WPR15FA268

The loss of engine power due to the mechanic's inadequate tightening of the crankshaft gear retaining bolts during an engine overhaul, which resulted in fatigue failure of the bolts and the crankshaft gear dowel pin that allowed the crankshaft gear to separate from the crankshaft.

The airplane, flown by a commercial pilot, had just departed for a local flight when witnesses observed it overhead and heard its engine sputtering and backfiring. One witness reported that he thought he heard the engine stop completely. Another witness reported seeing the airplane go out of sight behind a stand of trees and then hearing the sound of a crash. Postaccident disassembly and examination of the engine revealed that all four crankshaft gear retaining bolts had failed in their threaded portions, which had resulted in separation of the crankshaft gear from the crankshaft. Two of the crankshaft gear retaining bolts exhibited gross plastic deformation in their shank portions, and both of these bolts exhibited fractures consistent with overstress. The other two bolts did not exhibit gross plastic deformation, and both of these bolts exhibited flat fractures and crack arrest marks consistent with fatigue fractures. Further, the crankshaft gear dowel pin was fractured, and examination of the pin revealed that it had failed due to fatigue, which could only occur if the crankshaft gear bolts were loose during engine operation. The total time on the engine since its most recent overhaul was 41.4 hours. Although the mechanic who performed the overhaul stated that he had used a calibrated torque wrench to properly torque the bolts during the overhaul process, the loose condition of the crankshaft gear retaining bolts was most likely due to the bolts not being correctly tightened during the overhaul.

At 1315, the weather reporting facility at the Oak Harbor Airport (OKH), Oak Harbor, Washington, located about 14 nm northeast of the accident site, reported wind 340° at 8 knots, visibility 10 miles, sky clear, temperature 17°C, dew point 9°C, and an altimeter setting of 30.05 inches of mercury. According to the carburetor icing-probability chart, for the area conditions reported at about the time of the accident, moderate icing could be expected at cruise power, and serious icing conditions might be expected at descent power. The pilot, age 69, possessed a commercial pilot certificate with single-engine land, multiengine land, single-engine sea, and instrument airplane ratings. The pilot reported a total flying time of 3,000 hours, with 2,000 hours in the same make and model as the accident airplane, and 20 hours of multiengine time. Additionally, the pilot reported that in the last 90 days and 30 days, he had flown 20 hours and 1 hour respectively. The pilot's most recent flight review was conducted on September 8, 2014. The pilot received his third-class Federal Aviation Administration airman medical certificate on August 11, 2015, with the stated limitation, "Must have available glasses for near vision." A survey of the accident site revealed that the airplane's wings had impacted trees on a heading of north at the 40-foot level. This was followed by the propeller/engine colliding head-on with a 28-inch diameter tree in the same direction, and at the same height. The airplane then rotated about 90 degrees to the left prior to falling vertically to the ground, where it came to rest upright in a densely tree and bush covered area. The forward section of the airplane and both wings were substantially damaged, while the aft fuselage and empennage sustained only minor damage. There was no postcrash fire. All components necessary for flight were accounted for at the accident site. On October 15, 2015, under the supervision of the NTSB IIC and a Federal Aviation Administration aviation safety inspector/airworthiness, and with technical support provided by field representatives for Continental Motors Inc. and Textron Aviation, an examination of the engine and airframe was performed. The results of the examinations revealed the following: Airframe The right wing remained attached to the fuselage at all attach points, with the outboard one-third of the wing substantially damaged due to impact forces with trees. The wing strut remained in place, and was not damaged. The right flap remained attached to the wing's trailing edge at all attach points, and was observed impact damaged. The right aileron remained attached to the wing's trailing edge at all attach points, and was not damaged. The wing's fuel tank was intact, with the fuel cap observed in place and tight to touch. The left wing remained attached to the fuselage at all attach points. The outboard two-thirds of the wing was observed to have sustained heavy impact damage and deformation as a result of tree impact forces. The wing strut was not damaged. The inboard one-third of the wing sustained moderate impact damage, with some wrinkling observed. Additionally, the outboard two-thirds of the wing was bent down about 45°. The aileron remained attached to the wing's trailing edge at all attach points, with minimal damage observed. The wing's fuel tank was breached due to impact forces, with the fuel cap observed intact, not damaged, and tight to touch. The airplane's vertical stabilizer remained attached to the aft upper fuselage area at all attach points, and was not damaged. The rudder remained attached to the trailing edge of the stabilizer at all attach points, and was not damaged. The right horizontal stabilizer remained intact, and attached to the aft fuselage at all attach points. The outboard tip was damaged mid-span. The right elevator was intact, and remained attached to the right stabilizer's trailing edge at all attach points. The outboard tip of the elevator was observed separated. The left horizontal stabilizer, which was not damaged, remained intact, and attached to the aft fuselage at all attach points. The left elevator, which was not damaged, was intact, and remained attached to the right stabilizer's trailing edge at all attach points. The elevator trim tab remained connected to the elevator at all attach points, and observed in the neutral position. It was confirmed that all of the control cables were properly attached during the review of the wreckage, with control continuity confirmed throughout the airplane. A visual examination of the flap actuator revealed that the flaps were in the UP position. The flap position indicator was observed in a position beyond the FULL UP position. Fuel samples from the carburetor and the fuel strainer were observed light blue in color, and tested negative for water with water detection paste. The fuel handle was observed in the wreckage in the off position; however, the valve handle had been placed on the OFF position by the first responders. Air was applied to the right wing root fuel line with the wings removed and the fuel valve in the off position. Air was expelled out of the left wing root fuel line. With the fuel selector valve then placed in the ON position and the left wing fuel line held closed, expelled air was confirmed coming from the fuel strainer. The fuel vent line was tested with air, and the vent line was free of obstructions. The examination revealed no anomalies with the airframe that would have precluded normal operation. Engine At the direction of the NTSB IIC, the engine had been removed from the airframe prior to the examination. Additionally, the number three rocker box cover was removed, and all ignition leads were removed from their respective spark plugs. The starter motor was imbedded, and removed from the firewall for safety reasons. The examination revealed that the engine had remained intact, and displayed impact damage signatures. All four cylinders remained attached to the engine, and displayed varying degrees of impact damage. The crankcase remained intact, and was unremarkable. The two-bladed fixed pitch propeller remained attached to the crankshaft. The propeller blades displayed minor impact damage, with no bending or twisting deformation observed. Both magnetos remained attached to their mounting pads, and displayed minor impact damage. The ignition harness displayed impact damage signatures, with a few of the ignition leads partially severed. All of the spark plugs remained in their cylinders, and were visually observed to be unremarkable. The carburetor had broken free from the induction system, and displayed damage consistent with impact forces. The induction system displayed damage at the induction manifold in the form of bending. The right magneto remained attached to its mounting pad, and displayed minor impact damage. The magneto was removed, and the drive shaft rotated by hand. The impulse coupling operated normally with spark produced at all four distributor posts in normal order. There were no anomalies noted with the magneto. The left magneto remained attached to its mounting pad, and displayed minor impact damage. The magneto was removed, and the drive shaft was rotated by hand; the impulse coupling operated normally. During impulse coupling operation, it was observed that the magneto would produce a spark on all four distributor posts simultaneously. The magneto was retained by the NTSB IIC for further examination. The ignition harness sustained impact damage; several of the ignition leads for the right magneto were partially severed. The left magneto ignition harness was tested, and it was capable of conducting a spark from the magneto to the ignition lead end. There were no preimpact anomalies noted with the ignition harness. All spark plugs remained installed in their respective cylinders. The spark plugs were removed, and visually inspe