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.

The history of specialised hydraulic fluid for use in aircraft systems.

Aircraft hydraulic systems are capable of reliable unattended operation for long periods of time, but some periodic service is generally required. Such service will either be hydraulic fluid servicing using hydraulic fluid with specialised properties for use in aircraft systems or air bleeding. Hydraulic fluids MIL-H-5606, MIL-H-83282, and MIL-H-81019 are used in automatic pilots, shock absorbers, brakes, control mechanisms, servo control systems, and other hydraulic systems using seal materials compatible with petroleum-based fluids.

The original hydraulic fluid, dating back to the time of ancient Egypt, was water. Beginning in the 1920s, mineral oil began to be used more than water as a base stock due to its inherent lubrication properties and ability to be used at temperatures above the boiling point of water. Today most hydraulic fluids are based on mineral oil base stocks or in specialised hydraulic fluid for use in aircraft systems, synthetic base stock..


Natural oils such as rapeseed (also called canola oil) are used as base stocks for fluids where biodegradability and renewable sources are considered important.

Other base stocks are used for specialty applications, such as for fire resistance and extreme temperature applications. Some examples include: glycol, esters, organophosphate ester, polyalphaolefin, propylene glycol, and silicone oils.

Other components blended into specialised hydraulic fluid for use in aircraft systems.

Specialised hydraulic fluid for use in aircraft systems 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), organophosphate (e.g. tributylphosphate), silicones, alkylated aromatic hydrocarbons, polyalphaolefins (PAO) (e.g. polyisobutenes), corrosion inhibitors (including acid scavengers), anti-erosion additives, etc.

Widespread use of specialised hydraulic fluid for use in aircraft systems.

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 compressing this specialised hydraulic fluid for use in aircraft systems. 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 called a Ram Air Turbine (RAT) which is concealed under the fuselage. This provides electrical power for the hydraulic pumps and control systems as power is no longer available 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.

Specifications for specialised hydraulic fluid for use in aircraft systems.

Specialised hydraulic fluid for use in aircraft systems fall under various specifications:
Common petroleum-based specialised hydraulic fluid for use in aircraft systems:

• Mil-H-5606: Mineral base, flammable, fairly low flashpoint, usable from −65 °F (−54 °C) to 275 °F (135 °C), red color 
• Mil-H-83282: Synthetic hydrocarbon base, higher flashpoint, self-extinguishing, backward compatible to -5606, red color, rated to −40 °F (−40 °C) degrees. 
• Mil-H-87257: A development of -83282 fluid to improve its low temperature viscosity. 

Phosphate-ester based synthetic specialised hydraulic fluid for use in aircraft systems:

• US/NATO Military Specification - MIL-H-8446 
• Boeing Seattle - BMS3-11 
• Boeing Long Beach - DMS2014 
• Boeing Long Island - CDS5478 
• Lockheed - LAC C-34-1224 
• Airbus Industrie - NSA307110 
• British Aerospace - BAC M.333.B 
• Bombardier - BAMS 564-003 
• SAE - Ac974 
• SAE - AS1241 

Contamination of specialised hydraulic fluid for use in aircraft systems.

Special, stringent care is required when handling specialised hydraulic fluid for use in aircraft systems 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 (primarily C and D checks) to check contamination.

When considering specialised hydraulic fluid for use in aircraft systems it’s critical that reputable suppliers, such as Habot Oil, be approached for advice and products.