What happened
On 6 June 2014, a solo pilot conducting a cross-country flight from George to Tsitsikamma experienced a sudden mechanical emergency near Plettenberg Bay. While climbing through 2,500ft AMSL near Beacon Island, the pilot noted the engine was running roughly and felt vibrations through the control column. After reducing power and initiating a turn toward Plettenberg Bay aerodrome, the propeller suddenly detached from the crankshaft flange, flying over the cockpit.
The pilot declared a Mayday and, facing a loss of engine power, identified an open stretch of beach as a suitable landing site. The Jabiru SP, registration ZU-FTC, touched down on the beach and came to a halt within 60 meters. Local sea rescue teams arrived at the scene via rubber duck to assist. There were no injuries to the pilot.
The investigation
An investigation by the SACAA AIID focused on the mechanical sequence leading to the separation. An engine teardown revealed that the number two cylinder exhaust valve had become stuck in the open position due to heavy carbon build-up on the valve stem. This caused the valve to strike the piston, leading to a catastrophic engine seizure and a deformed crankcase.
Metallurgical analysis determined that the engine failure occurred first. The sudden seizure applied a massive reverse load to the still-rotating propeller, which sheared the six flange attachment bolts and caused the propeller to separate from the aircraft. The investigation also noted that while the aircraft had undergone recent compression tests, the non-mandatory Service Bulletin SL8, intended to prevent such carbon build-up, had not been implemented on this engine.
Findings
- The primary cause of the incident was a catastrophic engine failure triggered by an exhaust valve failure in the number two cylinder.
- Excessive carbon accumulation on the valve stem prevented the valve from sealing and eventually caused it to stick.
- The propeller separation was a secondary effect caused by the sudden mechanical shock of the engine seizing.
- The aircraft's maintenance records showed the engine had passed a differential compression test only 12.7 hours prior to the failure.