A nosewheel shimmy is usually caused by wheel imbalance or uneven tire wear, exacerbated by loose joints in the nose strut scissors and steering mechanism. Therefore, you should begin troubleshooting a shimmy issue by ensuring that all the bushings, bolts, and rod end joints are tight and free of excessive play before proceeding to the shimmy damper as the remedy.
The shimmy damper does not prevent shimmy oscillations at the source, per se. But it does play an important role in ensuring that minor vibrations and oscillations remain damped and under control. This reduces wear on every other joint and system in the aircraft (including the pilot). It’s a circular relationship: A tight nose gear system allows the shimmy damper to function properly and control minor oscillations, while a functioning shimmy damper protects the nose gear from excessive wear. If either system breaks down, the other will soon follow.
Most experienced mechanics will correctly point to worn nose gear components first when a pilot squawks about nose shimmy. However, it’s a mistake to simply repair the loose components and call the job done without servicing the damper at the same time. Doug Haughton of Avian Aeronautics specializes in Beechcraft damper overhauls and probably rebuilds more shimmy dampers in a year than most mechanics do in a lifetime. According to Haughton, about 90 percent of the shimmy dampers that come through his shop have not received the routine maintenance that would prevent the wear that ultimately scraps expensive components.
Shimmy dampers work on a simple principle: A shaft with a piston fixed at the midpoint slides fore/aft inside a cylinder containing hydraulic fluid. A small hole in that the piston allows the fluid to pass through at a metered rate. This forces the fore/aft movement of the rod to be slow and controlled, thereby damping rapid movements of the steering mechanism that it’s attached to. O-rings form seals around the piston, as well as the shaft and end caps at each end of the cylinder.
Air is the main enemy of a properly functioning shimmy damper. The O-rings are not perfect seals, especially as they harden and take a set over time. When hydraulic fluid is lost from the cylinder, it is replaced by air and the damping action of the piston is lost. Even a small amount of air in the cylinder will compromise proper damper function.
The typical Cessna damper has no built-in reserve of fluid, so it should be serviced proactively to ensure that it remains full and functional. Beechcraft dampers, by comparison, are more complex because they include a fluid reserve mechanism. The shaft that operates fore/aft is hollow and serves as a reservoir for the hydraulic fluid. The excess fluid is pressurized by tiny springs and pistons, replenishing any lost fluid in the main cylinder. The amount of reserve fluid in the shaft can be checked by poking a piece of safety wire into the shaft to determine the position of the reserve pistons. When the wire goes in past a certain point, the reserve fluid is getting low and it’s time to service the unit before the main cylinder is affected.
Moisture is the second enemy of shimmy dampers because moisture anywhere in the mechanism will cause corrosion and impede the damper’s ability to seal and work properly. This is another reason to disassemble, clean, and replace the seals and fluid on a regular basis.
As with many other aspects of aircraft maintenance, routine maintenance can prevent unplanned (and expensive) repairs and component replacement. In the case of shimmy dampers, routine maintenance can usually be done in under an hour and only requires a few dollars’ worth of O-rings and hydraulic fluid. However, left unchecked, the shafts and cylinders can corrode or score from lack of fluid and excessive movement—bringing the cost well into the hundreds of dollars in parts alone.
Under the supervision of an A&P, shimmy damper servicing is also a great owner-assisted maintenance task. Talk to your mechanic, crack open your aircraft’s maintenance manual, and give your nose gear a little TLC this winter. You’ll appreciate your work with every landing. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.