Three types of hydraulic fluids
1. Vegetable base fluid
MIL-H-7644 fluid was used in the past, when hydraulic system requirements were not as stringent as they are today. This fluid is essentially castor oil and alcohol, and is used primarily in older aircraft. Although similar to automotive brake fluid, it is not interchangeable, and is dyed blue for identification. Natural rubber seals are used with vegetable base fluid. If a vegetable base fluid system is contaminated with petroleum base or phosphate ester base fluids, the seals will swell, break down and block the system. The system may be flushed with alcohol. Vegetable base fluids are flammable.
2. Mineral-base fluid
MIL-H-5606 is the most widely used hydraulic fluid in general aviation aircraft today. It is basically a kerosene-type petroleum product, having good lubricating properties and additives to inhibit foaming and prevent the formation of corrosion. It is quite stable chemically and has very little viscosity change with temperature. MIL-H-5606 fluid is dyed red for identification, and system using this fluid may be flushed with naphtha, varsol, or Stoddard solvent. Neoprene seals and hoses may be used with MIL-H-5606 fluid. This type of fluid is flammable.
3. Synthetic base fluid
The most commonly used fluid of this type is MIL-H-8446. It can operate at very high temperature.
1. Vegetable base fluid
MIL-H-7644 fluid was used in the past, when hydraulic system requirements were not as stringent as they are today. This fluid is essentially castor oil and alcohol, and is used primarily in older aircraft. Although similar to automotive brake fluid, it is not interchangeable, and is dyed blue for identification. Natural rubber seals are used with vegetable base fluid. If a vegetable base fluid system is contaminated with petroleum base or phosphate ester base fluids, the seals will swell, break down and block the system. The system may be flushed with alcohol. Vegetable base fluids are flammable.
2. Mineral-base fluid
MIL-H-5606 is the most widely used hydraulic fluid in general aviation aircraft today. It is basically a kerosene-type petroleum product, having good lubricating properties and additives to inhibit foaming and prevent the formation of corrosion. It is quite stable chemically and has very little viscosity change with temperature. MIL-H-5606 fluid is dyed red for identification, and system using this fluid may be flushed with naphtha, varsol, or Stoddard solvent. Neoprene seals and hoses may be used with MIL-H-5606 fluid. This type of fluid is flammable.
3. Synthetic base fluid
The most commonly used fluid of this type is MIL-H-8446. It can operate at very high temperature.
