What happened
On the morning of 21 May 2025, a Cessna Cardinal C177B, registration ZS-FXT, departed Newcastle Airfield in KwaZulu-Natal, bound for New Tempe Airfield in the Free State. The flight was being conducted under private operations in clear weather conditions. Approximately ten minutes into the journey, while cruising at 1,500 feet, the engine began running roughly before losing power. As the aircraft descended, the engine ceased functioning entirely at roughly 80 and 800 feet above ground level. The pilot identified a suitable field near the Chelmsford Nature Reserve and executed a forced landing. There were no injuries and no damage to the aircraft.
The investigation
SACAA AIID examined the engine, a Lycoming O-360 A1F6, which had been recovered to a maintenance organization for a teardown inspection. The investigation focused on the engine's recent maintenance history, noting that the engine had undergone an overhaul only 29 hours prior to the incident. Investigators analyzed the physical condition of the cylinders and pistons, as well as the maintenance procedures used during the recent overhaul.
Findings
Technical analysis revealed that the failure of the exhaust valve spring key on the number three cylinder was the primary cause of the engine failure. This failure allowed the exhaust valve to become stuck and subsequently drop into the cylinder, where it was struck by the piston. The resulting impact caused extensive internal damage, including destruction of the valve and piston, and the dispersal of debris into the other cylinders and the oil sump.
While the maintenance organization had performed valve leak tests, they did not perform valve guide inspections because the cylinders were delivered from the supplier with components pre-installed. Notably, the supplier later suggested that a manufacturing defect in the supplied parts may have caused the valve spring key to fail. The investigation also noted that the engine had previously experienced rough running in February 2024, which had prompted a necessary overhaul.