Aircraft Hydraulic Fluids: Fire Resistance, Temperature Range, and Compatibility
HGW Hydraulics on Jul 1st 2026
Aircraft hydraulic systems need fluids that can transmit power reliably across a wide temperature range while resisting fire risk and remaining compatible with system materials. A fluid that works in a factory hydraulic unit may not be suitable for aircraft service.
Why Aircraft Fluids Are Different
Aircraft hydraulic fluids are typically thinner than many industrial fluids and must remain usable in very cold and very hot conditions. They also need fire resistance because hydraulic lines may operate near engines, brakes, actuators, and other heat sources.
Water and vegetable oil are not practical aircraft hydraulic fluids because freezing, boiling, oxidation, and compatibility limits would create reliability problems.
Common Aviation Fluid Families
MIL-H-5606 was used for many years, but its flammability limits its use in newer or higher-risk applications. MIL-H-83282 improved fire resistance compared with 5606, though low-temperature viscosity can still be a design consideration. MIL-H-87257 was developed to improve low-temperature flow while maintaining better fire resistance.
Phosphate ester fluids are another important aviation hydraulic fluid family. They are valued for fire resistance and are used in many commercial aircraft hydraulic systems. Compatibility must be checked carefully because these fluids require suitable seals, hoses, and system materials.
Compatibility Matters
The correct fluid is not chosen by viscosity alone. The system must consider elastomer compatibility, seal swell, hose construction, temperature range, pressure rating, corrosion behavior, and maintenance requirements. Mixing unapproved fluids can damage seals, change fire resistance, or reduce system reliability.
What Buyers Should Check
When selecting fittings, seals, hoses, or adapters for aircraft-related hydraulic work, confirm the approved fluid type, temperature range, pressure class, material compatibility, and relevant aircraft or maintenance specification. The fluid, seal, and fitting materials have to work as one system.