What happened
On July 5, 2013, a Cessna 150 was performing a low pass over the runway at the former military airfield in Sochaczew, following permission from FIS Warszawa. During the maneuver, the instructor, who was accompanied by a student, increased engine power, at which point audible knocking sounds were heard from the front of the aircraft. The engine RPM dropped to 1600.
In response to the power loss, the instructor initiated a turn to return to the airfield. The aircraft proceeded to land with the engine still running; however, the engine ceased operation during the flare. The aircraft completed a safe landing on the concrete runway and was subsequently towed away from the active surface. There were no injuries to the two occupants.
The investigation
On September 20, 2013, the PKBWL investigation team inspected the engine at the operator's facility. The examination revealed that the intake valve of cylinder number 3 had broken and fragmented. Investigators recovered several components, including the valve head with a portion of the stem, the spring retainer, the valve lock, and a fragment of the stem near the lock area. A portion of the valve stem remained missing.
Metallographic analysis was performed on the recovered stem fragments. The results indicated that the fracture in the valve stem occurred near the valve lock area and was characterized by fatigue. The nature of the fatigue crack suggested a long-term, cyclic cracking process that originated from the external surface of the stem. The investigation identified a mechanical notch as the likely initiator of the crack, though the specific time and origin of this notch could not be determined.