A landing gear strut - commonly called an oleo or air/oil strut - is the pneumatic air-oil hydraulic shock absorber that connects the airframe to the wheels. It is built from a cylinder fastened to the airframe and a piston fastened to the wheel, and it uses compressed gas combined with hydraulic fluid to absorb and dissipate the landing loads.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an oleo strut in landing gear?
An oleo strut is a pneumatic air oil hydraulic shock absorber used on landing gear of most large aircraft and many smaller ones. Shock struts, called oleo or air/oil struts, use a combination of nitrogen (or sometimes compressed air) and hydraulic fluid. The main structure of the oleo strut consists of a cylinder and a piston. The oleo strut's purpose is to absorb and dissipate landing forces, provide damping of landing shocks, minimize the chance of a bounced landing, and slow post-impact recoil while maintaining structural integrity under FAA-required sink rates of 10 feet per second (3.05 meters per second).
An oleo strut is a piston-and-cylinder shock absorber attached between the airframe and the wheel. It is the basic design of landing gear on most aircraft from Piper Cherokees to airliners. Inside the cylinder, compressed air forms a gas spring while hydraulic fluid provides the damping force. On touchdown, the piston attached to the wheel compresses, forcing fluid through a metered orifice from the lower chamber to the upper one. Simultaneously, the air compresses, increasing resistance. This action allows the oleo strut to absorb landing loads and dissipate the shock of landing by converting kinetic energy into thermal energy. As the strut extends during post-landing recoil, fluid is forced back through the orifice in the opposite direction, dampening recoil and preventing bounce. A tapered metering pin varies the effective size of the orifice so that compression and extension occur at a controlled rate. The rule-of-thumb compression ratio for a main-gear oleo strut is about 4:1 between the fully-extended and static (taxi) position.
The design centers on a cylinder attached to the airframe and a piston attached to the wheel. An orifice plate divides the lower fluid-bearing part into two chambers. The size of the orifice, combined with fluid viscosity, limits the rate at which the strut can compress or extend. The upper chamber contains compressed gas that delivers the spring force, while hydraulic fluid damps the action of the gas spring, allowing smooth energy absorption and release.
The landing gear leg itself is the strut: the outer metal tube is attached to the airframe, the piston is attached to the wheel, and the entire assembly compresses during landing and extends during takeoff or recoil. Maintenance includes occasional additions of hydraulic fluid and air, and the seal and scraper ring are typically retained by a snap ring.
What material is used inside of an oleo strut?
The oleo strut contains oil in its lower chamber and gas, usually nitrogen, above it. All struts contain hydraulic fluid in the bottom part; the cavity is divided into two chambers connected by a small orifice, and oil fills the bottom. The shock absorber purpose of both fluids is to slow post-impact recoil, and they damp the action of the gas spring. The compressed gas combined with hydraulic fluid absorbs and dissipates landing shocks and landing energy. This combination makes the oleo strut pneumatic and hydraulic: oil and nitrogen provide the combined materials needed to absorb the touchdown force safely.
