On 25 July 2009, a Boeing 737-7Q8 aircraft, registered VH-VBA, was taxiing toward the runway for departure at Melbourne aerodrome, Victoria, when the crew reported hearing a loud thud from the airframe. The crew of a passing company aircraft advised the crew of VH-VBA that they had lost a nose wheel tyre. It was subsequently discovered that the right wheel had detached from the nose landing gear (NLG) as a result of a fracture of the axle.
An Australian Transport Safety Bureau investigation of the NLG failure determined that the nose wheel had separated as a result of the initiation and propagation of a fatigue crack through the right, inboard bearing journal. The fatigue crack had originated under the influence of residual stresses in the steel surface associated with grinding damage during manufacture, and its initiation was probably hydrogen-assisted from plating processes applied to the journal bearing surfaces.
As a result of the occurrence, the aircraft operator conducted an immediate, fleet-wide inspection of axles with similar service history. To reduce the likelihood of future possible axle failures, the aircraft manufacturer conducted an audit of the landing gear supplier's processes and production records, in an attempt to establish the extent of the grinding problem. The aircraft manufacturer also released a communication to 737 operators and maintenance providers, detailing enhanced inspection recommendations for the identification of grinding damage.