Incident Overview
On 16 April 2011 at approximately 1300Z, South African Airways flight SA188, a Boeing 737-800 registered ZS-SJO, was taxiing on Taxiway 'A' at OR Tambo International Airport (FAJS) in Gauteng, South Africa, en route to Runway 03L for an international scheduled flight to Mwalimu Julius K Nyerere International Airport in Tanzania. The aircraft carried 2 cockpit crew, 5 cabin crew, and 100 passengers.
During taxi, the cockpit crew heard a loud thud and applied brakes to stop the aircraft. Fire services inspected the undercarriage and found that the right-hand side axle of the nose landing gear (NLG) had fractured, causing the entire right nose wheel to separate from the aircraft. South African Airways Technical engineers later inspected the NLG, and the aircraft was taxied back to the apron, where passengers disembarked normally without further incident. There were no injuries to occupants and no additional damage to the aircraft.
Aircraft and Component Details
The NLG inner cylinder assembly, which includes an integral axle with four wheel bearing journals, was removed and sent for metallurgical analysis to determine the cause of the fracture. The aircraft, a Boeing 737-800 (serial number 32357), was manufactured in 2001 and had accumulated 27,438.25 total airframe hours at the time of the incident. The NLG inner cylinder had been installed during production and had 13,577 cycles since new.
Metallurgical Findings
Initial observations showed that the NLG right-side inboard axle had fractured through the chromium-plated inner-bearing journal surface, beneath the area covered by the wheel bearing spacer. The outboard section of the axle remained attached to the right nose wheel assembly and was recovered. Both wheel bearings were found in satisfactory condition; the wheel nut was correctly lock-wired.
Local metallurgical analysis determined that the fracture originated from fatigue cracking. The cracking was linked to surface damage from grinding during the manufacturing process, assisted by hydrogen that evolved during the chrome plating process.
Probable Cause
The nose landing gear inner cylinder assembly, right-hand inboard nose wheel axle fractured through the chrome plated journal, causing the right-hand nose landing gear wheel assembly to separate from the nose landing gear. The fatigue cracking originated within the aircraft nose landing gear right axle as the result of surface damage associated with grinding during the manufacturing process, assisted by the initiation of hydrogen that evolved during the chrome plating process.