other performance criteria. Furthermore, refinements continued to be made to the technology behind the oleo strut. During 1954, hydropneumatic suspension was introduced, which utilizes the same principle of a gas that compresses (nitrogen) and a fluid that does not; in this application, an engine-driven pump is used to pressurize the hydraulic fluid. Another such example was a US patent filed by Jarry Hydraulics during 1958. During the 1960s, the British Ministry of Technology sponsored research into theoretical studies into improved oleo-dampening technology. In 2012, it was proposed that the vibration-dampening qualities of the oleo strut could be enhanced by using semi-active control to adjust fluid viscosity. The use of oleo struts for electric-powered automatic guided vehicles has also been evaluated. According to Engineering360, by 2019, the oleo-pneumatic strut had become the most common type of shock absorber in use on modern aircraft. In particular, the oleo strut has seen heavy use on the largest cargo airplanes in the world, such as the Antonov An-124 Ruslan; it reportedly provides for a rough-field landing capacity while carrying payloads of up to 150 tons. This design also cushions the airframe from the impacts of taxiing, resulting in greater levels of comfort for passengers and crew alike. In non-aircraft use, the Quadro range of motor scooters use the oleo strut, which is claimed to give favourable low speed lean characteristics. Operation 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 are connected by a small orifice of a precise, calculated size. When the aircraft is stationary on the ground, its weight is supported by the compressed gas in the cylinder. During landing, or when the aircraft taxis over bumps, the piston slides up and down. This movement compresses the gas, which acts as a spring, and forces oil through the orifice, which acts as a damper. A tapered rod is used on some designs to change the size of the orifice as the piston moves, providing greater resistance as compression of the strut increases. Additionally, a check valve is sometimes used to uncover additional