Oleo strut (aircraft suspension)

An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations.

It is undesirable for an airplane to bounce on landing—it could lead to a loss of control. The landing gear should not add to this tendency. A steel coil spring stores impact energy from landing and then releases it. An oleo strut absorbs this energy, reducing bounce.

As the strut compresses, the spring rate increases dramatically, because the air is being compressed, while the viscosity of the oil dampens the rebound movement.

The largest cargo airplanes in the world, such as the Antonov An-124 Ruslan, use oleo struts to allow for rough-field landing capacity with a payload of up to 150 tons. This design also cushions the airframe from the impacts of taxiing.

Cleveland Pneumatic Tool Company designed and introduced the first oleo-pneumatic shock-absorbing strut for airplanes, later called "Aerol" struts in 1926. Oleo struts have been in use for many decades.Oleo-pneumatic shock absorber technology has continued to be refined, for example with US patent US2959410 A, Jarry Hydraulics, in 1960.

Pneumatic systems like the oleo strut generally have long operating lives, and the construction is not unusually complex for maintenance purposes.

An oleo strut consists of an inner metal tube or piston, which is attached to the wheel axle, and which moves up and down in an outer (or upper) metal tube, or cylinder, that is attached to the airframe. The cavity within the strut and piston is filled with gas (usually nitrogen, sometimes air—especially on light aircraft) and oil (usually hydraulic fluid), and is divided into two chambers that communicate through a small orifice.

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