OAKDALE, CA, USA
N8661F
Hughes 369D
The pilot experienced a partial loss of engine power while established in an out of ground effect hover at 150 feet agl and landed hard in the resulting autorotation. Postcrash examination of the engine revealed that the turbine had experienced oil starvation due to a failure of the oil pump. The aft bearing (gear case cover) of the FCU/oil pump drive spur gear had disintegrated due to failure of the ribbon-type ball separator, which was found in the bottom of the gearbox in two pieces and deformed. The balls were ejected into the gear case and one was ingested by the oil pump, which caused internal jamming and the resultant shearing of the splined adapter which drives the oil pump. The ball separator was manufactured from an austenitic stainless steel, instead of AISI 1010 as specified by the manufacturer, and the material exceeded allowable hardness specifications. Allison CEB 1228, issued November 20, 1985, calls for bearings with the ribbon-type, pressed steel separator, to be replaced with bearings that incorporate a one-piece machined steel separator. Compliance was due at the next accessory gearbox repair when the gearbox was opened. The mechanic reported that he had replaced the number 2.5 bearing and the spur adapter gear shaft 265 hours and 4 months before the accident, which would have provided access to the gearbox to accomplish the bulletin.
On July 3, 1999, about 1000 hours Pacific daylight time, a Hughes 369D, N8661F, sustained substantial damage during a hard autorotative landing to a field 5 miles south of Oakdale, California, following a loss of engine power. The commercial pilot and two passengers were not injured. The helicopter was being operated by Capricorn Helicopters, Durango, Colorado, for a local power line repair flight, conducted under the provisions of 14 CFR Part 133 (Class A). The flight originated about 0900 from a nearby construction site. Visual meteorological conditions prevailed and no flight plan was filed. The pilot reported that he was working on contract to a company contracted to a local power company. On the accident flight, he was patrolling power lines for the purpose of installing new static/fiber lines. The two passengers onboard were employed as repair technicians. Both left-side doors and the front right-side door were off the helicopter. The seats had been removed from the back of the aircraft; the crewmembers worked from the back area. In accordance with 14 CFR Part 133, Class A, the pilot would hover next to a wire crossing that needed to be guarded, and the crewmembers would position themselves on the left skid to perform the repairs. Class A does not allow for external loads or objects to be attached to the airframe. While hovering at 150 feet agl with both crewmembers on the left skid, the pilot noticed that the engine rpm was decreasing. He stated that the low rotor rpm audio signal came on indicating that rotor rpm was decreasing below 98 percent. The pilot reported that he slightly reduced the collective and initiated a left turn away from the power lines. Once he was clear of the lines, he fully lowered the collective, rolled the throttle to flight idle, and entered an autorotative descent. The pilot stated that there was no communication possible between himself and the crew; they did not have headsets. When he started the turn away from the wires, both crewmembers immediately climbed into the back of the helicopter and lay flat on their stomachs. The pilot reported that the rpm was low during the deceleration flare and cushion. He landed in a cornfield and the aircraft slid forward approximately 15 feet after touchdown. The main rotor blades severed the tail boom. The engine was still idling so the pilot closed the fuel shutoff valve and throttle and turned off all electrical power. A postcrash examination of the airframe and engine was conducted by the Safety Board investigator at Aircraft Recovery Services in Compton, California, and at National Airmotive in Long Beach, on July 14, 1999. Flight control continuity was established. A battery was hooked up to the helicopter to check the instrument panel lights; the "Engine Chips" light illuminated. The "Fuel Low" light did not appear to function. The fuel gauge indicated approximately 275 pounds of fuel. Both auto reignition lights illuminated when tested; the pilot reported that he flew with the reignition system in the "armed" position. All fluid lines, including oil, fuel, and pneumatics, were intact and their fittings were secure. The oil tank was full, and there was evidence of oil in the oil lines. The top and bottom chip plugs were removed and inspected. The top plug was clean and absent of debris. The bottom plug displayed black paste and chunks of particulate. Approximately 1 pint of oil was drained from the lower chip plug port. The compressor would not rotate and evidenced minor foreign object damage to three of the first stage blades. Number 2 rotated freely, continuous to the output shaft. A pneumatics check of the engine controls system was performed with no leaks detected. The compressor, gearbox, and turbine modules were then separated for further examination. The number 2 bearing displayed a bluish coloration and melted metal was visible between the inner and outer races. The spur adapter gear shaft also showed heat distress signatures; the O-ring was burned and appeared to be decomposing. The gearbox was locked up; it could be rotated through the fuel control unit drive, but not through the N1 tachometer drive. The gearbox case was split open; it contained dark oil and had a strong burned odor. The aft bearing (gear case cover) of the fuel control unit/oil pump drive spur gear was disintegrated, with five bearing balls, as well as the outer race, found in the bottom of the gearbox. The bearing separator was also found in the bottom of the gearbox in two pieces and deformed. A sixth ball was found in the scavenge pump oil outlet port. No visible damage was noted on the spur gear bearing inner or outer race. The bearing housing was loose in the case and had rotated approximately 180 degrees; the retaining pin was dislodged and deformed. The oil pump was removed and disassembled. The splined drive adapter-retaining pin was sheared, and was disconnected from the fuel control spur gear shaft assembly, which provides the drive for the oil pump. Aluminum and magnesium chips and flakes were found throughout the scavenge pump chamber. The scavenge chamber wall (driven gear) was gouged circumferentially, from inlet to outlet port. The scavenge oil pump drive spur gear displayed a split fracture and a small, round imprint was evidenced on the gear tooth. The imprint appeared to match the size of a bearing ball. The scavenge oil pump idler spur gear also displayed a round imprint the size of a bearing ball on the gear tooth. One bearing ball from the spur gear drive bearing was found in the outlet port of the scavenge pump chamber. The ball had a surface indentation; the Allison representative opined that the scar was caused by the ball meshing with the gears. The turbine was rotated via the compressor to turbine coupling; it was very rough and did not rotate freely. The failed bearing was examined further at Seal Laboratories, El Segundo, California, under the supervision of the Safety Board. The bearing separator was a two-piece, ribbon-type separator, which consisted of two "ribbons" held together with two overlapping tabs. The tabs had broken off from the bearing separator and the fracture surfaces were smeared. The tabs were not recovered. Testing revealed that the side of the separator that held the tabs was magnetic and the adjacent half of the separator (without tabs) was non-magnetic. According to the Rolls-Royce Allison metallurgist, the engineering specifications required that the separator be AISI 1010 steel, which was magnetic. Qualitative analysis (XEDA) indicated that the non-magnetic bearing separator was made from an austenitic type stainless steel. A microstructure and hardness evaluation was performed. Both halves of the separator exceeded the drawing specification maximum allowed hardness of 95 HRB. ADDITIONAL INFORMATION Allison Commercial Engine Bulletin (CEB), issued November 20, 1985, advises that bearings with the ribbon-type, pressed steel separator, should be replaced with bearings that incorporate a one-piece machined steel separator. The bulletin stated that compliance was due at the next accessory gearbox repair when the gearbox was opened. The pilot, who is also a mechanic and who performs the maintenance on the helicopter, reported that he had replaced the number 2.5 bearing and the spur adapter gear shaft approximately 265 hours and 4 months before the accident. According to the Allison representative, the gearbox would have been opened to replace those components.
The disintegration and failure of the oil pump drive spur gear aft bearing, which resulted in failure of the oil pump and subsequent oil starvation to the engine's turbine module. A factor in the accident was the failure of company maintenance personnel to comply with the manufacturer's service bulletin regarding replacement of the bearing.
Source: NTSB Aviation Accident Database
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