What happened
On July 22, 2017, at approximately 18:30 LMT, a Cessna U206G Stationair was performing its tenth flight of the day, transporting skydivers from Bielsko-Aleksandrowice Airport (EPBA). During the climb phase at FL115, the engine suddenly failed. The skydivers successfully exited the aircraft and landed safely.
Following the initial failure, the engine restarted but operated unevenly with significantly reduced power before failing again. The pilot attempted several corrective measures, including activating the electric pump and adjusting the fuel valve, which temporarily restored engine operation. However, after the pilot closed the mixture cock, the engine stopped once more, and cylinder head temperatures dropped rapidly toward the lower limit of the green arc. By adjusting the throttle and enriching the mixture, the pilot managed to regain engine operation. The pilot noted that the engine performed poorly under low power settings, so they maintained a higher manifold pressure (22-25 inches) to facilitate a normal landing at the airport.
Upon landing and reducing engine RPM, the engine failed a final time, and the propeller came to a stop. The pilot confirmed that sufficient fuel remained in both tanks (approximately 40 liters in the left and 50 liters in the right) and that no fuel starvation had occurred.
The investigation
An inspection of the Continental IO-520F engine was conducted by a PART-145 maintenance organization between July 24 and July 28, 2017. The engine had a total time of 1,610.2 hours since new and only 9.8 hours since its last major overhaul.
Technical findings revealed that the intake valve stem in cylinder number 1 had seized within its valve guide. This seizure caused the valve stem to bend, preventing the valve from closing properly and resulting in the engine's uneven performance and subsequent failures.
Findings
- The primary cause of the incident was engine oil contamination, which caused the valve lash adjuster on the intake valve of cylinder number 1 to stick.
- This contamination led to the seizure of the valve stem in the valve guide and the subsequent bending of the stem.