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Care and Inspection of landing gear shock struts

Many aircraft landing gear designs incorporate oleo assemblies in various configurations depending on the aircraft type. The oleo is essentially a piston and cylinder assembly that contains an air/oil mix to provide a shock absorbing response upon landing. Most small aircraft utilize an oleo assembly in the nose gear position for tricycle gear models. The main gear can also be a similar oleo design or made of tubular or flat spring steel. Most of the Piper low wing style aircraft use the three oleo design whereas most of the Cessna 100 and 200 series singles use the spring steel on the mains. In the larger twin category of aircraft, we see the oleos in all three positions. Older fabric covered aircraft like the Piper Cub and Pacer designs were manufactured with low cost bungee cords as the primary means of shock absorption from landings – mostly on grass airstrips. Some of the basic features of these earliest models did not have bungees and relied on low pressure tires for minimal shock absorption.

The standard oleo design is common to various airframe manufacturers although exact size and shape may vary from one type to another. These assemblies are regularly exercised by the normal forces of landing and are therefore subject to a detailed servicing schedule although regular servicing is sometimes overlooked simply due to the inherent reliability of their strong design. This should not dissuade the owner from remaining attentive to landing gear condition as recommended. Keep in mind that maintenance and servicing requirements may demand more frequent attention depending on the aircraft operating environment such as gravel strips or extremely cold temperatures.

The most common defects associated with oleo maintenance are air leakage, seeping fluid and nose gear shimmy. Even under normal operations, the main seal becomes compromised due to abrasion from dirt and other contaminants adhering to the chromed piston. These oleo units see all the loose gravel and dirt first hand even on well-groomed surfaces. Once the main seal becomes compromised by foreign debris, it is likely that leaking air and fluid will soon follow. In most cases, nose gear steering is actioned by steering rods which are linked to pedal movement from the cabin. Other affected parts of the oleo include the shimmy dampener and the torque link attachments. Nose gear shimmy is a condition that can be influenced by a combination of factors related to worn parts. This sometimes elusive condition is more pronounced with weight on the nose wheel during landing and is recognized through transmitted vibration often felt throughout the entire aircraft.

The integrity of the overall assembly is usually determined through regular 100 hour inspections where technicians examine each of the moving segments in relation to each other. Minor adjustments can usually be done on the aircraft whereas a more intensive overhaul of the unit may require removal from the aircraft. Maintenance crews will prepare for landing gear removal by jacking and supporting the aircraft according to the service manual instructions. Aircraft jacking and support points are determined by the manufacturer and can differ between aircraft types. 

Removing the air charge from the unit is necessary before preparing the unit for disassembly otherwise it may cause the unit to explode apart upon removal of the securing hardware. This nasty surprise is often accompanied by a sudden splash of hydraulic fluid as well causing a potential mess and risk of serious injury. A thorough repair or overhaul of the unit will require replacement of all installed seals particular to the assembly. Oleo seals come in various forms and are represented by O-rings, back-up rings, scrapers, and other attaching parts required for proper assembly. Other attachments may require bushings and/or bearings that tend to wear out throughout regular loading and stresses.

Keep in mind that the disassembly process is also an opportunity for closer inspection of the fork, steering collar, and cylinder tube for cracks and/or damage. It is not unusual to clean and dry all parts for detailed inspection and even strip paint to better evaluate suspect areas. When replacing the rubber seals throughout the unit, it is important to use new products that have not exceeded their shelf-life. This can be determined by experienced personnel who can identify the packaging and/or cure date information printed on the packaging itself. It is best to keep these seals in their packaging until it is time for them to be installed in order to prevent product contamination or mixing up similar sizes. Extra care is taken when installing the seals to ensure that they are not damaged during this phase of the repair. These seals require adequate lubrication in order to prevent rolling or twisting while being installed.

As mentioned earlier, one of the causes of nose wheel shimmy may be directly related to the shimmy dampener and is not necessarily due to worn parts. The dampener is also filled with hydraulic fluid and is prone to leaks through service severity and lack of routine servicing. Fluid leakage will be evident on the exterior of the unit as the residue also attracts dirt and dust to create a sticky mess. The best solution to a troublesome shimmy dampener has been the replacement of the unit with a maintenance-free design that utilizes rubber as the interior cushioning agent rather than the typical fluid. This innovative technology has been available for many years now and has even been adopted by some manufacturers.

Re-assembly of the oleo is done methodically and with great care not to damage the seals as already mentioned. Once the unit has been re-assembled and installed back onto the aircraft, the next step would be to service the oleo again with new fluid and air. It is imperative to use new fluid as not to re-introduce contaminants back into the system. Nitrogen is used as a cleaner source of air to prevent contamination and long term corrosion. Only trained technicians should be permitted to pressurize the oleo assembly using Nitrogen from a high-pressure bottle due to the critical nature of this seemingly simple task. The method used to determine proper oleo extension is found in the aircraft service manual and/or POH. Experienced maintenance providers usually have a good sense of how it all works.

Once in service, troubleshooting of landing gear oleos is usually required when the exposed chrome extension appears to be low or feels too bouncy. A low oleo condition may simply be the result of a gradual air leak and can often be topped up occasionally or until a faulty seal or valve core is replaced. Sometimes a sticky oleo makes the gentle inflation process more difficult and requires another person to help lift the wing or nose of the aircraft a bit to let the air in. This action is hit and miss when aiming for the ideal height of the oleo and it may end up sitting too high. In this case, the technician can bleed off some of the air and try to settle the aircraft to the proper height. A bouncy oleo is usually the result of the fluid seeping out over time and the cushioning is provided by mainly air. At that point, the oleo needs to be serviced and its overall condition assessed by an experienced maintainer.

Regular care and servicing of oleo assemblies should include frequent cleaning or wiping of the chromed piston to remove excess dirt and grime that could otherwise become a menace to the main seals. Lubricate bearings at the grease points provided and ensure that these grease fittings are not plugged up with dirt. Maintain proper fluid level and inflation as much as possible. The oleo may require more frequent air servicing in extremely cold climates. Aircraft owners should also become familiar with any manufacturer’s bulletins concerning additional maintenance requirements affecting oleo condition, especially on older aircraft. Paying attention to the state of your oleos is worth considering even though they are quite resilient. As they say, “An ounce of prevention is worth a pound of cure”. 

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