Background
On 5 July 2013, a Cessna 150 was conducting a low pass over the runway of the former military airfield in Sochaczew, Poland, with an instructor and a student on board. The flight was being operated under the supervision of FIS Warsaw.
Incident Sequence
During the low pass, after adding power, the instructor heard knocking sounds from the front of the aircraft and observed a drop in engine RPM to 1600. The instructor decided to execute a turn to return to the airfield. An approach for landing was made with the engine still running. During the flare, the engine stopped. The aircraft landed safely on the concrete runway and was subsequently towed off the runway.
Investigation
The Polish State Commission on Aircraft Accident Investigation (PKBWL) conducted an inspection of the engine at the operator's facility on 20 September 2013. The inspection revealed that the intake valve of cylinder #3 had broken and fragmented. Found components included a fragment of the valve head with a portion of its stem (element A), a spring retainer (element B), a valve collet (element C), and a fragment of the stem from the retainer groove area (element D). A missing portion of the valve stem was not located.
Metallurgical examination was performed on elements A and D. The examination showed that the fracture of the valve stem occurred in the area of the retainer groove and was fatigue in nature. The fatigue fracture covered almost the entire cross-section of the stem, indicating a long-term cracking process. The crack initiation point suggested that the damage progressed from the outer surface of the stem, and the likely initiator was a mechanical notch. The valve head and remaining stem fragment were free of material defects; their damage was secondary, occurring after the primary fracture at the retainer groove. The time and place of formation of the mechanical notch were not determined.
Probable Cause
The fatigue fracture of the stem of the intake valve of cylinder #3 in the area of the valve retainer groove, initiated by a notch, resulted in reduced engine power and forced a precautionary landing. The time and place of formation of the notch were not determined.