What happened
On the morning of 21 April 2024, a Boeing 737-800, registration ZS-FGE, was performing a scheduled domestic flight from O.R. Tambo International Aerodrome (FAOR) to Cape Town International Aerodrome (FACT). During the takeoff rotation, the left outer mainwheel detached from its axle. A cabin crew member noticed the separation through a window and alerted the flight deck.
Following the incident, the crew entered a holding pattern west of Swartkop Air Force Base to reduce aircraft weight by burning off fuel. During this period, the crew requested a low-altitude fly-pass at their maintenance facility to allow technicians to inspect the landing gear. After confirming the left inner mainwheel remained attached, the crew proceeded to land on Runway 21R at FAOR. While the landing itself was successful, the left inner mainwheel tire burst approximately 30 meters before the aircraft came to a complete stop. There were no injuries among the 178 passengers and 6 crew members on board.
The investigation
SACAA AIID investigators examined the failed wheel hub components and the aircraft's maintenance history. The investigation included a metallurgical analysis of the hub and an audit of the maintenance organization responsible for the wheel assembly. The inquiry also reviewed similar historical occurrences involving the Boeing 737-800 to identify failure patterns.
Findings
Technical analysis revealed that the failure originated from a fatigue crack at the bearing cup seat transition of the inboard wheel hub bearing bore. This crack led to a loss of the interference fit, causing the bearing cup to rotate. This rotation generated excessive heat and fretting, which accelerated the crack's propagation and eventually caused the inboard hub half to fail catastrophically.
As the inboard hub failed, the outboard bearing was forced to carry the entire load of the wheel. This created a severe overload and misalignment, leading to the disintegration of the outboard bearing. During the takeoff rotation, as the aircraft's weight was transferred from the wheels, the remaining components were no longer held in place by the aircraft's weight, allowing the wheel to wobble and eventually separate from the axle.