Aircraft Maintenance: Servicing struts

Aside from simply keeping an aircraft off the ground, struts dampen landing forces and protect critical aircraft structures from damage. The typical general aviation landing gear system is designed to safely absorb landing forces up to 3 Gs. However, if the full 3G-force of a landing were absorbed instantaneously, it is likely that the aircraft, and probably its occupants, would be damaged in the process. Therefore, these loads must be absorbed incrementally.

Enter the oleo strut. Oleo-pneumatic (oil/gas) shock struts combine the compressibility of a gas with the non-compressibility of a fluid to both cushion and dampen the vertical landing forces. In its most basic form, the system consists of a tube within a tube where a lower tube (the chromed part that you see during your preflight) acts like a piston sliding into the upper strut housing. The bottom of the assembly contains hydraulic fluid (MIL-H-5606), and the upper portion is filled with pressurized nitrogen. Between the two are an orifice and a cone-shaped metering pin. As the strut is compressed, fluid flows around the metering pin, through the orifice, and into the upper part of the strut. As the strut is compressed and the metering pin is pushed farther into the fixed orifice, there is progressively less space for the fluid to pass around the pin into the upper chamber. The result is a damping action where the more the strut is compressed, the more resistance it has to further movement. It’s a simple but brilliant solution that has been around since 1915 when it was adapted for aircraft from the recoil damping mechanism used on World War I machine guns.

Although the system is fairly straightforward, it does require maintenance to work properly. Oleo struts need to be serviced with the proper fluid amount and gas pressure, they have to seal properly, and they must be able to move freely without restriction or excessive lateral play.

Servicing struts can be very dangerous due to high gas pressures involved, typically hundreds of pounds per square inch. Strut servicing also generally requires the aircraft to be suspended on jacks, with the mechanic supporting the gear and moving it up and down to get the proper amount of fluid into it. Mishaps during this process can have dire consequences, as Richard Bogert of Bogert Aviation discovered while doing this very task. As Bogert was working on the gear struts, he stepped out from under the wing just as a support jack seal blew out, and the wing came crashing down. Realizing that he could have broken his neck if he had been under the wing when the jack failed, he set out to create a tool that made the process simpler and safer. The result was Bogert’s pressurized strut service tool, which replaces the hydraulic fluid and fills the strut without ever having to place the wing on jacks. I use the tool myself and can attest that it is a far safer and more efficient method of strut servicing (which is saving my back as well).

If a strut is leaking fluid, going low/flat, or not going up and down smoothly, it needs more than just a simple servicing. In this case, the aircraft must be placed on jacks and the strut deflated and thoroughly inspected. As noted before, this is a dangerous maintenance task and must be done by a qualified mechanic following proper safety procedures. Once pressure has been removed from the strut, the technician can determine if the issue is fluid loss, pressure loss, or problems with the seals or bearings.

Most struts have a series of O-rings that form the seal between the cylinders, as well as a top cap and fittings for the Schrader valve where the gas is added. Additionally, there are bearing surfaces that ensure that the cylinder and piston cannot wobble fore or aft. Finally, there are usually felt and wiper seals at the bottom of the union, forming the bottom portion of the hydraulic assembly. Everything must be in excellent shape to function as intended without excessive play, binding, or leakage. Whenever a strut is disassembled for maintenance, it makes sense to replace all wear items including seals, O-rings, and the Schrader valve. Be sure to use the correct, high-pressure Schrader valves as these may look similar to low-pressure tire valves, but are absolutely not the same part. In addition to the seals, pitting/corrosion and wear can prevent the system from operating properly. There are several shops that specialize in re-chroming, re-bushing, and repair of oleo strut components, so don’t despair if you find an issue.

Routine strut inspection and proper servicing are the key to keeping the system healthy. Always use nitrogen to fill a strut. Nitrogen contains no moisture and its pressure is stable over a wide temperature range. Shop air or manual strut pumps can introduce moisture into the strut and cause internal corrosion over time. In addition, 5606 hydraulic fluid is hygroscopic, readily attracting and absorbing moisture from the air…again, leading to corrosion. So, it pays to replace it with fresh, clean fluid every few years or as recommended by the manufacturer.

As an aircraft owner, there are a few key items you can include on every preflight:

• Look for leaks: Inspect the strut as well as the tire for any signs of hydraulic fluid leaks. (Hydraulic fluid is red when new, but can turn brown over time.)
• Inspect the strut for proper height: Keep a small note card in your kneeboard with markings for the proper strut heights for the nose gear and mains. Use the card as a reference on every preflight to monitor strut condition. Never fly with a collapsed strut, because doing so could lead to aircraft structural damage.
• Keep your struts clean: Wipe down the strut cylinder with a clean cloth, preferably slightly dampened with fresh 5606 fluid.

Healthy struts will help protect your aircraft from damage during landing, and they also improve your landing performance and comfort. As with many aircraft components, a little preventive maintenance can save you a lot of money in the long term, avoiding corrosion, wear, and costly damage. Until next time, I hope you and your families remain safe and healthy, and I wish you blue skies.