding around the area of the bolt holes. Measurements of the diameter of the bolt holes were made on the intact (right) skid strut. The smaller bolt hole was found to have an average diameter of 0.5057 inch (the maximum diameter allowed per the maintenance manual is 0.500 inch). The larger bolt hole was found to have an average diameter of 0.5336 inch (the maximum diameter allowed per the maintenance manual is 0.5313 inch). In addition to being oversize, the smaller of the two bolt holes had evidence of cold working on the interior surface of the bolt hole. This cold working was consistent with the bushing, which would have been installed through the bolt hole, repeatedly pressing against the bore of the hole due to relative movement between the two. On the exterior surface of the intact strut, adjacent to the area of cold working in the bore of the bolt hole, a crack is visible. This crack was approximately 2 mm long and could be seen with the unaided eye. The full Materials Laboratory Factual Report is available in the official docket of this investigation. Following examination by the NTSB Materials Laboratory, the struts were brought to the MD Helicopter facility in Mesa, Arizona, under supervision of the Safety Board IIC, for a functional installation check on a MD 369E airframe, in accordance with MD Helicopters, Inc. Maintenance Manual (CSP-HMI-2, Chapter 32-10-00, page 602, Landing Gear Strut Inspection and Chapter 32-10-00, page 401, Landing Gear Replacement). The landing gear strut inspection is incorporated into the 100-hour or annual inspection. The aft left side of the exemplar airframe was fitted with a new strut, the aft right side of the airframe was fitted with the right strut from the accident helicopter. According to the Maintenance Manual, the inspection procedure is to jack the helicopter until the landing gear dampers are fully extended, shake the landing gear assemblies, and note any play or looseness at pivot bearings, inboard strut assembly fittings, and drag brace holes. If any play or looseness is noted the landing gear is to be removed and the strut requires further inspection. The results of the inspection revealed that the right strut exhibited some fore/aft and vertical play of the strut, while the new strut on the left side was completely rigid, exhibiting no play under the test conditions. The MD Helicopters Maintenance Manual describes the installation of the drag brace with the attaching hardware as follows.”Attach brace to strut with attaching hardware. Torque nut to 5 inch-pounds (0.56 Nm) maximum.” A caution note precedes the installation step. The caution states, “Do not over-tighten brace-to-strut hardware. Maximum permissible nut torque is finger-tight; then backed off to nearest castellation for installation of cotter pin.” The actual torque on the brace-to-strut hardware on the accident helicopter was not measured.
The fatigue failure of the landing gear strut and maintenance personnel’s failure to perform an adequate inspection of the landing gear.
Source: NTSB Aviation Accident Database
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