oleo

An oleo strut is a landing gear assembly that absorbs the kinetic energy of an aircraft touching down. When wheels hit the runway, the strut compresses; oil is forced through a metered orifice, and the resulting viscous resistance slows the compression and converts impact energy into heat. Modern transport aircraft rely almost entirely on oleo struts rather than rigid or spring-based gear because they provide smooth, predictable energy dissipation across a wide range of landing weights and sink rates.

The strut contains two chambers separated by a piston. The upper chamber holds compressed nitrogen gas, typically pressurized to 600 to 1200 psi depending on aircraft type and design. The lower chamber contains hydraulic oil and connects to the metering orifice, a precisely sized passage that controls flow rate. When the aircraft lands and the gear compresses, nitrogen pressure is reduced; as the strut extends again, nitrogen pressure pushes the piston down to expel oil, creating the rebound damping action that prevents bouncing.

Maintenance and failure modes

Oleo struts require periodic servicing to remain effective. Nitrogen charge must be checked and topped up according to maintenance schedules; undercharging causes harsh landings and excessive tire wear, while overcharging reduces stroke and can cause gear collapse. Oil levels must be verified and fluid changed at specified intervals, as contamination or breakdown of hydraulic oil degrades the viscous damping and increases internal friction that generates unwanted heat.

Common failures include nitrogen leakage, which occurs at the gas valve seal and results in a slow loss of springback; oil leakage past the piston rod seal, which reduces damping and leaves hydraulic residue on the runway; and corrosion of the piston rod from salt spray or moisture, which roughens the rod surface and accelerates seal wear. A failed oleo strut must be replaced or serviced before flight; a collapsed gear or one that cannot extend properly is a ground event.

The term "oleo" derives from the Latin word for oil and originally referred to margarine made with vegetable oil. In the early aviation era the term was adopted to describe this type of gear because of its central use of oil as the energy-absorbing medium. The oleo strut design was refined through the mid-20th century and remains essentially unchanged; modern variants use more sophisticated piston seal materials and improved nitrogen charging methods, but the operating principle has proven robust across aircraft from regional twins to heavy freight haulers.