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ASN Wikibase Occurrence # 198776
Last updated: 25 September 2020
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Type:Silhouette image of generic SONX model; specific model in this crash may look slightly different
Sonex Sonex
Registration: N393SX
C/n / msn: 393
Fatalities:Fatalities: 0 / Occupants: 1
Aircraft damage: Substantial
Location:Lodi, WI -   United States of America
Phase: Unknown
Departure airport:Lodi, WI (9WN5)
Destination airport:Lodi, WI (9WN5)
Investigating agency: NTSB
During a local flight, the pilot of an experimental amateur-built airplane had a partial engine power loss. He said that the engine sounded different but remained running. He turned the airplane back to the airport and noted he was high and fast. He indicated that he bled off airspeed, as he wanted to avoid buildings before landing. He subsequently reduced power and performed a landing with calm wind present. The airplane's nose and left main landing gear collapsed after touchdown, the airplane skidded to the north side of the runway, and exited the runway into a ditch where it sustained the substantial damage. The airplane was powered by an engine that the pilot/builder assembled from a kit. The airplane accumulated 25 hours of flight time at the time of the accident. The kit manufacturer engine assembly and installation manual had specific rocker shaft instructions that included procedures on how to adjust and install rocker arms. An examination revealed that the threaded shaft of a rocker arm swivel pad had separated at a point on the shaft where there was a cross drilled hole. The remaining rocker arms did not exhibit the appearance of arm adjustment in accordance with the kit manufacturer's assembly manual. Detailed examination of the failed valve adjuster showed it separated into two portions approximately mid length in the shank area between the two threaded areas. This location was coincident with a hole drilled crosswise through the shank of the valve adjuster. The fracture surfaces exhibited crack arrest patterns consistent with a fatigue fracture. The origin of the fatigue fracture appears to be coincident with the edge of the cross-drilled hole. Examination of the exemplar valve adjuster and specifically the cross-drilled hole revealed a roughly finished surface with a pronounced burr around the circumference of the hole. A professional materials engineering publication, in part, stated, "The fatigue strength of components can be reduced merely by the presence of a drilled hole; it is further reduced by failure to remove burrs (incurred during drilling) from the hole edges. Fractures originating at drilled holes are common in complex parts containing internal, intersecting machined passages because of the difficulty and expense of providing adequate break-edge radii at such locations. It could not be determined if the failure of the rocker arm was due to the misassembly of the rocker arm assembly and/or the toolmarks left by the manufacturing process.
Probable Cause: The separated rocker arm assembly for undetermined reasons, which resulted in the partial loss of engine power and led to the landing gear collapsing during the subsequent forced landing.



Other occurrences involving this aircraft

15 Dec 2018 N393SX Private 0 West of Lodi Lakeland Airport (9WN5), Lodi, WI sub

Revision history:

19-Aug-2017 15:08 ASN Update Bot Added

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