N9159F

HISTORY OF FLIGHT

On February 9, 2022, at 1628 eastern standard time, a McDonald Douglas MD-369D helicopter, N9159F, was substantially damaged when it was involved in an accident near Bel Air, Maryland. The commercial pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 positioning flight.

The pilot stated that while conducting powerline inspection work earlier in the day, line personnel reported that the helicopter was making a strange "whistle" noise. The pilot inspected the helicopter and no anomalies were noted or observed. The pilot continued with normal operations, but the noise continued and one of the operator’s superintendents took a video, where a “whistle”-like sound could be heard. The pilot landed and ceased all human external cargo operations. He then reviewed the video, re-examined the helicopter, and spoke with company maintenance personnel. Though no obvious mechanical issues were observed, 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 not to reposition the helicopter back to its normal base of operations. He and another company helicopter departed as a flight of two. Several minutes into the flight, the pilot said the ENGINE CHIP light illuminated. He told the other pilot that even though the engine seemed to be operating normally, he would need to land as soon as practicable. Shortly after, the engine began to make a “grinding” noise along with an odor of engine oil, which eventually became smoke in the aft section of the passenger compartment. With the presence of smoke and the potential for an inflight fire, the pilot initiated an emergency descent-to-land to a suitable landing area. During the descent the engine noise and smoke in the aft section of the cabin intensified and began moving to the forward section of the cockpit. Descending through the landing flare, as the pilot leveled the helicopter to land, the engine stopped producing power and smoke in the cockpit reduced his visual reference to the ground. 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. Residual oil was observed on the interior and exterior surfaces of the engine access doors and on the interior of the engine compartment.

AIRFRAME AND ENGINE EXAMINATION

The helicopter was recovered by the operator and taken to their facility in Gettysburg, Pennsylvania. Before the engine was removed, an external examination of the lubrication system between the airframe and the engine was conducted. Neither the aircraft-mounted oil reservoir nor the oil cooler were damaged, and no residual oil was noted within the cabin area (where the cooler and reservoir were located). The engine was then removed from the airframe and shipped to Keystone Turbine Services, Coatesville, Pennsylvania, where a full engine examination was conducted under the supervision of the NTSB.

Examination of the engine revealed that each of the external oil lines were secure except for the oil line which supplies pressure oil to the turbine sumps. The line connected to a horizontal fire shield and a T-fitting near the Nos. 6 and 7 bearing sump. The line was fractured at the horizontal fire shield and misaligned. Also, the clamp that secured the line to the turbine module was fractured.

The scavenge oil filter was clean and full of oil. A small amount of ferrous debris was observed on the filter. The pending bypass button was not extended. Both the upper and lower engine magnetic chip detector plugs were removed, and ferrous debris was observed on both.

The gas producer turbine rotor (N1) was seized but the power turbine (N2) turned and was connected to the powertrain. No damage was noted on the first stage compressor blades or compressor inlet. The fourth stage turbine wheel was normal in appearance when viewed from the exhaust collector.

The accessory gearbox was intact except for one fractured compressor mount pad. As the compressor was removed, the mount was found liberated from the gearbox housing. The gearbox housing and cover were split apart, and all the internal gears and bearings were intact, except for the No. 2 ½ bearing. The No. 2 ½ bearing was missing 6 rollers, consistent with having fallen out during the compressor removal, and were recovered from the bottom of the engine stand. The rollers were undamaged. The compressor bore on the gearbox cover displayed a wear step between the 1:00 to 12:00 position.

The combustion section was not damaged. The inner surface of the outer combustion case barrel was missing material and was cracked. Several pieces of barrel material were recovered from the turbine inlet. The combustion liner was covered in carbon, but no mechanical damage was observed.

Several areas within the turbine module displayed evidence of oil starvation:

• No. 8 bearing cavity

• No. 8 bearing sump oil scavenge line (was also clogged with debris/dry residue)

• No. 8 bearing sump oil supply tubes

• No. 6/7 bearing cavity

• No. 6/7 bearing scavenge orifice

• No. 6/7 bearing oil supply tube

• No. 6/7 bearing external sump can

The No. 6 bearing rollers were disintegrated, which precluded inspection of the No. 7 bearing.

The No. 8 bearing balls were also disintegrated and the remaining components (inner race, outer race, and separator) were removed. The bearing components displayed thermal signatures consistent with overtemperature operation.

The gas producer (GP) rotor tie bolt nut was loose but remained in position within the locking feature; the GP rotor was intact. The trailing edge blade tips of the first stage turbine wheel displayed mechanical damage and the upstream face of the wheel was circumferentially gouged and smeared. The energy absorbing ring location tabs and corresponding slots in the GP support displayed heavy wear.

The No. 5 bearing turned freely and smoothly and displayed some brown discoloration. The N2 rotor was removed and all airfoils were intact. The upstream face of the third stage turbine wheel was coated in a gray carbon-like substance. The N2 shaft was intact with some discoloration and carbon noted on the exterior surface. The N1 shaft was intact and exhibited some slight bulging at the end of the shaft.

The engine compressor turbine assembly, gearbox housing, oil pump, oil supply line with fittings and clamp assembly, N1 coupling, GP turbine assembly, GP turbine support (which included a sump nut, retaining ring and plate, and the No. 8 oil supply jet), No. 8 bearing, No. 8 rotating and stationary seal, No. 8 bearing spanner nut, and the outer combustion chamber, were sent to the NTSB Materials Laboratory for examination.

Examination of these components by the NTSB Materials Laboratory determined numerous instances of high heat damage and fracture due to high-cycle fatigue.

The gearbox housing support lug located on the upper left side of the gearbox exhibited fracture surfaces consistent with high-cycle fatigue. The oil pump, which was installed on the interior of the gearbox, was intact. There were no fretting contact marks noted on the pump mating to blended area on gearbox housing. Pitting consistent with cavitation damage were noted on gear teeth in the pressure body.

The oil supply line that was found fractured during the engine exam, and its associated support clamp, also exhibited fracture surfaces consistent with high-cycle fatigue. This oil line supplied oil to the Nos. 6 and 7 bearings in the turbine section. The fracture of this line most likely led to the rapid deterioration of the bearings from oil starvation.

The GP support and its related components revealed the retaining plate shear pin was fractured. The fracture surfaces showed curving crack arrest lines and dark tinting consistent with fatigue. The retaining plate was installed on the aft side of the GP support hub and had multiple recesses machined into the outer diameter, including those for accommodating the shear pin and the No. 8 oil supply jet. A mark was observed on the clockwise side of the recess for the shear pin corresponding to contact with the aft piece of the shear pin that was not recovered. A separate mark was observed near the clockwise end of the recess for the No. 8 oil jet, and a corresponding contact mark was observed on the lower inboard side of the No. 8 oil supply jet body. The marks on the retaining plate recesses corresponded to contact with the shear pin and the oil jet, respectively, as the retaining plate rotated counterclockwise relative to the GP support hub.

On the forward side of the retaining plate, damage from fretting contact with the No. 8 bearing outer race was observed near the retaining lug on the forward face next to the inside diameter. The surfaces of the retaining lug on both the clockwise and counterclockwise sides showed damage from fretting contact with the No. 8 bearing outer race. The contact damage was more extensive on the lower (clockwise) side of the lug, consistent with the bearing outer race rotating counterclockwise relative to the retaining plate. (The GP turbine and No. 8 bearing inner race normally rotate clockwise).

Fretting contact marks were observed on the sump nut face corresponding to contact with the outer diameter of the retaining plate. A wear contact mark was observed on one of the castellation surfaces on locking flange on the aft side of the sump nut. The wear mark corresponded to contact with the retaining ring with the sump nut flange approximately flush to the aft side of the GP hub.

The retainin...