As is well known, to ensure the normal operation of mechanical hydraulic systems, hydraulic fluid is required for lubrication, cooling, and rust prevention to maintain the system in optimal condition. This liquid is known as hydraulic fluid.
There are many types of hydraulic fluids, but they are mainly divided into three broad categories: mineral oil-based, emulsion-based, and synthetic-based.
Aviation hydraulic fluid is a specialized type. The selection criteria are extremely stringent and must meet the following requirements:
(1) Proper viscosity and good viscosity-temperature characteristics.
(2) Good lubricity and strong anti-rust performance.
(3) High purity with minimal impurities.
(4) Good anti-foaming and demulsibility properties.
(5) Good compatibility with aircraft metals and sealing materials.
(6) Low coefficient of volumetric expansion and high specific heat capacity.
(7) Low pour point and freezing point; high flash point and fire point.
(8) Harmless to humans and low environmental pollution.
Therefore, selecting the correct hydraulic fluid is essential to ensure the normal operation of aircraft systems and prevent damage to non-metallic components in the hydraulic system.
There are three main types of hydraulic fluids commonly used on aircraft:
(1) Vegetable-based hydraulic fluid is a mixture of castor oil and alcohol, possessing a strong alcoholic odor. For easy identification, the fluid is dyed blue. It is compatible with natural rubber seals and was originally used in older aircraft.
(2) Mineral-based hydraulic fluid is refined from petroleum. For easy identification, it is dyed red, also known as "red oil." It has good lubricating properties and contains additives for anti-oxidation and high-temperature resistance to prevent foaming and corrosion. It is widely used in light aircraft brake systems, hydraulic power systems, and shock absorbers.
(3) Phosphate ester-based hydraulic fluid is a non-petroleum-based, synthetic fluid. It has exceptional fire-resistant properties; when sprayed into a 6,000°C welding flame for a fire-resistance test, it will not sustain combustion, occasionally exhibiting only slight flare-ups. It was introduced in 1948 for use in high-performance piston engines and turboprop aircraft. It is purple in color and features excellent low-temperature operational characteristics and low corrosiveness. It is now widely used in the hydraulic systems of modern large transport aircraft.
When refilling aircraft hydraulic systems, these three types of aviation hydraulic fluids must not be mixed. Otherwise, it will damage the hydraulic system and lead to accidents. Refilling must be strictly based on the fluid grade specified in the aircraft maintenance manual or the oil brand stipulated in the manuals of the reservoir or accessories.
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2026-10-30