A Boeing 737-7Q8 was being prepared for a scheduled flight from Melbourne aerodrome, Victoria (Australia) on this day in 2009. With all preparations complete and passengers on board, the doors were closed and after obtaining clearance the engines were started.
Figure 1: The nose landing gear with the axle stump in the yellow dashed box
(Source and © ATSB)
While taxiing toward the runway for departure, a loud (and abnormal) thud was heard by the crew from the nose section of the aircraft. The crew of a passing company aircraft advised the crew they were missing a wheel from the nose landing gear. The aircraft was stopped immediately and engineers were dispatched to inspect the aircraft, and the nose landing gear specifically. They found that the right-hand nose landing gear axle had fractured and as a result, the right-hand nose wheel had separated. The aircraft taxied back to the gate where the passengers disembarked. There were no injuries to the crew and passengers and no further damage was sustained by the aircraft.
The nose landing gear (See Figure 1 above) has a telescopic oleo arrangement, comprised of:
an airframe-mounted outer cylinder assembly
an inner cylinder assembly that slid vertically within the outer cylinder
a torsion link unit joining the two.
Sliding movement of the cylinders was resisted and damped by compressed nitrogen and oil, which provided the shock absorption properties of the gear. The two nose gear axles were part of an integral forged assembly at the base of the inner cylinder. Each axle consisted of an inner and outer hard chromium-plated journal surface, upon which the corresponding wheel bearings were supported.
The incident was investigated by the Australian Transportation safety board (ATSB). The NLG inner cylinder right axle had fractured through the chromium-plated inner-bearing journal surface, underneath the area covered by the wheel bearing spacer (Figure 2). The outboard section of the axle remained in and was subsequently recovered with the right wheel. Both wheel bearings were in good condition and the wheel nut was lock-wired in place.
Figure 2; close up of fractured axle (Source and © ATSB)
They determined the cause of the nose landing gear axle failure was the 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.
Figure 3; Close-up of the failure area of the axle (Source and © ATSB) {Ratchet marks represent the boundary between cracks in different failure planes.}
The ATSB report on this axle failure is available by clicking here, it contains an extensive description of the metallurgical details, as well as detailed photographs of the axle failure.
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