Bettles, AK, USA
N744AK
Robinson R44
According to the commercial pilot, about 4 minutes after liftoff for the public flight, the helicopter lost all engine power. He entered an emergency autorotation and landed the helicopter in a bog. During the emergency landing, the tips of the helicopter's skids broke through the vegetative matt on the bog's surface, and the main rotor blades impacted the tailboom. The helicopter sustained substantial damage to the fuselage and tailboom. Postaccident engine examination revealed that the crankshaft idler gear and gear shaft were loose in their mount. Both the bolt and stud for the idler gear shaft had separated from the crankcase, and their respective bores were elongated. Fragmented remains of the safety wire, stud, nut, and bolt were found in the oil sump. Examination of images of these components revealed that there was impact wear and deformation around both of the crankcase mounting holes and the shaft flange mounting face, which was consistent with the partial or complete loss of bolt and stud clamping force of the shaft flange to the crankcase. The insufficient preload on the bolt and stud for the idler gear shaft likely resulted in the dismounting of the idler gear shaft assembly. A review of maintenance records revealed that the last maintenance activity during which the fasteners could have been adjusted was during an engine overhaul performed about 3 years before the accident. The engine had accumulated about 727 operating hours since that time. It is likely that maintenance personnel applied insufficient preload to the bolt and stud at that time, which allowed them to wear and deform over time. The oil filter element from the most recent 100-hour inspection, which was conducted 18.1 hours before the accident, was examined and found to contain more than 60 small pieces of shiny, flakelike nonmagnetic metallic material; some of the flakes were estimated to be more than 1/16 inch in diameter. The operator reported that the mechanic who performed the inspections did not find any metallic material. The engine manufacturer had issued Mandatory Service Bulletin No. 480F, "Oil Servicing, Metallic Solids Identification After Oil Servicing, and Associated Corrective Action," which stated, in part, if "pieces of shiny flake-like, material (larger than 1/16 inch in diameter) with no copper tint" are found in the oil filter, the engine should be removed or disassembled for repair. The mechanic's failure to identify the metallic particles contained in the oil filter element at the most recent 100-hour inspection likely contributed to the accident because it allowed the helicopter to be returned to service when it should not have been.
On July 29, 2018, at about 0912 Alaska daylight time, N744AK, a Robinson R44 II helicopter, sustained substantial damage during a forced landing following a total loss of engine power shortly after takeoff from the Bettles Airport (PABT), Bettles, Alaska. The commercial pilot and two passengers were not injured. The helicopter was registered to and operated by Quicksilver Air Inc., under the provisions of 14 Code of Federal Regulations Part 91 as a public-use flight. Visual meteorological conditions prevailed and company flight following procedures were in effect. The flight departed PABT about 0908. According to the pilot, he had computed the weight and balance and performed a preflight inspection prior to departure. After the passengers boarded the helicopter, the engine was started, and all ground run-up checks of the engine were satisfactory. He stated that about 4 minutes after liftoff the helicopter lost all engine power. He entered an emergency autorotation and landed the helicopter in a bog. During the emergency landing, the tips of the helicopter's skids broke though the vegetative matt on the bog's surface and the main rotor blades impacted the tailboom. The helicopter sustained substantial damage to the fuselage and tailboom. Examination of the recovered Lycoming IO-540-AE1A5 engine revealed that it remained attached to the airframe via all its mounts. All fuel lines, throttle, and mixture control cables were intact and remained attached to the engine. The bottom spark plugs were removed and remained free of mechanical damage. The bottom spark plugs exhibited coloration consistent with normal operation. The cooling fan nut was rotated in the direction of normal rotation. No movement of the intake and exhaust rocker arms was observed. The oil suction screen was removed and found to contain metallic fragments. The engine was removed from the airframe for further examination. The left and right magneto, vacuum pump, oil filter and rear accessory housing was removed from the engine. The crankshaft idler gear and gear shaft was observed loose from their mount. Both the bolt and stud were separated from the crankcase and their respective bores were elongated. Heavy fretting, gouging, and smearing was observed around the mounting surface for the crankshaft idler gear shaft. Fragmented remains of the safety wire, stud, nut and bolt were located in the oil sump. Examination of images of the components by the National Transportation Safety Board (NTSB) Materials Laboratory revealed impact wear and deformation around both crankcase mounting holes and the shaft flange mounting face, was consistent with the partial or complete loss of bolt and stud clamping force of the shaft flange to the crankcase. The NTSB's Materials Laboratory Factual Report is available in the public docket for this accident. Review of the helicopter's maintenance records revealed that the overhauled engine was installed on the accident helicopter on July 10, 2015 at a total time in service of 6,600.0 hours. The last logged maintenance activity in which the fasteners could have been manipulated was performed at the engine overhaul, about 3 years prior to the accident. The engine had accumulated about 727 operating hours since that time. The most recent 100-hour inspection was conducted 18.1 hrs before the accident on July 21, 2018, at an engine total time in service of 7,309.3 hrs, and 709.3 hrs since major overhaul. No logbook entries were observed concerning the idler gear shaft mounting stud and fasteners. The oil filter element which had been cut and examined at the most recent 100-hr inspection was located and reexamined and found to contain greater than 60 small pieces of shiny flake like metallic material with some of those flakes estimated to be greater than 1/16 inch in diameter. According to the operator, the mechanic who completed the most recent 100-hr inspection examined the oil filter with the aid of a magnet and found no ferrous material and did not observe any significant amount of non-ferrous particles. In addition, the oil screen was examined, and no metallic material was found. Lycoming Mandatory Service Bulletin No. 480F, Oil Servicing, Metallic Solids Identification After Oil Servicing, and Associated Corrective Action, states in part: Oil Filter Element Inspection 1. Cut open the removed oil filter element with an approved tool (e.g., for full-flow filters, use Champion Tool CT-470) per the tool manufacturer's instructions. 2. Remove the paper element from the oil filter. 3. Carefully unfold the paper element to prevent loss of collected particles which can compromise the integrity of this inspection. 4. Examine the material trapped in the filter. Look for shiny metallic particles/residue, shavings or flakes. Refer to the sections: "Identification of Metallic Solids After Oil Servicing" and "Visual Inspection of the Oil Filter Element, Oil Pressure Screen, and Oil Suction Screen" in this Service Bulletin. 5. Record all inspection findings and any corrective action in the engine logbook. Table 3 Guidelines for Particle Quantity and Size on Oil Filter, Oil Pressure Screen, or Oil Suction Screen Recommended Corrective Action Options Table 4 Recommended Corrective Action Options (Refer to the applicable Lycoming Engine manual for procedures) The closest weather reporting facility is Bettles Airport (PABT). At 0853, an METAR from PABT was reporting in part: wind, calm; visibility, 10 statute miles; sky condition, few clouds 11,000 feet; temperature, 66° F; dew point 55° F; and an altimeter setting of 30.11 inches of mercury.
Maintenance personnel's application of insufficient preload on the attachment hardware for the idler gear shaft during engine overhaul, which resulted in the dismounting of the idler gear shaft assembly due to wear and deformation and the subsequent total loss of engine power. Contributing to the accident was a mechanic's failure to identify metallic particles contained in the oil filter element at the most recent 100-hour inspection.
Source: NTSB Aviation Accident Database
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