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, involving a student pilot and a flight instructor, was part of a Part 141 training program.
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 landing light to navigate and executed a successful forced landing on Moloto Road. While the aircraft sustained minor damage to the right wing tip due to contact with a passing truck, there were 0 fatalities and no injuries to the occupants.
The investigation
SACAA AIID examined the Lycoming O-320E2D engine through a teardown inspection at an approved maintenance organization. The investigation focused on the engine's mechanical integrity and maintenance history.
Records indicated that the engine had undergone regular maintenance, including a 100-hour annual inspection in March 2022 and a 50-hour oil inspection in late March 2022. The engine was also well within its time between overhaul (TBO) limits, having completed only 447.3 hours since its last overhaul. Furthermore, a non-destructive testing (NDT) procedure, including magnetic particle and fluorescent penetrant inspections, had been performed on the crankshaft in May 2021 with no cracks detected.
Findings
Technical analysis of the engine revealed 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.
Investigators determined that the fatigue crack likely developed over a period of at least 51 engine start/stop cycles, which had occurred since the last magnetic particle inspection. The physical evidence of the fracture included 'step-like' progression marks, which are typically associated with changes in the alternating loading spectrum during engine start and stop sequences.