What happened
On 20 May 2001, a Cessna T206H, registration ZS-OXI, was conducting a private pleasure flight departing from Lanseria Aerodrome. Shortly after the aircraft left the ground, the pilot observed the engine running roughly. Monitoring of the engine parameters revealed fluctuating cylinder head temperatures, rising oil temperatures, and a decreasing fuel flow. This was immediately followed by a loud bang, after which the engine failed completely and the propeller ceased rotation.
The pilot issued a may-day call on the Lanserila tower frequency and successfully executed a forced landing in a ploughed field in the Hartebeeshoek area near Hartebeespoort Dam. There were no injuries and no damage to the aircraft was reported.
The investigation
The investigation examined the engine's recent history and mechanical condition. It was noted that the aircraft had been involved in a previous engine failure during a practice forced landing on 14 February 2001. At the time of this second incident, the engine had only flown 14 hours since its previous repairs. The most recent Mandatory Periodic Inspection (MPI) had been completed on 25 April 2001.
Following the event, the engine manufacturer, Lycoming, requested the unit be returned to their facility in the United States for analysis. A physical inspection of the crankshaft identified two circumferential cracks. Subsequent metallurgical analysis determined that the crankshaft fractured due to fatigue, likely caused by over-stresses.
Findings
- The engine failure was caused by a fracture of the crankshaft through the cheek and counterweight ear forward of the #6 crankpin journal.
- The fracture was attributed to an overstress condition.
- Although the crankshaft met all dimensional, material, and chemical specifications, it was later noted that Lycoming introduced a new crankshaft forging process in 2003 to address risks associated with improper forging in previous parts.