Aircraft hydraulic fluid is the base of every powered flight control, landing gear and brake system. Its chemistry therefore decides both safety and performance. Fluids fall mainly into mineral-based and fire-resistant phosphate-ester types. Mineral-based oils are the most common fluids in aircraft hydraulics, and within this group MIL-H-5606 is a mineral based hydraulic fluid while MIL-H-6083 is a rust-inhibited version of MIL-H-5606.
Phosphate ester hydraulic fluids are the mandatory choice for modern commercial aviation, and in this category Skydrol is a phosphate ester-based Fluid and Hyjet is a phosphate ester-based Fluid. Between the two families stands MIL-H-83282, a synthetic hydrocarbon base fluid that offers higher flash point and self-extinguishing properties, with MIL-H-46170 acting as a rust-inhibited version of MIL-H-83282. Specifications like Type IV and Type V fluids denote performance levels, assuring that each aircraft receives the precise thermal, fire-resistant and lubricating characteristics its hydraulic circuit demands.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What type of hydraulic fluid is used in aircraft hydraulic systems?
Mineral-based hydraulic fluids like MIL PRF 5606, a petroleum oil refined to Group 1 base oil, are the most common fluids in aircraft hydraulics; Royco 756 MIL PRF 5606H and AeroShell Fluid 41 are typical examples. Military aircraft hydraulic systems are filled with MIL PRF 83282, a polyalphaolefin-based synthetic hydrocarbon fluid that is fire-resistant and self-extinguishing. MIL H 46170 is the rust-inhibited version of the same specification.
Civil transport category aircraft are normally operated on phosphate ester-based hydraulic fluids - Skydrol types IV and V and Hyjet IV-A PLUS or Hyjet V - to obtain fire resistance. Boeing uses BMS3-11, Airbus uses NSA307110, Bombardier uses BAMS 564-003 and McDonnell Douglas uses DMS2014. Water-glycol mixtures provide a water-based, non-flammable option yet they are rare in flight hydraulic circuits. Whatever the exact brand, every seal, gasket and hose installed in the system must be compatible with the type of fluid in use. The three main choices are therefore mineral-based, synthetic hydrocarbon, and phosphate ester-based, giving operators a continuum from low-cost general viscosity to maximum fire resistance.
What is aircraft hydraulic oil?
Aircraft hydraulic oil is a mineral-based fluid, which is the most common type in aircraft hydraulics, and it serves as a reliable solution for systems where operators prioritize fire safety. These fluids function effectively in small aircraft, and they are relatively inexpensive to produce. Hydraulic systems convert pressurized fluids to transmit power.
Aircraft hydraulic oil is the sealed, fire-resistant fluid that lets every modern airplane move its flaps, brakes, and landing gear. Inside rigid lines it carries power so that a small electrical pump can place tons of controlled force on actuators, wheels, and cargo doors. While it transfers this energy it also acts as a lubricant, a coolant and a sealant, protecting the pumps, valves and pistons from wear, heat and leaks. The three types used are mineral-based, synthetic hydrocarbon, and phosphate ester. Phosphate esters are chosen for extreme fire resistance because fluid heated above 180°C (356°F) sustains combustion if the system is compromised. Brand-named examples include Skydrol LD-4, Skydrol 500B-4 and Hyjet IV and V, all of which can withstand extreme temperatures and pressure conditions.
What are the specifications of aircraft hydraulic oil?
Aircraft hydraulic fluid specifications begin with the governing standard MIL-PRF-5606-H, the current specification the US Military uses for a grade of hydraulic oil to be used in military aircraft. The oil has ISO viscosity level 15, is thin compared to most oils and can flow at colder temperature. This grade is NATO-code H-538, identified by NATO Code No. H-538, and is intended for use in brakes, shock absorbers, flap-control mechanisms, and automatic pilots within the temperature range -54°C (-65.2°F) to +200°C (392°F).
Physical limits are set: pour point -64°C (-83.2°F), flash point (PMCC) of 90°C (194°F), evaporation loss 6 wt % at 71°C (159.8°F) for 6 h, and dielectric strength 35 kV (35 kV) at manufacture. The hydraulic fluid shall have viscosity measured in centistokes (cSt) at various temperatures, minimum viscosity change over range of operating temperatures, and a viscosity index of 300+. Low viscosity base stock provides extremely high viscosity index, which is pivotal for a lubricant or hydraulic fluid operating over the very broad temperature range of -54°C (-65.2°F) to 135°C (275°F) at pressures up to 3,000 psi (20 684.3 kPa).
Cleanliness and compatibility requirements state that the fluid shall contain no admixture of fatty oils, resins, gums, soaps, or oxidized hydrocarbons, and no formation of resinous gums, sludge, or insoluble material shall occur. The hydraulic fluid shall have particulate contamination measured by FTM 3012, gravimetric filtration of 0.45 microns (0.000018 inches) at 25°C (77°F), and super clean filtration processes to prevent clogging of ports and orifices. The fluid is incompressible and shall be miscible with MIL-PRF-5606, MIL-PRF-6083, MIL-PRF-46170, and MIL-PRF-83282 fluids in all proportions from -65.2°F (-54°C) to 392°F (200°C).
Performance tests include four-ball wear measured by ASTM D4172, foam resistance measured by ASTM D892, water content measured by ASTM D6304, acid number measured by ASTM D664, copper corrosion measured by ASTM D130, and density 11.99 lbs/gal @ 60°F (1.44 kg/L @ 15.6°C). The hydraulic fluid shall have seal compatibility and protect against rust and corrosion. It shall exhibit oxidation resistance, resists deposit formation and viscosity increase due to oxidation, and has long service life and shear stability for longer fluid life. Military inspection and grade approval mean manufacturers pay for the military to inspect the oil, which is why hydraulic oil price is higher.
X/C 5606J and X/C 5606A, Phillips 66 products, are mineral oil-based, ashless (zinc-free), and designed to meet the severe demands of aerospace and industrial applications. They are recommended for pressurized systems operating between -54°C (-65.2°F) and 135°C (275°F) at pressures up to 3,000 psi (20.7 MPa) and for non-pressurized systems operating between -54°C (-65.2°F) and 90°C (194 ° F). These fluids have a very high viscosity index, outstanding low-temperature properties, excellent oxidation resistance, good foam resistance, excellent wear protection, and protect against rust and corrosion. They do not contain zinc or other heavy metals and resist deposit formation and viscosity increase due to oxidation.
Higher-performance fluids are addressed by additional specifications: MIL-PRF-83282 offers higher flash point and self-extinguishing properties. MIL-PRF-87257C is a performance specification for fire-resistant low-temperature synthetic hydrocarbon base hydraulic fluid, approved for use by all Departments and Agencies of the Department of Defense, and supersedes MIL-PRF-87257B. MIL-H-46170 is a rust-inhibited version of MIL-H-83282, while MIL-H-6083 is a rust-inhibited version of MIL-PRF-5606. Phosphate ester-based fluids are extremely fire-resistant but are not fireproof and will burn under certain conditions. They are very susceptible to contamination from moisture in the atmosphere, and fluid density influences selection of phosphate ester technology.
What is the color of aircraft hydraulic fluid?Aircraft hydraulic fluid is not limited to one color. Hydraulic fluid comes in clear, yellow, red, green, or dark. Mineral oil-based MIL-H-5606 is red. MIL-H-83282 and MIL-H-87257 oils are red-dyed for easy identification. Fire-resistant Skydrol 500B is dyed purple. Skydrol 5 and Skydrol PE-5 are purple, while Skydrol 7000 is dyed green. Phosphate ester fluid starts as straw yellow. When artificially dyed it turns purple for leak detection, but if fluid leaks and becomes oxidised it turns back to straw yellow.
Color of hydraulic fluid indicates composition and condition: overheated phosphate ester aviation hydraulic fluid turns from purple to dark purple to grey to black, oxidized or contaminated fluid turns dark brown or black, and water ingress causes milky, cloudy, or lightened color. Brightly colored hydraulic fluids are easier to detect for leaks, making visual inspection straightforward.
