What happened
On August 19, 2010, a Cessna 172M, registration CS-AVT, operated by Aero Clube de Chaves, was performing a recreational flight from Chaves to Portimão. After completing the southbound leg and refueling, the aircraft departed Portimão at 11:40 UTC for the return trip to Chaves.
While cruising north of the Douro River, near the Vila Real aerodrome, the pilot experienced abnormal vibrations and a partial loss of engine power. The pilot attempted standard troubleshooting procedures, including applying carburetor heat, cycling the magnetos, and adjusting the throttle, but the engine performance did not stabilize. To ensure safety, the pilot contacted Lisbon Approach (LISACC) and declared an intention to perform a precautionary landing at the nearest suitable airfield. The aircraft landed safely at Alijó aerodrome at 13:40 UTC with no injuries and no damage to the aircraft.
The investigation
Following the landing, a maintenance technician was dispatched to inspect the engine. The initial examination revealed that the pushrod protection tube for the exhaust valve of cylinder number 2 was bent. Upon further disassembly of the cylinder head and rocker arm, investigators confirmed that the exhaust valve was stuck in the open position due to the presence of carbon deposits on the valve seat. This prevented the valve from closing properly, which in turn caused the deformation of the valve pushrod and the resulting engine vibrations.
Investigators also reviewed the aircraft's flight logs, noting that the engine had experienced long periods of inactivity during the winter months. The aircraft averaged only about 30 flight hours per year, with significant gaps between operations.
Findings
- The engine vibration and power loss were caused by the sticking of the number 2 cylinder exhaust valve.
- The valve remained stuck in the open position, which led to the bending of the valve pushrod.
- The valve sticking was caused by the accumulation of contaminants and carbon deposits on the valve seat.
- These deposits formed due to long periods of engine inactivity and the lack of specific maintenance measures to remove residues created during infrequent operations.