CEN15FA126

An in-flight failure of the helicopter's #2 main rotor spindle due to undetected fatigue cracking, which resulted in an in-flight breakup. Contributing to failure were the nonconforming thread root radius of the spindle and the manufacturer's failure to include a bending moment within the spindle threads when performing the fatigue analysis during initial design of the spindle.

The flight instructor and the student pilot were conducting a local instructional flight in the helicopter. The helicopter was on final approach to land when one of the three main rotor blades (#2 blade) separated from the main rotor head. The main transmission and main rotor head (with #1 and #3 blades still attached) then separated from the helicopter, and the helicopter descended to ground impact. Examination of the wreckage revealed that the separation of the #2 blade was due to fracture of the #2 main rotor blade spindle. The #2 spindle fractured at the inboard end of the threads. Metallurgical analysis of the fractured spindle revealed signatures consistent with a fatigue crack initiating from multiple origins that propagated across 92% of the spindle cross-section; the remaining 8% of the fracture surface exhibited signatures consistent with overload. The high percentage of stable fatigue fracture growth versus overload suggested that low loading propagated the crack. Further, corrosion was visible on the fracture surface in the fatigue initiation area, which indicated that the crack had been present and growing for some time. Similar fatigue cracks were observed emanating from thread roots on the #1 and #3 spindles; the crack in the #3 spindle emanated from the inboard-most thread, similar to the crack in the #2 spindle, and the crack in the #1 spindle emanated from the cotter pin hole. Both of these cracks were shorter than the crack in the #2 spindle, and the fatigue on their fracture surfaces had only propagated a small amount through the #1 and #3 spindle cross-sections. The root radii of the threads on all three of the spindles did not meet the thread form specified on the manufacturer's drawing. Cross-sections of the threads from the three spindles showed that the #2 spindle and the #3 spindle had flat-bottomed threads; the #1 spindle did not have flat-bottomed threads but had threads with a sharper than specified root radius. A subsequent finite element analysis determined that the flat-bottomed threads and the sharper than specified root radius threads would result in higher stresses at the threads, which likely contributed to the crack initiation within the threads. Further, the investigation determined that the predicted loads used in the original fatigue analysis during the design of the spindle did not account for a bending load at the spindle threads. The omission of bending loads during design and the flat-bottom quality of the threads led to stresses greater than used in the original fatigue life calculation by the manufacturer. Further, the finite element analysis of the spindle revealed that, once a crack has initiated, the bending loads at the spindle threads would be sufficient to propagate the crack at a rate similar to that of the total time accumulated on the #2 spindle (about 9,300 flight hours). Before the accident, the spindle was not a life-limited part, and there were no recurrent inspections specified for the spindle threads, resulting in a low likelihood of the operator detecting the fatigue fracture within the spindle threads before the accident. Following this accident, the manufacturer released a service directive bulletin and the Federal Aviation Administration released an airworthiness directive that require a magnetic particle inspection of spindles with 1,500 hours or more time in service.

HISTORY OF FLIGHT On January 26, 2015, about 1202 mountain standard time (MST), an Enstrom 280FX, N86235, was destroyed when it impacted terrain during final approach for landing at Erie Municipal Airport (EIK), Erie, Colorado. Both the instructor pilot and student pilot were fatally injured. The helicopter was registered to New Course Aviation Company and operated by Mountain One Helicopters as a 14 Code of Federal Regulations Part 91 instructional flight. Visual meteorological conditions prevailed for the local flight, which operated without a flight plan. According to a witness interview, the helicopter was on final approach to land on the runway at what seemed like a steep angle of descent. As the helicopter descended, she heard a loud "pop." After the pop, the helicopter began to rotate and then the main rotor (MR) blades separated from the top of the helicopter. The remainder of the helicopter descended to ground impact and then "exploded." All three MR blades were found during the on-scene investigation and were identified as blades #1 through #3. The #2 MR blade was found separated from the MR head, which still contained MR blades #1 and #3. Post accident examination revealed that the #2 MR blade had separated due to a fractured spindle. PERSONNEL INFORMATION AIRCRAFT INFORMATION The helicopter, an Enstrom 280FX, was a three-seat, single-engine helicopter (serial number 2002) manufactured in 1985. The three-bladed MR system was powered by a turbo-charged Lycoming HIO-360-F1AD engine rated at 225 horsepower at 3050 rpm. The inboard end of each main rotor blade is attached to its respective grip and drag link. Each rotor blade is attached to the outboard side of its respective grip via a retention bolt. At the inboard side of each grip, a pitch arm and drag link are attached to the leading and trailing sides, respectively, of the grip. Two feather bearings, which accommodate rotor blade pitch change motion, a bearing spacer, and a spindle are installed within the grip. A Lamiflex bearing, a spindle retention nut, and a cover are installed at the threaded outboard end of the spindle. Two flanges at the inboard end of the spindle attach to a universal block, the latter of which is attached to the main rotor hub, that accommodate rotor blade flapping and lead-lag motions. See figure 1. Figure 1: Enstrom spindle and grip assembly According to the maintenance records, the most recent 100-hour inspection was completed on March 17, 2014, at an airframe total time of 9,167 hours. During this annual inspection, all three Lamiflex bearings were replaced. Additionally, all three MR grip assemblies were removed to "purge and clean" the flapping hinge bearings, then were reassembled and preloaded per the Enstrom Maintenance Manual. Lastly, the leading-edge tape was replaced on one of the MR blades; the helicopter was then flight tested and subsequently adjusted for proper track and balance. No other maintenance to the MR grips or blades was performed during this inspection or at any other time between the inspection and the accident. According to the maintenance records, the #2 spindle, part number (P/N) 28-14282-13, serial number (S/N) 03-014-84N, had not been replaced since the helicopter's manufacture. At the time of the accident, the spindles were not life limited, and there were no recurrent inspections specified for the spindle threads. According to the Enstrom Maintenance Manual, the Lamiflex bearings required replacement every 5 years. When the MR is assembled, the spindle threads reside under the retention nut retaining the Lamiflex bearing. The retention nut must be removed in order to examine the entire threaded region of the spindle for damage. It is unknown whether a thread crack, once grown to a sufficient depth, would produce an out-of-track MR blade. Examination of the maintenance records for the accident helicopter revealed at least 9 instances, from the time the helicopter was manufactured, of Lamiflex bearing replacement for the #2 spindle. (The Enstrom Maintenance Manual the Enstrom Maintenance manual requires the lamiflex bearing to be replaced every 5 years.) However, there was no pattern in the Lamiflex bearing replacement intervals or interrelated note of the helicopter's MR track and balance or the helicopter's flying characteristics. During manufacture, the cotter pin hole in the spindle is match-drilled with the spindle retention nut installed. There is no specific requirement on the dimension for the cotter pin hole in the engineering drawing to break the sharp edges or chamfer the edges at the cotter pin hole after it is match-drilled. Breaking or chamfering the sharp edges is typically used to reduce stress concentrations that may arise from the matched-drill process. METEOROLOGICAL INFORMATION The 1155 weather observation at EIK reported no wind, visibility 10 miles, clear skies, temperature 09ºCelsius (C), dew point 11ºC; barometric altimeter setting 30.18 inches of mercury. AIRPORT INFORMATION EIK is a publicly-owned airport without an air traffic control tower. The airport was equipped with one runway designated as runway 15/33. Runway 15/33 was a 4,700-ft-long by 60-ft-wide runway and was reported as "in good condition." The elevation of the threshold of runway 15 was about 5,076 ft above mean sea level. WRECKAGE AND IMPACT INFORMATION The helicopter came to rest upright with the right skid low off the east side of the runway. The main wreckage contained the main fuselage (flight deck, skids, tail boom) and engine. The aircraft wreckage displayed severe damage from a high energy vertical impact and post impact fire. The MR head (with the #1 and #3 MR blades still attached) and MR gearbox were located about 299 ft from the main wreckage at a heading of 350º. The threaded portion of the spindle and associated retention nut on the #2 blade was missing; however, the inboard portion of the spindle was still attached. The #2 MR blade assembly, including the blade, the grip, the threaded portion of the spindle, the retention nut, and the drag link, was located about 657 ft from the main wreckage at a heading of 263º. The #2 blade did not exhibit any damage or indications of contact with any part of the aircraft. An approximate 18-inch section of the leading edge spar and balance weight for the #1 rotor blade was located about 493 ft from the main wreckage at a heading of approximately 248º. The tail rotor blades were found about 290 ft and 666 ft from the main wreckage, at headings of 276º and 325º, respectively. Several other pieces of wreckage were scattered around the approach end of runway 15. The tail rotor gearbox was not located. Flight control continuity was established in the cabin area from the controls up to the MR gearbox and MR hub. The Hobbs meter located in the center pedestal indicated 9,332.7 hours. The tailboom exhibited a MR blade strike. Examination of the MR gearbox mounts indicated that the gearbox mounts were intact and separation from the airframe occurred at the pylon. The overrunning clutch operated normally. During the wreckage examination at the recovery facility, the #2 MR blade assembly (S/N 3469) was disassembled by removing the blade from the grip and removing the Lamiflex cover. The retention nut was found behind the cover with the fractured threaded end of the spindle still attached to the nut, and the cotter pin installed. Additionally, the Lamiflex bearing and shims were located behind the cover. The Lamiflex bearing (S/N 9383) had a manufacturing date of February 7, 2013. TEST AND RESEARCH Materials Laboratory Part Examination The three sets of spindles, nuts, shims, grips, and Lamiflex bearings from the helicopter's rotor arm assemblies were sent to the NTSB Materials Laboratory in Washington, DC for detailed examination. #2 Spindle The threaded end of the #2 spindle had separated in-flight a