Fall Creek, OR, USA
N511ZZ
LANCAIR ES
The pilot was conducting a personal cross-country flight when he descended from cruise flight in preparation for landing. Shortly thereafter, the engine lost power and started misfiring. The pilot attempted to restart the engine but was unsuccessful despite his actions. While over heavily wooded terrain, the pilot deployed the parachute system. During the descent through the heavily wooded terrain, the airplane collided with trees resulting in substantial damage. The airplane came to rest a few feet from the ground, still supported by the parachute, which remained stuck in the trees. Postaccident examination of the engine revealed the camshaft gear failed due to fatigue-induced fracturing of multiple teeth. Fatigue cracks initiated at the fillet radii between the flanks and bottom lands of multiple gear teeth. The fillet radii exhibited more roughness and surface discontinuities, likely acting as stress concentration areas for crack initiation. Once the fatigue cracks had propagated through over half the material cross-section, the teeth fractured in overstress. The exact cause of the crack initiation could not be concluded. There were no material defects, such as voids or inclusion, at the initiation sites. However, as previously stated, the fillet radii exhibited more pronounced surface features, which would lead to a higher chance of crack initiation. Once cracking started, propagation would be in high cycle with moderate loading modes, as evidenced by the depth of the fatigue cracks. The gear was a nitrided alloy steel in the quenched and tempered condition. This should be a suitable material and manufacturing process for gear applications, not lending itself to any material incompatibility or excessive wear or corrosion concerns.
On October 14, 2021, about 1815 Pacific daylight time, a Lancair ES, experimental airplane, N511ZZ, was substantially damaged when it was involved in an accident near Fall Creek, Oregon. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that during cruise flight he descended to 4,500 ft mean sea level in preparation to land at Mahlon Sweet Field Airport (EUG), Eugene, Oregon. Soon after disconnecting the autopilot and turning to the North, he felt the engine “decelerate suddenly, and it began misfiring badly.” He realized that he was too far from the airport to land on the runway. The pilot looked at his surroundings for a place to make a forced landing but saw only forest surrounding his route of flight. He decided to focus on restarting the engine; however, his attempts were unsuccessful. The pilot reported that smoke had begun to enter the cockpit and the airplane continued to lose altitude. About 1,000 ft above ground level, he checked his airspeed and pulled the ballistic recovery system handle. The parachute deployed and during the descent through the heavily wooded terrain below, the airplane collided with trees, resulting in substantial damage to the left wing and the separation of the empennage. The airplane came to rest a few feet from the ground, still supported by the parachute, which remained stuck in the trees. The pilot exited the airplane and was able to walk to a nearby road. Postaccident examination of the wreckage revealed that the engine exterior was undamaged, and the oil dip stick indicated about 7 quarts of oil. The lower spark plugs were removed from the engine. The cylinders combustion chambers were examined with the use of a lighted borescope and each cylinder combustion chamber revealed a normal amount of combustion deposits and the valve heads had normal operational signatures. The six piston heads had impact marks from each valve. The crankshaft was rotated by hand from the propeller assembly, and cylinder thumb compression was obtained to only four cylinders. The cylinder rocker covers were removed from each cylinder and the cylinder overhead components were undamaged and covered in an oil residue. The crankshaft was again rotated, and no movement was noted from each of the cylinder overhead components. The engine was removed from the airframe and disassembled. The oil sump contained metal fragments. The crankcase halves were separated, and the crankcase mating surfaces were undamaged. Small metal fragments were found throughout the interior of the engine. The valve lifters installed in both crankcase halves were undamaged. The camshaft drive gear (part number 631845) had missing gear teeth and had material deformation. About two-thirds of the camshaft drive gear teeth were damaged. The camshaft lobes had normal lifter contact wear signatures and several lobes had mechanical damage on their edges. The crankshaft was removed, and the main bearings were undamaged and showed normal operating wear signatures. The camshaft drive gear, five separated gear teeth and gear teeth remnants were sent to the National Transportation Safety Board Materials Laboratory for forensic examination. The examination revealed that 36 of the camshaft gear teeth had fractured and separated from the gear. Three of the separated gear teeth fracture surfaces exhibited a sloping, angled orientation, whose edges were located along the fillet radius below the flank and were consistent with post-fracture smearing. Another gear tooth’s fracture surface was consistent with having fractured from a fatigue crack that propagated about halfway through the tooth cross section before the remainder fractured in overstress. Located along the fillet radius of the tooth, the initiation site exhibited several concentric convex features, consistent with emanating or propagating from the surface site. Another fatigue crack initiation site was near a flat edge surface with radial lines consistent with crack propagation. The last gear tooth showed similar signatures of fatigue crack propagation but only had one initiation site located at the fillet radius, between the bottom land and the flank. The initiation site was collocated with a gouge or machine mark. The composition of the gear was examined using energy dispersive x-ray spectroscopy (EDS) and x-ray fluorescence (XRF). The chemical composition was consistent with a nitrided alloy steel. The gears exhibited a surface layer with a lighter contrast. Examination of this surface layer with EDS found it to contain nitrogen, consistent with a nitride surface treatment. The hardness of the tooth cross section was inspected with a microindentation hardness tester per ASTM E384.2. The hardness of the base alloy exhibited a hardness of 41 HRC (401 HV500). The hardness data and microstructure were consistent with a quench and tempered condition for this alloy composition. Aircraft documentation revealed that the engine was overhauled on July 30, 2015. A serviceable camshaft gear (part number 631845) was installed during the overhaul on the accident engine. Service bulletin (SB) 97-6B was in effect at the time of the accident airplane’s engine overhaul. The engine had about 351 hours of operation at the time of the accident. On August 9, 2005, the engine manufacturer issued SB05-8, which was later superseded by SB05-8A in August 2009, to introduce into service an improved camshaft gear applicable to the accident engine. The improved camshaft gear was nominally 0.060-inches wider than the previous camshaft configurations. The manufacturer recommended the replacement of the camshaft gear during the next engine overhaul or whenever the replacement of the camshaft gear was required. In November 2009, the manufacturer issued revised SB97-6B to identify parts to be replaced during maintenance, preventative maintenance, and overhaul which included the camshaft gear (part number 631845) as a mandatory replacement item at overhaul. In August 2018, the manufacturer revised SB-05-8A and issued Critical Service Bulletin CSB05-8D to eliminate the possibility of camshaft gear tooth fracture, resulting in power loss or in-flight shutdown, and to inspect and remove from service multiple camshaft gears, including part number 631845.
Total loss of engine power due to fatigue failure of the camshaft gear.
Source: NTSB Aviation Accident Database
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