Nordman, ID, USA
N286M
Kaman K-600
While ascending in a hover to take the slack out of a long-line, the Part 137 logging helicopter experienced the complete failure of its right main rotor mast (shaft). An NTSB metallurgical inspection determined that the mast had failed due to the propagation of a fatigue crack that originated on the inside surface of an opening in the shaft wall (L-crank slot). The fatigue crack originated in two shallow corrosion pits (0.0033 and 0.0029 of an inch wide) on the inner surface of the L-crank slot. Although the shaft had been overhauled/inspected approximately 1,000 hours prior to the accident (1,200 hour interval), the Military Technical Order that regulates the overhaul of this mast does not address a specific inspection for corrosion, nor how to address any corrosion that is detected.
On November 20, 2003, approximately 1545 Pacific standard time, a Kaman Husky K-600 helicopter impacted the terrain about 8 miles northeast of Nordman, Idaho. The commercial pilot received fatal injuries, and the aircraft, which is owned and operated by JC Helicopter of Newport, Washington, sustained substantial damage. The 14 CFR Part 133 commercial long-line logging flight was being operated in visual meteorological conditions. The flight was being conducted at a remote logging site, and there was no filed flight plan. According to the individuals who were hooking the load of logs to the helicopter, after having its long-line hooked to multiple chocker cables, the helicopter was ascending in a hover in order to take the slack out of the long-line. About the time that the ends of the first logs of the load were going to be lifted off the ground, witnesses heard a loud bang and then saw numerous pieces of the rotor blades flying through the air. Immediately thereafter the helicopter dropped straight down from an altitude of between 150 and 200 feet above the ground. At the accident site, the right rotor mast was found to have separated on a fracture line that passed through the lower portion of one of the L-crank slots, and then proceeded around the remainder of the shaft's circumference, passing below the L-crank slot on the opposite side of the mast. The mast was recovered and shipped to the NTSB's Materials Laboratory division, where it underwent further examination and testing. That examination revealed fatigue arrest markings on the portion of the fracture that extended from one side of the L-crank slot and preceded downward at approximately 45 degrees. The initial 0.5 to 0.6 inch of the fracture propagation was smooth and finely textured, consistent with high cycle fatigue. From that point the texture of the fracture surface became rougher, consistent with a transition to a higher stress lower cycle fatigue propagation. The total length of the fatigue measured about 1.5 inches on the inner diameter of the shaft, and about 2.0 inches on the outer diameter. The texture of the remainder of the fracture surface was consistent with overstress separation stemming from the region of fatigue. After the primer and paint were chemically stripped from the location of the initiation of the fracture, the origin of the fatigue crack propagation was examined with a scanning electron microscope (SEM), and it was determined that the fatigue initiated in two immediately adjacent shallow corrosion pits. One pit measured 0.0033 across, and the other measured 0.0029. Both had a maximum depth of about 0.0013 inch. Additional corrosion pits were found on the surface of the L-crank cutout adjacent to the origin of the subject initiation point. Most of these pits measured less than 0.0022 inch, but some were as wide as 0.0077. During the investigation it was determined that this helicopter had previously been operated by the Department of Defense (United States Air Force), and had been sold to the public as a surplus asset. Because it was formerly maintained through a military maintenance program, and was now being operated in the restricted category by a civilian company, portions of its current maintenance and inspection program were determined by a series of Military Technical Orders. Specifically, the rotor shaft assembly inspection process was defined by the requirements of Technical Order 3R12-2-3 (Rotor Shaft And Housing Assembly Overhaul). The overhaul inspection described in section 2-16 of Technical Order 3R12-2-3 calls for the shaft to be "visually inspected" for damage and "dimensionally inspected" for the fit of the mating surfaces. It also calls for a magnetic-particle inspection in accordance with Military Specification MIL-I-6868 for the purpose of detecting "...defects resulting from service use." The Technical Order inspection process and its accompanying illustration does not specifically address an inspection for corrosion, nor does it address corrosion limits or actions to be taken to deal with corrosion if detected. A further review of the aircraft's maintenance records revealed that the mast and housing assembly (Part Number K774504-605, Serial Number AK4-912) was last overhauled on August 8, 2002, at which time it had accumulated 1,424.8 hours in service. At that time mast Serial Number A16-62, which had undergone non-destructive testing on April 29, 2002, was installed in the assembly. This assembly was then disassembled, lubricated, and reassembled on August 5, 2003, at which time it had accumulated 2,044.6 hours in service, and 619.8 hours since overhaul. At the time of the accident, the mast and housing assembly was approximately 1,000 hours into its 1,200-hour overhaul cycle. As part of the investigation, the Federal Aviation Administration Bioaeronautical Science Research Laboratory completed a toxicological examination on samples taken from the pilot. This examination determined that there was no carbon monoxide or cyanide detected in his blood, and no ethanol detected in his vitreous fluid. The same examination detected Pseudoephedrine (a common medication used to treat symptoms associated with hay fever) in the pilot's blood and liver. The level of the concentration of the Pseudoephedrine was not stated.
The total failure of the helicopter's right main rotor shaft while in an ascending hover due to the propagation of a fatigue crack that initiated in a corrosion pit on the surface of a slot in the wall of the subject shaft. Factors include accumulation of corrosion on the surface of the shaft.
Source: NTSB Aviation Accident Database
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