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 crosswind 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 crosswind conditions. The crew report for a previous landing incident with this aircraft at Norfolk Island, indicated that crosswind 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.