Hughes 369D N9159F

The pilot reported that he had heard a whistling sound with the engine while conducting powerline operations and landed out of precaution. He examined the engine and consulted with maintenance personnel, but no mechanical anomalies were found. The helicopter was grounded for the workday; however, at the end of the day, a decision was made to try and return the helicopter back to its base of operations. During the flight, the engine chip light illuminated followed by the smell of engine oil and a grinding noise. The pilot attempted a precautionary landing to a field, but smoke filled the cabin, reducing his visibility while in the landing flare. The pilot attempted to slow the rate of descent and impacted the ground in a near-level attitude. During the ground run the front portion of the skids dug into the ground, causing the helicopter to pitch forward. The pilot applied aft cyclic to keep the helicopter level. During the landing sequence the main rotor blades struck the tail boom, which resulted in the horizontal and vertical stabilizers and the tail rotor assembly separating from the helicopter. Postaccident examination of airframe and engine revealed residual oil on the interior and exterior of the engine access doors and on the interior of the engine compartment. The oil supply line that feeds oil to the Nos. 6 and 7 bearings was fractured along with its support bracket. Numerous other components including the gearbox housing, N1 coupling, gas producer (GP) turbine support assembly (which included a sump nut, retaining ring and plate, the No. 8 oil supply jet, and a fractured shear pin), No. 8 bearing, No. 8 rotating seal, No. 8 stationary seal, and the outer combustion chamber, had also fractured and/or sustained high heat damage. Evidence of fretting damage was also observed on multiple components. The National Transportation Safety Board (NTSB) Materials Laboratory analyzed these components and determined that the engine most likely failed due to bearing failures in the turbine section resulting from the high-cycle fatigue fracture of the oil supply line that fed oil to the Nos. 6 and 7 bearings. The oil line failure led to rapid deterioration of the bearings from oil starvation, resulting in misalignment of rotating components and interference with stationary components within the engine, producing the grinding noise noted by the pilot. The oil line fracture was also likely associated with the smell of oil followed by the smoke in the cockpit reported by the pilot. Further examination of the engine revealed that the engine failure likely started with the No. 8 bearing stationary seal. The outside diameter of the seal was undersized, so it did not have the specified interference fit with the GP support hub. The improper fit likely led to insufficient support for the No. 8 bearing and excessive flexing of the No. 8 stationary seal cup wall. As a result, the stationary seal developed fatigue cracks and eventually fractured. The lack of interference fit with the No. 8 stationary seal likely affected the effectiveness of the seal between the stationary and rotating seals, which could have allowed oil to escape forward past the seal and into the gas path. The fracture of the No. 8 stationary seal reduced the support for the rotating turbine components at the No. 8 bearing, which likely led to increased vibrations in the engine. Fractures in the outer combustion chamber, oil line clamp, and gearbox case housing all had indications of high-cycle fatigue fracture from vibration loading. These failures likely resulted from excessive vibrations associated with the reduction in support for the turbine section rotating components. The fractured oil line support clamp failed followed by the oil supply line. According to overhaul records, the turbine section was last overhauled in July 2020, when the GP turbine wheels were replaced due to service time limits. A review of the engine manufacturer’s overhaul maintenance manual (OHM) revealed the condition of the No. 8 stationary seal should have been inspected; however, there was no indication in the overhaul records that the No. 8 stationary seal had been inspected, removed, or replaced. The turbine module was removed by the operator a few months after the overhaul due to a N2 lockup and sent to a repair facility. The repair facility ended up removing the GP support and sending the unit to another facility where the 4th stage wheel was replaced. According to the engine manufacturer, the repair facility should have been following the same OHM inspection criteria that included inspection of the No. 8 stationary seal. According to the repair facility, they had no record that the No. 8 stationary was repaired/replaced at the time the GP support/4th stage wheel was replaced.

Quoted verbatim from the NTSB record.