What happened
On 16 April 2011, a South African Airways Boeing 737-800, registration ZS-SJO, was taxiing toward Runway 03L at OR Tambo International Airport for an international flight to Tanzania. While moving along Taxiway “A”, the flight crew heard a loud impact. The pilots immediately applied the brakes to halt the aircraft.
Upon inspection by airport fire services, it was discovered that the right-hand side axle of the nose landing gear had fractured. This failure caused the entire right nose wheel assembly to separate from the aircraft. Following an inspection by technical engineers, the aircraft was moved back to the apron, where all 100 passengers and 7 crew members disembarked safely. There were no injuries resulting from the incident.
The investigation
Investigators examined the fractured nose landing gear inner cylinder assembly, which contains the integral axle. Metallurgical analysis was conducted locally and by the National Transportation Safety Board (NTSB) in the United States, with further detailed study performed by Boeing Research and Technology.
Testing revealed that the fracture originated at the chrome-plated inboard wheel bearing journal. The investigation focused on the history of the component, noting that the part was installed during the aircraft's initial production. While a Boeing service letter had previously recommended inspections for heat damage during overhauls, this specific axle was not due for overhaul until December 2011.
Findings
The investigation concluded that the primary cause of the failure was fatigue cracking that began at a pre-existing site within the metal. The root cause was traced back to the manufacturing process, where abusive grinding of the chrome plating caused heat-induced damage to the base metal.
This initial damage was further compromised by hydrogen embrittlement, which occurred during the subsequent plating operations. Once the aircraft entered service, the structural integrity of the axle degraded through continued fatigue propagation until the final ductile separation occurred, leading to the wheel separation.