NTSB Identification: CEN12LA524
14 CFR Part 91: General Aviation
Accident occurred Tuesday, August 07, 2012 in Carthage, TX
Probable Cause Approval Date: 07/30/2014
Aircraft: CESSNA 177B, registration: N17201
Injuries: 1 Uninjured.
NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.
The engine lost partial power without warning during a pipeline patrol flight at 500 feet above ground level. The airplane impacted logs during a forced landing on a logging site. A postaccident examination revealed that the No. 1 cylinder intake valve was not present under its rocker arm tip and that its retainers were worn, deformed, and segmented in the bottom of its rocker box. The tip of the rocker arm was worn consistent with the rocker arm contacting the upper valve stem and its retainer. Inboard of the tip, the rocker arm exhibited a wear mark consistent with contact with the valve spring retainer. Further, the piston exhibited impact marks consistent with valve contact, and the No. 1 intake port exhibited a hole consistent with a liberated valve stem impacting the side of the intake port. It is likely that, when sufficient rocker arm wear occurred, the intake valve was released. The liberated valve was found in pieces in the Nos. 3 and 4 intake tubes. The No. 1 intake valve examination revealed no indications of fatigue in any of the fractures, and the valve face did not exhibit erosion or wear. The intake valve and its guide exhibited no evidence indicating that the valve was stuck. The Nos. 2 through 4 intake valve rocker arms were sectioned, and they met the minimum overhaul hardness specifications, but the depth of the carburized case layer was somewhat shallow consistent with its having been resurfaced during overhaul, which the engine had undergone 878.5 hours before the accident. The wear damage to the failed rocker arm tip precluded the determination of the depth of its carburized layer. However, it is likely that the rocker arm tip’s wear occurred because of the thin carburized case layer.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The loss of engine power during a low-level aerial observation flight as the result of the wearing of the rocker arm tip due to a thin carburized case layer, which resulted in a forced landing. Full narrative available
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