Pecan Island, LA, USA
N501PH
BELL 407
The operator reported that the helicopter air taxi flight was in cruise about 1,000 ft above ground level when the pilot felt an impact and a strong vibration. The pilot completed an instrument and functional control check and could not immediately identify any anomalies. The pilot stated that, as he slowed the helicopter for landing, he noticed a "heavy mechanical sound and strong vibration." The vibration worsened, and the pilot began to have difficulty controlling the helicopter; he subsequently initiated an autorotation and deployed the helicopter's floats. The helicopter touched down in a marshy area, and the pilot and passengers egressed. During the landing, the main rotor blades contacted the tail boom and one of the tail rotor blades, resulting in separation of the tail rotor gearbox (TRGB) support structure, which was subsequently located in the marsh. A postaccident examination and metallurgical analysis revealed that fatigue fractures on two of the four TRGB attachment studs likely existed before the accident flight. As the fatigue fractures grew larger through the first two attachment studs, their load-carrying capability lessened, and the additional load was transferred to the remaining attachment studs. The progressive failure of the fatigued TRGB attachment studs led to the vibrations felt by the pilot and, ultimately, the uncommanded right yaw and subsequent loss of tail rotor control. The reverse- bending, high-cycle fatigue fracture initiation mode observed on two of the TRGB studs suggests the fatigue fractures were a result of a loss of torque of the attachment stud nuts. The reason for the loss of torque could not be determined based on the available information.
On June 8, 2015, at 1432 central daylight time, a Bell 407 helicopter, N501PH, made an autorotation to the ground near Pecan Island, Louisiana. The airline transport rated pilot and four passengers were not injured. The helicopter sustained substantial damage. The helicopter was registered to and operated by PHI Inc., Lafayette, Louisiana, under the provisions of 14 Code of Federal Regulations Part 135 as an air taxi flight. Visual meteorological conditions prevailed at the time of the accident and a company flight plan was filed. The flight originated from Vermilion Block 256-E in the Gulf of Mexico about 1400 and was en route to Pecan Island. The pilot and operator stated that the helicopter was in cruise flight about 1,000 ft above ground level when the pilot felt an impact and a strong vibration of the helicopter. The pilot completed an instrument and functional control check and could not immediately identify any anomalies. Soon after, the pilot initiated an airspeed and power reduction and noticed a heavy mechanical sound and strong vibration. The vibration worsened and the helicopter began a slow right turn so the pilot entered an autorotation. The pilot noticed that as the engine power was reduced further, the helicopter began to oscillate and he experienced difficulty maintaining directional control. With the floats inflated, the pilot made a hard forced landing into a marsh with tall grass. During the landing, the tail rotor gear box (TRGB) separated from the helicopter and was later located in the marsh. The helicopter was equipped with Outerlink, which recorded several of the helicopter's parameters, including GPS location, at 30 second intervals. The data was used to correlate the pilot's recollection of the anomalous vibrations, helicopter location, and timeline. On June 24, 2015, representatives from the FAA, Bell Helicopter, PHI, and the NTSB convened at Bell Helicopter facilities in Hurst, Texas, to examine the recovered tail rotor head, tail rotor blades (TRB), TRGB, TRGB support structure, and remnants of the flexible coupling that was still attached to the TRGB input flange. The exterior of the gearbox exhibited light damage and dirt consistent with immersion in the marsh. The bottom surface of the four mounting feet exhibited evidence of corrosion from exposure to the brackish water. All four TRGB attachment studs were fractured. The four attachment stud locations were labeled "A", "B", "C", and "D" for the purpose of the examination. All gearbox-side attachment studs remained within the gearbox housing; the mating half from attachment stud A was recovered from the accident site. Three of the four TRGB attachment studs exhibited signatures of fatigue fracture. Attachment stud A exhibited reverse bending fatigue through the majority of its cross-section. Attachment stud B exhibited reverse bending fatigue through about 2/3 of its cross-section, and exhibited signatures of low cycle fatigue and overload through the remaining 1/3 of its cross-section. Attachment stud C exhibited signatures of low cycle fatigue and overload. Attachment stud D exhibited signatures of overload. The reverse bending fatigue found on attachment studs A and B were primarily in the lateral axis. Multiple tool ratchet marks were observed at the reverse bending fatigue origins. The recovered TRGB support structure exhibited multiple fractures consistent with overload. Impact damage consistent with main rotor blade contact was observed on the forward end of the structure. Mechanical damage and rotational scoring was observed near the forward end near the area where the TRGB input flange and flexible coupling are normally located; the damage exhibited a shiny, silver-colored appearance. The four TRGB mount spot faces exhibited evidence of fretting damage adjacent to the TRGB mount bores. Fractures were observed through the thickness of the mounting bores for attachment studs A and B; the fractures exhibited signatures consistent with overload. Additionally, the bores for attachment studs A and B exhibited thread impressions along the length of the bore and the entirety of the bore circumference. Lastly, the bores for attachment studs A and B, normally circular in shape, exhibited elongation in the same direction as the reverse bending fatigue observed on the studs. The bores for attachment studs C and D exhibited thread impressions along the length of the bore along the fore-aft axis. Evidence of sealant was observed on the TRGB mount spot faces. Remnant pieces of flexible coupling remained attached to the TRGB input flange (driveshaft adapter) at its two bolted locations. The fracture surfaces of the remnant flexible coupling exhibited signatures consistent with low cycle reverse bending fatigue. When disassembled, evidence of corrosion and corrosion byproducts were observed in the interior surfaces of the TRGB housing and the input quill duplex bearing assembly. The TRGB input flange could not be rotated manually. The two TRBs remained attached to the tail rotor hub; one was relatively intact with minor exterior damage and the other exhibited leading edge damage and the outboard section was separated near midspan. The damaged TRB exhibited signs of high-energy contact with a main rotor blade. On August 16, 2016, the TRGB mounting studs were re-examined for evidence of striations. The fracture surfaces of attachment studs A and B were examined under a scanning electron microscope (SEM) and revealed a significant amount of mechanical contact and corrosion damage. The damage precluded a striation count to estimate a crack growth rate for attachment stud A. On attachment stub B, a localized area of intact striations was observed between the mechanical contact damage, which revealed a localized fatigue striation spacing of about 0.000016 inches. On September 21, 2016, the damaged TRB was re-examined for evidence of proper bonding. Examination of the tip block bond line on the outboard portion revealed pieces of chopped fiberglass, which is a material used to manufacture the tip block. The tip block adhesive exhibited signatures consistent with a cohesive failure. The tip block adhesive showed evidence of adequate tip block adhesion to the blade skin. The operator's maintenance personnel performed a postaccident damage assessment and found a small hole in the aft bulkhead fairing. The hole was continuous to the aft baggage compartment where a brass threaded stud was found inside. The stud was identified as a tail rotor tip block weight. About three weeks prior to the accident, on May 13 and 15, 2015, the tail rotor blades were repaired and inspected by an outside vendor. On May 21, 2015, the accident tail rotor assembly was reinstalled and balanced by the operator, at an aircraft total time of 16,624 hours, 13 flight hours prior to the accident.
The fatigue fracture of the tail rotor gearbox attachment studs, which resulted in a loss of tail rotor control and a subsequent hard landing.
Source: NTSB Aviation Accident Database
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