Specialised hydraulic fluid for use in aircraft systems in the 21st century.

Specialised hydraulic fluid for use in aircraft systems came to the fore as aircraft performance increased in mid-20th century. The force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort.

Referring to the hydraulic principles, the hydraulic actuators are controlled by valves; these in turn are operated directly by input from the aircrew (hydro-mechanical) or by computers obeying control laws (fly by wire), all requiring specialised hydraulic fluid for use in aircraft systems.