History of Flight
On February 9, 2022, at 1628 eastern standard time, a McDonnell Douglas MD-369D helicopter, registration N9159F, was substantially damaged during an accident near Bel Air, Maryland. The commercial pilot was not injured. The helicopter was operating as a Title 14 Code of Federal Regulations Part 91 positioning flight.
Earlier in the day, while conducting powerline inspection work, line personnel reported a strange whistling noise from the helicopter. The pilot inspected the aircraft but found no anomalies. The noise persisted, and a company superintendent recorded a video capturing the whistle-like sound. The pilot landed and ceased human external cargo operations. After reviewing the video, re-examining the helicopter, and consulting company maintenance personnel, no obvious mechanical issues were noted. The pilot parked the helicopter for the remainder of the workday.
At the end of the workday, the pilot again inspected the aircraft and found no mechanical reason to prevent repositioning to the normal base. He and another company helicopter departed as a flight of two. Several minutes into the flight, the ENGINE CHIP light illuminated. The pilot informed the other pilot that, despite normal engine operation, a landing was needed as soon as practicable. Shortly after, a grinding noise and the odor of engine oil emanated from the engine, followed by smoke in the aft passenger compartment. Suspecting an inflight fire, the pilot initiated an emergency descent to a suitable landing area. During the descent, the engine noise and smoke intensified, spreading to the forward cockpit. As the pilot flared while landing, the engine stopped producing power, and smoke reduced visual reference to the ground. The helicopter impacted in a near-level attitude. During the ground run, the front of the skids dug into the soil, causing a forward pitch. The pilot applied aft cyclic to level the helicopter. The main rotor blades struck the tail boom, separating the horizontal and vertical stabilizers and the tail rotor assembly. Residual oil was observed on the interior and exterior surfaces of the engine access doors and within the engine compartment.
Engine Examination
The helicopter was recovered and taken to the operator's facility in Gettysburg, Pennsylvania. The engine was removed and shipped to Keystone Turbine Services in Coatesville, Pennsylvania, for a full examination under NTSB supervision.
All external oil lines were secure except for the oil line supplying pressure oil to the turbine sumps. This line was fractured at a horizontal fire shield and misaligned; its clamp was also fractured. The scavenge oil filter was clean and full of oil, with a small amount of ferrous debris. Both upper and lower magnetic chip detector plugs showed ferrous debris. The gas producer turbine rotor (N1) was seized, while the power turbine (N2) turned and remained connected to the powertrain. The accessory gearbox was intact except for a fractured compressor mount pad. The No. 2½ bearing was missing six rollers, which were undamaged and recovered from the engine stand.
The combustion section was undamaged, but the inner surface of the outer combustion case barrel had material loss and cracks; barrel fragments were found in the turbine inlet. The combustion liner was carbon-covered with no mechanical damage.
Multiple areas within the turbine module showed evidence of oil starvation, including the No. 8 bearing cavity and sump, supply tubes, the No. 6/7 bearing cavity and scavenge orifice, and the external sump can. The No. 6 bearing rollers were disintegrated, and the No. 8 bearing balls were also disintegrated, leaving only the inner race, outer race, and separator. These components displayed thermal signatures consistent with overtemperature operation.
Examination of the oil supply line and its support clamp revealed fracture surfaces consistent with high-cycle fatigue. This line supplied oil to the Nos. 6 and 7 bearings, and its fracture likely led to rapid bearing deterioration from oil starvation.
The No. 8 stationary seal was fractured due to fatigue, with multiple origins around its circumference, consistent with torsional loading. The seal's outside diameter was found to be undersized by at least 0.008 inch compared to the specified interference fit with the gas producer support hub. The part number of the seal could not be determined due to heat and impact damage.
Maintenance Records
The turbine section was last overhauled on July 2, 2020, when the gas producer turbine wheels were replaced per service time limits. The engine manufacturer's overhaul manual required inspections including the No. 8 stationary seal, but overhaul records did not indicate that this seal was inspected, removed, or replaced. In September 2020, the turbine module was removed due to an N2 lockup; the gas producer support was sent to another facility where the 4th stage wheel was replaced. No maintenance record showed inspection or replacement of the No. 8 stationary seal during that work.