AS THE WRENCH TURNS
David Vital, Director of Maintenance email@example.com
Hello all. Spring is upon us and the good flying weather is ahead. This month we will talk about what causes the infamous nose shimmy. Nose wheel shimmy conditions may be caused by any one of the following, but are usually the result of a little of all three problems.
1. The torque links (scissors)
The torque link bushings and bolts should be checked for looseness and wear. One way to determine if the torque links are the cause is to replace the three castellated nuts on the torque link bolts with regular nuts, and perform this test: Tighten the nuts tighter than you would normally tighten a castellated nut to squeeze the links together to remove any excessive looseness. Make sure that the shimmy dampener and the nose gear strut are properly serviced. A high speed taxi test should tell you if the torque links are the cause. If they are, replace bushings and bolts as necessary and reinstall with castellated nuts.
2. The steering arm assembly shims (fixed gear aircraft only)
These shims are located between the bottom of the steering arm and the bottom of the shock strut outer tube. After a period of time, these shims will wear, allowing the steering arm to move up and down. When this happens, any shimmy movement becomes a vertical movement rather than a horizontal movement that can be dampened out by the shimmy dampener. These shims are available in various thicknesses and are used as necessary to obtain a snug fit. Access to the shims requires removal of the shock strut assembly from the aircraft.
3. The shimmy dampener
The shimmy dampener should be properly serviced and the attach points checked for looseness. The shimmy dampener should provide the same amount of resistance from one end of the piston rod travel to the other. Since most of the shimmy dampener piston travels in the center of the housing, the center portion of the housing will usually show the most wear. When the housing becomes worn to the point that the piston can no longer provide a good seal, the fluid will bypass around the piston instead of going through the orifices in the piston. When this happens you lose the restriction that provides the dampening action for the nose wheel assembly.
Many times the nose wheel fairing is blamed for a shimmy condition. Unless the fairing is actually loose, it will not cause a shimmy. However, the added weight of the wheel fairing may allow the nose wheel assembly to resonate after a shimmy has started.
So that is a brief description and reasoning of the infamous nose wheel shimmy.
Reminder the MX team will be holding a Pizza Party in the hanger on Feb 22 @ 12:00 pm. There will be a couple of planes opened up for your observation. The MX team will also be there and available to answer your questions. We hope to see you there.