What happened
Upon touchdown at Calgary, the left main landing gear of a Fokker F28-Mk1000 immediately began to oscillate. In an effort to stabilize the aircraft and suppress the shimmy, the crew applied the brakes. However, the vibrations intensified, eventually causing both the left main wheels and the brake assemblies to detach from the axles. The separation occurred approximately 1,450 feet from the touchdown point, while the initial failure of the upper torque link happened within the first 200 feet of the landing roll.
Following the incident, the aircraft came to a halt, and all passengers and crew were evacuated through the forward main cabin door without injury. The aircraft sustained significant damage, including impacts to the left inboard and outboard flaps, as well as damage to the tires, wheels, brakes, and the lower sliding member of the oleo.
The investigation
Investigators examined the mechanical state of the landing gear and the maintenance history of the aircraft. It was noted that the aircraft had recently undergone maintenance to address a similar shimmy issue on the right main gear. While a flight test had been successful, the left gear developed the oscillation during the landing in question.
Technical analysis focused on the torque links, which are designed to maintain wheel alignment and prevent rotation. The investigation found that the upper torque link had failed under the stress of the oscillations. Furthermore, the investigation looked into the nitrogen pressure within the oleo and the condition of the pins and bushings within the torque link assembly.
Findings
- The primary cause of the failure was the failure of the upper torque link due to excessive torsional movement.
- The shimmy was amplified by excessive clearances in the torque link pins and bushings, which reduced the system's ability to dampen oscillations.
- Nitrogen pressure in the oleo was higher than the levels specified in the maintenance manual, creating excessive stiffness that prevented the oleo from compressing immediately upon touchdown.
- The landing gear design was found to be highly sensitive to any slack or looseness in the component build-up.
- The operator was not aware that the torque link pins and bushings had been installed using non-approved, loose-fitting parts during a previous overhaul.
Safety action
Following the investigation, the operator implemented three maintenance changes across their fleet:
- Nitrogen pressures in all landing gear must be verified and serviced strictly according to the maintenance manual within 21 days.
- A mid-life inspection must be performed on all undercarriages where the torque link overhaul status cannot be confirmed as meeting "new" limits.
- Mid-life inspections will be integrated into the initial inspection process for all future aircraft added to the fleet.