What happened
On the evening of 6 April 2022, a Cessna 172M Skyhawk, registration ZS-SBN, was conducting a night-rating training flight from Wonderboom Aerodrome (FAWB) in Gauteng. The flight, which included a student pilot and a flight instructor, was intended to be a routine training session within the Pretoria general flying area.
While cruising at approximately 6,000 feet near the Rooiwal Power Station, the flight instructor observed a high engine oil temperature indication. In response, the instructor took control to initiate a return to Wonderboom Aerodrome. Shortly after passing the N1 Highway, an audible 'click' was heard from the engine, immediately followed by a total engine stoppage. The instructor utilized the aircraft's glide capability to steer toward Moloto Road, where a successful forced landing was executed. There were 0 fatalities and 0 injuries to the occupants. The aircraft sustained minor damage, including a slight impact to the right wing tip caused by a passing truck after the landing.
The investigation
SACAA AIID examined the Lycoming O-320E2D engine through a detailed teardown inspection at an approved maintenance organization. The investigation focused on the engine's internal components to identify the source of the sudden failure. While the oil level was found to be adequate and no external leaks were present, the inspection revealed significant internal damage.
Investigators reviewed the aircraft's maintenance history, noting that the engine had undergone regular 50-hour and 100-hour inspections, and the last major overhaul had occurred approximately 447 hours prior to the incident. A non-destructive testing (NDT) procedure performed in May 2021 had previously shown no cracks in the crankshaft. However, the investigation tracked the progression of the failure through the engine's operational cycles.
Findings
Technical analysis confirmed that the engine crankshaft failed due to fatigue cracking. The fracture originated at the transition between the No.6 journal surface and the crankarm fillet, eventually progressing to the No.4 connecting rod/cylinder journal area.
It is estimated that the fatigue crack developed over at least 51 engine start/stop cycles following the last magnetic particle inspection. The physical evidence of the fracture included 'step-like' progression marks, which are characteristic of alternating loading patterns typically seen during engine start and stop sequences. This structural failure caused the crankshaft to shift, subsequently breaking the idler gear between the camshaft and crankshaft.