Robinson R44 N744AK accident near Bettles, Alaska

Probable cause

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for ANC18LA058 for the full record.