Serious Incident Involving a Cessna U206G Stationair at Bielsko-Aleksandrowice (SP-ZKU)

Incident Overview and Context On 22 July 2017, at approximately 18:30 local mean time, a Cessna U206G Stationair operated by Skoki BB was conducting its tenth parachute drop flight of the day from Bielsko-Aleksandrowice Airport (EPBA). The aircraft was carrying six passengers, all of whom were parachute jumpers. The pilot was the sole crew member on board. During the climb after takeoff, the engine suddenly stopped at flight level 115. The jumpers exited the aircraft safely and landed without incident. The engine subsequently restarted but ran unevenly and with noticeably reduced power before stopping again. The pilot activated the electric fuel pump and switched the fuel valve, which restored engine operation. When the pilot closed the cowl flaps, the engine stopped again, and cylinder head temperatures rapidly dropped to the lower limit of the green range. The pilot slightly retarded the throttle and enriched the mixture, after which the engine restarted. The pilot noted that the engine performed poorly at lower manifold pressure settings and therefore set a higher manifold pressure of 22–25 inches before making a normal landing at the airport. Upon reducing engine speed after landing, the engine stopped again and the propeller came to a halt. The pilot confirmed that fuel levels in both tanks were adequate, with approximately 40 liters in the left tank and 50 liters in the right tank, and that the tanks had not been exhausted. ## Technical Investigation and Findings Between 24 and 28 July 2017, a PART-145 organization inspected the Continental IO-520F engine, serial number 564914. The inspection revealed that the engine had a total time in service of 1,610.2 hours and 9.8 hours since its last major overhaul. Investigators found that the stem of the intake valve in cylinder number 1 had seized in its valve guide and was bent, preventing it from closing properly and causing uneven engine operation. The organization replaced the cylinder number 1 assembly, checked the integrity of all cylinders, changed the oil, inspected the full-flow filter for metal particles, and performed a borescope inspection of the cylinder interiors, with particular attention to the intake and exhaust valves. The removed cylinder number 1 assembly and valve were sent to the engine manufacturer to determine the cause of the malfunction. ## Probable Cause and Conclusions The most probable cause of the incident was oil contamination in the engine, which caused the valve clearance compensator on the intake valve of cylinder number 1 to hang up and led to the seizure of the valve stem in its guide, accompanied by bending of the stem. No contributing circumstances were identified. The operator did not take preventive actions, and the investigation committee issued no safety recommendations.