What happened
On July 30, 2013, an incident was reported at Kaniów Aerodrome (EPKW) involving a Cessna C-172M. During a routine check of the aircraft's technical log, a leak from the pushrod cover was identified. Subsequent inspection of the engine revealed a leak and deformation of the exhaust valve pushrod shroud on cylinder number 2. Upon removing the valve covers, investigators discovered steel fragments within the engine. Further disassembly showed that components of the hydraulic lash adjuster body had fallen out, and the exhaust valve pushrod was deformed with visible wear and damage caused by steel debris being forced through the shroud.
The investigation
The investigation examined the maintenance history of the engine. Records showed that on December 2, 2011, cylinders 2 and 3 had been replaced following a valve clearance inspection. Subsequent 400-hour and 100-hour inspections were performed, including checks of the cylinders, valve covers, and pushrod housings; however, no defects were noted during the most recent 100-hour inspection on July 11, 2013. The investigation established that the mechanical failure mechanism was the seizing of a valve stem within its guide, which subsequently destroyed the hydraulic lash adjuster, the pushrod, and the cam due to the pressure exerted by the cam.
Findings
- The primary cause was the seizing of the exhaust valve stem in the guide.
- The engine design is inherently susceptible to this type of failure, a known issue for O-320 engines.
- Factors that can contribute to the reduction of valve-to-guide clearance include specific flight profiles (such as low-power operations), improper operating ranges, high ambient temperatures, or high lead content in the fuel.
- All documented maintenance tasks for the engine had been performed in accordance with requirements.