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Know your Plane

May 24, 2017

 

Nose Gear SHIMMY

The demands placed on your landing gear under different loads and operating conditions are vast. It pays to look after it.

 

 

A nose gear shimmy is an unstable swivelling oscillation of the nose wheel with a severe shudder feedback on the rudder pedals and airframe, and is a common snag. It happens on almost all aircraft and will probably be experienced by every pilot at some stage in their flying career. It is, however, not normal, is destructive and will lead to catastrophic failures. A shimmy generally happens for a short time, mostly on paved runways/taxiways and at different speeds. It will progressively get worse if not attended to. 

Although nose wheel shimmies are common and usually quickly attended to, the root causes are often overlooked, causing aircraft to be returned to the maintenance facility more often than is needed, adding to much frustration for owners and operators.

It is important to accept that a shimmy dampener is simply not strong enough to prevent a nose wheel shimmy when a nose tyre has established a shimmy wear pattern in the tread, or when nose landing gear and wheel components are worn beyond acceptable limits. In most cases, by repairing the shimmy dampener you are merely treating the symptoms and not the cause.

Although most manufacturers supply service information and troubleshooting procedures, the following is a compilation of what to look for to prevent and rectify a nose shimmy:

A good starting point is to check for the correct tyre pressure and nose oleo inflation. An uneven tyre wear pattern will be a good indication of a cause, while a wobble or out of round tyre can be checked by spinning the tyre. When fitting a new tyre, manufacturers paint a red dot on the outside to indicate the light spot of the tyre. A yellow stripe on the tube or the valve stem should be aligned to the red dot during assembly for static balance. However, the preferred method is to dynamically balance the tyre by spinning the wheel and adding balance weights as needed. Not all maintenance facilities have this equipment and this is mostly overlooked when new tyres are fitted.

With weight on the nose wheel, check for any un-dampened nose gear movement on torque links, the steering collar and shimmy dampener. Torque links are the connections between the nose wheel assembly fork and the steering collar. Any play on the torque links should be removed by replacing the attaching hardware and bushes. Simply tightening the torque link bolts can lead to over tightening which will cause the collar ears and torque links to crack. This is often done by inexperienced mechanics as a quick solution, but can be detrimental to the owner’s pocket.

The upper ends of the torque links attach to the steering collar. It is vital that the collar only move in a rotational motion around the strut. Any up, down or cocking motion should be shimmed.

Bad wheel bearings also allow undampened tyre movement. Wheel bearings should be in a good condition and properly lubricated.

It is imperative that the nose gear rigging is correct. Damaged or worn steering rods and bungee’s can lead to nose wheel shimmy.

A shimmy dampener has closed chambers filled with hydraulic fluid. When a shimmy occurs the motion is damped by hydraulic fluid being forced through a small hole in a movable piston, much like a shock absorber in a motor vehicle. Fluid leakage past the shaft can be caused by faulty or worn O-rings or a bent piston shaft, due to over steering or excessive shimmy. 

All attachment hardware should also be checked for condition, paying special attention to elongated holes on the attachment points. 

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