Accident Details
On July 29, 2018, at approximately 0912 Alaska daylight time, a Robinson R44 II helicopter, registration N744AK, sustained substantial damage during a forced landing following a total loss of engine power shortly after takeoff from 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 14 CFR Part 91 as a public-use flight. Visual meteorological conditions prevailed, and company flight following procedures were in effect. The flight had departed PABT about 0908.
According to the pilot, he had computed weight and balance and performed a preflight inspection. After boarding, the engine was started, and all ground run-up checks were satisfactory. About 4 minutes after liftoff, the helicopter lost all engine power. The pilot entered an emergency autorotation and landed in a bog. During the landing, the skid tips broke through the vegetative mat, and the main rotor blades impacted the tailboom. The fuselage and tailboom sustained substantial damage.
Engine Examination
Examination of the recovered Lycoming IO-540-AE1A5 engine revealed it remained attached to the airframe via all mounts. All fuel lines, throttle, and mixture control cables were intact. Bottom spark plugs were removed, free of mechanical damage, and exhibited coloration consistent with normal operation. The cooling fan nut rotated in the normal direction. No movement of intake and exhaust rocker arms was observed. The oil suction screen contained metallic fragments.
The engine was removed for further examination. The left and right magneto, vacuum pump, oil filter, and rear accessory housing were removed. The crankshaft idler gear and gear shaft were found loose from their mount. Both the bolt and stud were separated from the crankcase, and their bores were elongated. Heavy fretting, gouging, and smearing were observed around the mounting surface for the idler gear shaft. Fragmented remains of safety wire, stud, nut, and bolt were located in the oil sump. Examination by the NTSB Materials Laboratory revealed impact wear and deformation around both crankcase mounting holes and the shaft flange mounting face, consistent with partial or complete loss of bolt and stud clamping force.
Maintenance History
Review of maintenance records showed the overhauled engine was installed on July 10, 2015, at a total time in service of 6,600.0 hours. The last maintenance activity where fasteners could have been manipulated was during the engine overhaul, about 3 years before the accident. The engine had accumulated about 727 operating hours since then. The most recent 100-hour inspection was conducted 18.1 hours before the accident on July 21, 2018, at an engine total time of 7,309.3 hours (709.3 hours since major overhaul). No logbook entries concerning the idler gear shaft mounting stud and fasteners were observed.
The oil filter element from the most recent 100-hour inspection was located and reexamined. It contained greater than 60 small pieces of shiny flake-like metallic material, some estimated greater than 1/16 inch in diameter. According to the operator, the mechanic who performed the inspection examined the oil filter with a magnet and found no ferrous material and no significant non-ferrous particles; the oil screen also showed no metallic material.
Lycoming Mandatory Service Bulletin No. 480F outlines procedures for oil filter element inspection, including cutting open the filter, removing the paper element, and examining for shiny metallic particles. Guidelines in Table 3 and Table 4 recommend corrective actions based on particle quantity and size.
Weather at Bettles Airport at 0853 reported wind calm, visibility 10 statute miles, few clouds at 11,000 feet, temperature 66°F, dew point 55°F, and altimeter 30.11 inches of mercury.