What It Looks Like

The Shimmy was a Roaring Twenties-era jazz dance characterized by a whole lotta shakin' goin' on. As a dance, shimmying looks like it could be a lot of fun. The shimmy most of us pilots are likely to experience, however, is the unnerving high-frequency shaking or wobbling of the front wheels of our cars and trucks-or airplanes.

An airplane nosewheel is subject to shimmying if the tire is out of balance or components in the nosewheel strut assembly are worn or bent. A slack rudder cable also can result in nosewheel shimmy on an airplane that has interconnected rudder and nosewheel steering.

You notice shimmying as a pronounced wobble or vibration that begins when the airplane gains speed on the takeoff roll or touches down on landing. Severe shimmy can lead to failure of the nosewheel assembly, much the way flutter can destroy a horizontal stabilizer.

Manufacturers try to prevent nosewheel shimmy by attaching a small shock absorber, called a shimmy dampener, on the nosewheel strut assembly. The dampener consists of a small shaft-mounted piston encased in a cylinder filled with hydraulic fluid. The piston shaft is attached to a fixed portion of the nose gear strut, while the cylinder is attached to the strut's steerable section.

The piston has a small calibrated hole that allows the hydraulic fluid to "leak" through when the cylinder moves slowly-when the nosewheel turns in response to the pilot's steering inputs. However, the fluid cannot pass when the cylinder is subjected to the high-frequency vibration of a shimmy. Instead, the fluid absorbs the vibration without transferring it to the rest of the nosewheel strut and airframe.

During your preflight inspection, you should check the shimmy dampener for leaking fluid and loose attachment hardware. If you see either, it's a good idea to consult a mechanic before flying the airplane.

If you experience nosewheel shimmy during the takeoff roll, it's prudent to abort the takeoff and return to the ramp for a more detailed inspection. If it first occurs on the landing roll, report it immediately to the FBO or your mechanic before the airplane departs on another flight. Tracking down the cause of shimmy can involve eliminating any number of possible problems until the culprit is positively identified.

Some shimmy dampeners can be dismantled and repaired if defective; others must be replaced. Regardless, you don't want to find yourself accelerating down the runway only to discover that, suddenly, there's a whole lotta shakin' goin' on.