On 4 July 1999, a Fokker F100 aircraft, on a direct service from
Brisbane to Norfolk Island with 43 persons on board, experienced a
severe vibration from the left main landing gear as the brakes were
applied during the landing roll. The crew were able to bring the
aircraft safely to a halt on the runway and then conducted an
inspection of the aircraft. This revealed that the left main
outboard wheel was missing. There was no other damage to the
aircraft and no injuries to any of the occupants. The crew reported
there was approximately 15 knots of crosswind for the landing.
The wheel was later located on the flight strip, where it was
examined and then forwarded to the Australian Transport Safety
Bureau (ATSB) for specialist fracture analysis.
Examination of the maintenance documentation showed the wheel
had completed 99.8 hours and 77 cycles since it was last
overhauled. During overhaul, the wheel had also undergone an
approved repair to remove scoring from the hub caused by rubbing
contact with the brake heat shield during service. The repair
involved reducing the hub diameter by 0.02 inches by machining. The
repair instructions specified that after the material was removed
the repaired region was to be shot-peened and the shot-peening
parameters were to be adjusted to produce a specific surface
quality.
The specialist fracture analysis report concluded that the wheel
failed because of a fatigue crack, starting at the surface of the
metal, in the repaired region of the axle-hub to wheel web
transition. It found that no single stress point concentrator had
started the cracking. It had begun at numerous closely spaced
points around the circumference of the hub, known as ratchet marks.
This was consistent with sideways flexure of the wheel web, and
with crack growth from the repaired surface of the hub. There was
no indication the growth had started at any crack that had been
present prior to the repair.
The manner in which the fatigue crack spread was also consistent
with sideways flexing of the wheel web. Such flexing would occur
when a turning moment (torque) was applied to the main landing gear
while the wheels were rotating, such as during ground turning or
cross-wind landings. The crew reported that high crosswind
components are regularly experienced during take-off and landing at
Norfolk Island.
A comparison of the surface of the repaired region with the
original surface of the wheel hub revealed that the intensity of
shot peening was lower than that applied during manufacture. This
variation would be expected to lower the resistance of the wheel to
fatigue cracking. The lower level of compressive residual stress
associated with the less intense shot-peening process applied to
the repaired region would also increase the likelihood of fatigue
failure under normal loading conditions. As part of the
investigation into this incident, the operator's maintenance
facility found that a part of the left main landing gear shimmy
damper had been incorrectly re-assembled during last overhaul.
Analysis of the wheel fracture surface evidence determined that
this would have had minimal if any influence on the fatigue crack
starting or spreading.
On 9 October 1999, this aircraft had a similar incident
involving the failure of the left main landing gear upper torque
links while landing at Norfolk Island, ATSB occurrence number
199904802. The ATSB specialist fracture analysis report found the
torque link failure had started in similar circumstances to those
for the wheel hub failure.
CONCLUSIONS
The left main landing gear wheel failed as a result of fatigue
cracking. Although the cracking started at the site of the repaired
area of the wheel hub, the cracking did not start as a result of
any surface changes, such as scoring.
The surface treatment of the repaired region was significantly
different to the "as manufactured" condition of the hub. The
repaired region had evidently been shot peened by a lower intensity
process.
A reduction in the intensity of shot peening, with an
accompanying reduction in the magnitude of residual compressive
stress, had made fatigue cracking more likely under normal loading
conditions.
The wheel finally failed during the early stage of landing at
Norfolk Island Airport while torque was being transmitted through
the landing gear assembly. These significant landing gear torque
loads were probably associated with cross-wind take-off and
landings.
RECOMMENDATION R20000310
The ATSB recommends that the UK Civil Aviation Authority review
the repair and overhaul processes for the failed wheel and also for
the failed torque links identified in occurrence 199904802, to
ensure they conform to the appropriate airworthiness
requirements.