On 9 October 1999, a Fokker F28-100 aircraft, on a direct
service from Brisbane with 84 persons on board, experienced severe
vibration through the airframe during landing at Norfolk Island.
The crew stopped the aircraft on the runway and, after a
preliminary examination, taxied the aircraft to the terminal where
the passengers disembarked normally. There were no injuries.
Investigation revealed that the left main landing gear upper
torque link attachment lugs had broken. The upper torque link
attachment point on the landing gear main fitting was an integrally
forged double lug with a stiffening web between the two lugs. The
maintenance documentation showed that the main landing gear had
completed 16,579 cycles since new and 658 cycles since last
overhaul.
The Australian Transport Safety Bureau (ATSB) conducted
specialist fracture analysis of all the broken landing gear
components. The specialist report concluded that the failure of the
torque link attachment lugs was associated with the extension of
pre-existing cracking in the lug-stiffening web while torque was
transmitting through the torque links. The initial cracking in the
web was caused by stress corrosion. The propagation of the fatigue
crack was consistent with a loading regime that involved the
sideways flexing of the wheel rim. This will occur when a turning
moment (torque) is applied to the main landing gear while the
wheels are rotating, such as during ground turning or cross-wind
landings.
The evidence showed that the region of the pivot pin bore and
locating pinhole had been reworked during overhaul. At that time
material had been removed by localised surface grinding to remove
corrosion. The pivot bore surface was then shot peened and
repainted with a chromate based paint primer. However, the paint
primer exhibited poor adhesion and the shot peening coverage was
haphazard. Consequently, these measures had been ineffective in
preventing stress corrosion.
The final failure of the torque link attachment lugs occurred
during the initial stage of the landing, and occurred while the
landing gear was being subjected to significant torque loads. It is
likely that the torque loads were associated with cross-wind
conditions. The crew report for a previous landing incident with
this aircraft at Norfolk Island, indicated that cross-wind
components of 15 knots or higher are regularly experienced during
operations at Norfolk Island.
The operator's maintenance facility reported that part of the
left main gear shimmy damper was found to have been wrongly
re-assembled during last overhaul. The fracture analysis evidence
indicates this would have had minimal if any influence on the start
or development of the fatigue cracks that led to the failure of the
torque link attachment lugs.
A previous incident occurred on 4 July 1999, involving the left
main landing gear of this aircraft, also while landing at Norfolk
Island. In that incident, ATSB occurrence number 199903327, the
outboard main landing gear wheel broke away from the wheel hub
during the landing roll. The ATSB specialist fracture analysis
report (see below) found the wheel failure had started and
progressed in similar circumstances to those for the torque link
attachment lugs.
