What happened
On the morning of 1 August 2021, a student pilot and an instructor were conducting a routine circuit training flight at Lanseria International Airport (FALA). While operating on the downwind leg for Runway 07, the crew of the Cessna 172N, registration ZS-SPB, experienced severe vibrations and a rough-running engine. Shortly after, a loud knocking sound was heard, and the engine ceased operation entirely.
Following the loss of power, the crew attempted to restart the engine without success. After declaring a Mayday call, the pilot identified taxiway Charlie as a suitable landing site. The aircraft performed a successful forced landing on the taxiway, resulting in no injuries to the two occupants and no damage to the airframe.
The investigation
An investigation by the SACAA AIID focused on the mechanical failure of the engine. The engine was removed and inspected at an approved workshop. The examination revealed that the crankshaft had fractured into two separate pieces at the Number 4 piston connecting rod journal area.
Investigators also reviewed the engine's maintenance history, noting that the engine had been overhauled in December 2019. Records indicated that during this overhaul, the journals were polished to accommodate specific bearing sizes. The investigation compared the failure pattern to similar documented cases where the polishing process created a surface irregularity.
Findings
- The primary cause of the in-flight engine stoppage was a failed crankshaft that broke into two halves due to fatigue cracks.
- The fatigue cracks originated at the transition between the journal bearing surface and the crank web fillet.
- A contributing factor was the potential introduction of a notch or material weakness during the polishing of the connecting rod journal during the engine's last overhaul.