Cessna Oleo servicing and nose gear shimmy

By Stuart McAulay

Many of today’s aircraft landing gear designs incorporate oleo assemblies to ensure the softest possible landings. The basic aircraft oleo assembly is essentially a unique cylinder and piston combination commonly used on post-war era tricycle gear aircraft. The mechanical oleo assembly incorporates a mixture of hydraulic oil and air (usually nitrogen) to aid in the shock absorbency response upon each touchdown. Older fabric covered aircraft like Piper Cubs and Pacers were manufactured with heavy bungee cords on a hinged gear leg as the primary means of absorbing the shock of landings. Of course, many of those types were lighter and slower than most of today’s aircraft and were often flown off of grass fields as opposed to hard surfaced runways. The harder the landing (or larger the aircraft), the gear must absorb more energy as a result of that landing. This is where tougher landing gear oleo assmblies become increasingly necessary than the older style bungee designs.

The oleo design (referring to the air/oil mixture) had first been adopted during wartime aircraft production of the 1930’s and has continued, more or less, with the same design theory and reliability ever since. Many of the modern day low-wing Piper and Beechcraft designs utilize oleos in both the main and nose gear positions. Mooney has often incorporated a series of stacked rubber donuts on their designs as well. A majority of the modern day high-wing Cessna fleet use oleos in the nose position only and flat or tubular spring steel struts for the mains. As the aircraft models increase in size and weight, we see all of the popular twins use oleo assemblies corresponding to the overall size and weight of the aircraft. These oleo assemblies are extremely reliable considering the abuse they are subject to from repeated landings on mainly hard-surfaced runways. They are also extremely resilient to water, snow, dirt and grime. Regular servicing and maintenance ensures that these units continue to serve in all types of weather and operating conditions. Their inherent reliability often permits regular servicing to become postponed or overlooked for longer periods, but keep in mind that these servicing requirements may actually demand more frequent attention depending on the aircraft operating environment.

The typical oleo is serviced with fluid on the bottom and air (usually nitrogen gas) on top. As the strut gets compressed during landing, the volume of the fluid is displaced under pressure via the internally calibrated metering pin and orifice serving to regulate the overall energy transfer of the hydraulic oil within the unit. The high pressure also compresses the air enabling it to act as a shock absorber while the fluid also dampens the recoil action of the air so that the landing energy does not cause the aircraft to bounce back into the air. The air/oil mixture within the tube shaped outer cylinder and inner piston components is sealed by a combination of an O-ring and back-up rings for support. The unit also has a scraper ring which glides along the chromed piston to wipe away any debris as the oleo is repeatedly compressed. These components must be kept clean and in good condition to function properly.

Common maintenance routines include cleaning of the oleos so that the invasive elements collected in the form of dirt and moisture are removed before they begin to compromise the bearings, bushings and/or rubber seals. Using industrial strength degreasers and solvents are best for a thorough cleaning of the oleo from top to bottom. The area must also be blown dry with compressed air to ensure that all contaminants are removed from the harder to reach areas. Maintenance of the oleo unit ensures that the seals are good, the air/oil mixture is of the proper ratio, and that all mechanical connections are torqued, shimmed and lubricated as required by the aircraft service manual. Maintenance requirements also include close inspection of the gear components, attachment lugs and the fork assembly for cracks.

The most common defects associated with the care and servicing of oleos usually point towards air leaks, fluid seepage, and nose gear shimmy. Dirt and grime are always attracted to the exposed piston due to its close proximity to the ground and the light traces of hydraulic oil on the chromed strut. Dirt, foreign debris or solvent contamination can all contribute to the eventual deterioration of the main seal on most oleo installations if not addressed during routine servicing and maintenance.

Air leaks often originate around the seals if they become compromised or twisted in any way. Air leakage is also possible around the Schrader valve which is normally installed at the top of the oleo for servicing with fluid and adding the recommended air charge. In some cases, the high pressure valve core may not have seated properly within the valve body and a simple removal and re-installation could solve the problem. Ensure that the valve core is properly rated for the type of installation as high pressure cores are often marked with an H on the valve stem of certain models. Air leaks can usually be identified quite easily once you understand how the components all fit together. Re-charging the unit with a nitrogen regulator is usually performed by trained technicians who are familiar with the proper operation of the equipment.

Fluid leaks may also be the result of compromised seals and are easily recognized by excess fluid residue on the chromed piston area of the oleo. In the case of a noticeable fluid leak, the only course of action is to replace the main seal, then replenish the unit with new fluid and air. The leaked fluid is easily recognized as red in color (commonly known as 5606 or Fluid 41) and creates a sticky film on the outer parts of the oleo which soon become covered in dirt and debris. This same fluid type is common to most other light aircraft hydraulic gear and brake systems.

The caster movement of the nose wheel often leaves it vulnerable to shimmy especially when the oleo assembly is not maintained in optimal condition. Whenever the wheel veers from the center line due to wheel imbalance, tire deformity or uneven runway conditions while rolling, it tends to generate a restorative force to center it again but the resulting overcompensation often initiates a noticeable front end vibration (aka nose wheel shimmy). A very bad shimmy could resonate throughout the aircraft causing excessive vibration of the flight instruments and other components. Nose gear shimmy may actually be the result of any combination of factors. These additional factors may include worn torque link attachments or steering collar shims, a faulty shimmy dampener or even a loose wheel installation or out-of-balance wheel condition. The inclusion of wheel fairings (aka wheel pants) may also add to the concern of a shimmy condition.

There are various model applications for shimmy dampeners according to manufacturer design but they are all prone to leaking fluid at some point due to service severity and lack of routine servicing. The shimmy dampener (or damper) also uses a piston and cylinder design with small holes in the piston allowing pressurized fluid to transfer from one side of the piston to the other within the cylinder to help dampen the induced shimmy of the lower half strut (piston). Once the fluid inside the unit starts to seep out, the dampening action becomes less effective. A leaky fluid condition is usually obvious as the fluid sticks to the outside of the dampener and onto the oleo itself. The remedy would be to replace the affected seals and service with new fluid while ensuring that all remaining air has been purged out of the unit. There is an after-market design of shimmy dampener from the Lord Corporation who are known for their well-known history with engine rubber isolating mounts. This proven design has been around for several years now and replaces the fluid with a patented rubber core making this a truly maintenance free alternative. These designs are very common to most Cessna models and have applicability to many other models as well.

Regular servicing of the oleo usually includes topping up the air charge within the limits recommended by the manufacturer. The aircraft service manual usually calls for the strut to be fully extended for the charging process or the nitrogen charge can also be administered from a high pressure bottle and regulator with the aircraft weight on the oleo. Maintenance providers use Nitrogen as a more stable alternative to shop air to prevent moisture from entering the assembly and contaminating the metal components. A minimal or gradual slow air leak sometimes occurs from a minor seal defect, a worn piston, or an extreme change in outdoor temperature. It is not unusual for an oleo to sit lower than normal when exposed to colder temperatures. Recharging the unit with Nitrogen usually solves the problem. It is this air charge that ultimately determines the height of the oleo in its static position.

An oleo which has been depleted of its air charge will sit fully compressed with only the fluid possibly remaining. Aircraft manufacturers often reference a specified measurement of exposed chrome as the standard for oleo inflation. While oleo height is more specific to make and model of aircraft, a general rule for most light Cessna’s is the width of 4 fingers when placing your hand against the exposed chrome piston. Once it has been properly serviced, the oleo should respond to weight input that causes the aircraft to gently bounce without bottoming out (which occurs when the upper cylinder contacting the top of the fork assembly). An oleo that bounces too easily has sufficient air but lacking fluid. It’s important to get this air/oil balance right for optimal performance of the oleo assembly. An oleo which is nearly depleted of fluid can be detected while taxiing as the unit tends to feel bouncy allowing the upper half of the oleo assembly to bottom-out on the lower half causing a clunking sensation easily heard from the cabin. A properly maintained amount of air charge will sufficiently cushion the mild loads of oleo movement experienced while taxiing to and from the runway.

Servicing of aircraft oleos by owners should include regular cleaning of the exterior surfaces to remove excess dirt and grime that otherwise becomes a menace to the main seals. Check for sufficient lubrication of the torque links and steering collar grease points and note any defective grease fittings. Keep the oleo inflated to the proper POH specifications and use only nitrogen whenever possible as opposed to shop air. Have your maintenance provider check the fluid level and color of the fluid which darkens from contamination over time. Sometimes we tend to overlook these basic preventative measures simply because the components are generally very reliable. Oleo maintenance represents a good example of the old adage that “an ounce of prevention could be worth a pound of cure”

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