Hydraulic fluid

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Hydraulic fluids are a large group of fluids used as the motive medium in hydraulic machinery. Fluid types include synthetic compounds, mineral oil, water, and water-based mixtures. The fluids are found in machinery and equipment ranging from brakes, power steering systems, and transmissions to backhoes, excavators, garbage trucks and industrial shredders. Hydraulic systems are very common in aircraft flight control systems.

Hydraulic systems like the ones mentioned above will work most efficiently if the hydraulic fluid used has low compressibility.


Base stock

Base stock may be any of: castor oil, glycol, esters, ethers, mineral oil, organophosphate ester, polyalphaolefin, propylene glycol, or silicone.

Other components

Hydraulic fluids can contain a wide range of chemical compounds, including: oils, butanol, esters (e.g. phthalates, like DEHP, and adipates, like bis(2-ethylhexyl) adipate), polyalkylene glycols (PAG), phosphate esters (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors, etc.

Biodegradable hydraulic fluids

Environmentally sensitive applications (e.g. farm tractors and marine dredging) may benefit from using biodegradable hydraulic fluids based upon rapeseed (Canola) vegetable oil when there is the risk of an oil spill from a ruptured oil line. Typically these oils are available as ISO 32, ISO 46, and ISO 68 specification oils. ASTM standards ASTM-D-6006, Guide for Assessing Biodegradability of Hydraulic Fluids and ASTM-D-6046, Standard Classification of Hydraulic Fluids for Environmental Impact are relevant.

Brake fluid

Brake fluid is a subtype of hydraulic fluid with high boiling point and low freezing point. It is intentionally hygroscopic, so that it will absorb water which could otherwise cause corrosion of brake system components.


Because industrial hydraulic systems operate at hundreds to thousands of PSI and temperatures reaching hundreds of degrees Celsius, severe injuries and death can result from component failures and care must always be taken when performing maintenance on hydraulic systems.

Fire resistance is a property available with specialized fluids.

Trade names

Some of the trade names for hydraulic fluids include Durad, Fyrquel, Houghton-Safe, Hydraunycoil, Lubritherm Enviro-Safe, Pydraul, Quintolubric, Reofos, Reolube, and Skydrol.

Aircraft hydraulic systems

The use of hydraulic systems in aircraft almost certainly began with braking systems.Template:Fact As aircraft performance increased in mid-20th century, the amount of force required to operate mechanical flight controls became excessive, and hydraulic systems were introduced to reduce pilot effort. 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). See flight controls.

Hydraulic power is used for other purposes. It can be stored in accumulators to start an auxiliary power unit (APU) for self-starting the aircraft's main engines. Many aircraft equipped with the M61 family of cannon use hydraulic power to drive the gun system, permitting reliable high rates of fire.

The hydraulic power itself comes from pumps driven by the engines directly, or by electrically driven pumps. In modern commercial aircraft these are electrically driven pumps, should all the engines fail in flight the pilot will deploy a propeller driven electric generator which is concealed under the fuselage.[1] This provides electrical power for the hydraulic pumps and control sysytems as power is no longer avaliable from the engines. In that system and others electric pumps can provide both redundancy and the means of operating hydraulic systems without the engines operating, which can be very useful during maintenance.


Aircraft hydraulic fluids fall under various specifications:

Common petroleum-based:

  • Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from -65F to 275F, red color
  • Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color
  • Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity.

Phosphate-ester based:

  • BMS 3-11: Skydrol 500B-4, Skydrol LD-4, Skydrol 5 and Exxon HyJetIV-A plus - Typically light purple, not compatible with petroleum-based fluids, will not support combustion.


Special, stringent care is required when handling aircraft hydraulic fluid as it is critical to flight safety that it stay free from contamination. It is also necessary to strictly adhere to authorized references when servicing or repairing any aircraft system. Samples from aircraft hydraulic systems are taken during heavy aircraft maintenance checks to check contamination.

See also

External links

  1. Discovery channel-'seconds from disaster'