What happened
On 24 May 2013, a Mooney M20E, registration ZS-CBW, departed from Springs Aerodrome for a repositioning flight to its home base at Heidelberg Aerodrome. The flight followed recent maintenance performed to address high oil consumption and engine roughness. While the pilot was performing a right-hand orbit over the Overkruin residential area to manage local traffic, an audible clanking sound emerged from the engine, followed by total engine failure approximately 30 seconds later.
The pilot immediately initiated emergency procedures, maintaining a best glide speed of 100 knots and broadcasting a Mayday call. In an attempt to reach the runway, the pilot extended the undercarriage early. This action increased the aircraft's descent rate, making it impossible to reach the threshold of Runway 24. The aircraft struck an embankment, causing the right main gear to detach, and skidded across a public road before coming to rest 300m short of the runway. Despite the substantial damage to the airframe, all four occupants evacuated the aircraft without injury.
The investigation
SACAA AIID investigators conducted a thorough teardown of the Lycoming IO-360-A1A engine to identify the source of the power loss. The inspection revealed that the engine could not be manually rotated due to internal seizure. Specifically, the investigators found scoring on the number 4 cylinder and noted that the piston pin had seized within the small end bush. This mechanical failure led to a fracture in the number 4 bearing journal of the crankshaft.
Metallurgical analysis of the crankshaft confirmed that the break was a fatigue mode failure. While the exact trigger for the fatigue was not definitively identified, investigators considered several contributing factors, including potential propeller imbalance, improper bearing seating, or machining-induced scoring during previous overhaul processes. The investigation also noted that the crankshaft was approximately 32 years old and had been in service since 1981.
Findings
- The engine failure was caused by a fracture in the crankshaft resulting from fatigue.
- The pilot's decision to extend the landing gear early during the glide increased the descent rate, preventing a successful landing on the runway.
- The internal damage to the crankshaft was not detectable during the aircraft's recent maintenance or the pilot's pre-flight inspection.