Occurrence
Upon touchdown at Calgary, the left main landing gear of the Fokker F28-Mk1000 immediately began to shimmy. The crew applied brakes to slow the aircraft and control the oscillation, but the shimmy persisted until both left main wheels and brake assemblies separated from the axles. The aircraft came to a stop, and all passengers and crew evacuated through the forward main cabin door without incident.
Examination
Site examination revealed that the upper torque link failed within the first 200 feet of the landing roll, and the wheels separated approximately 1,450 feet from touchdown. Substantial damage occurred to the oleo lower sliding member, wheels, tires, brakes, and left inboard and outboard flaps. The crew reported that the lift dumpers (spoilers) had been armed during the approach and were deployed when the aircraft stopped, but they were uncertain when deployment occurred.
The aircraft had recently been out of service due to a shimmy problem isolated to the right main gear, which was rectified by replacing an out-of-balance inboard wheel and tire. A successful flight test and positioning flight preceded the occurrence flight.
Investigation
The original excitation of the oscillations leading to the shimmy could not be determined. However, two factors allowed the oscillations to continue and amplify: additional clearances in the torque link pins and bushings, which reduced the dampening capability of the torque links; and excessive nitrogen pressure in the oleo, preventing compression immediately at touchdown. With the torque links extended in a position of minimum mechanical leverage, unchecked torsional movement of the sliding member overloaded the torque link to failure.
The operator was unaware of the non-approved fit of the torque link pins and bushings. The aircraft had not been in service long enough to require the operator's mid-life inspection on the gear, which would have revealed the excessive clearances.
Safety Actions
Following the occurrence, the operator implemented three changes across its fleet: rechecking and servicing all landing gear nitrogen pressures per the maintenance manual within 21 days; performing mid-life inspections on undercarriages whose torque link overhaul status could not be confirmed; and incorporating mid-life inspections as part of initial inspections for future fleet additions.