Hydraulic Fluids and Contamination

Desirable Properties of Hydraulic Fluids

Hydraulic fluids for aircraft must perform reliably under extreme pressure and temperature variations. Key properties include:

• Thermal Stability: The fluid must remain effective from very low to high temperatures (about -50°C to 100°C), with a low freezing point and a high boiling point.
• Corrosion Resistance: It should not corrode or damage the metal components it contacts, preserving precision and preventing expensive repairs.
• High Flashpoint & Low Flammability: To reduce fire risk, the fluid must resist ignition and have a high flashpoint.
• Low Volatility: The fluid should not easily vaporize, remaining liquid throughout its operational temperature range.
• Low Viscosity: It must flow freely to ensure responsive system operation, but not be so thin as to cause leaks or poor lubrication.
• Incompressibility: Hydraulic fluid is considered incompressible at system pressures, ensuring precise and immediate force transmission.

Types of Hydraulic Fluids

Aircraft hydraulic fluids fall into two main categories:

• Mineral-Based Fluids: Typically dyed red, compatible with synthetic rubber seals, but not natural rubber. These are flammable but chemically stable at high temperatures.
• Synthetic-Based Fluids: Commonly phosphate-ester types like Skydrol (dyed purple), offering superior fire resistance. These require compatible seals and must never be mixed with mineral fluids.

Mixing different types of hydraulic fluids is strictly prohibited, as it can cause chemical reactions, seal damage, and system failure.

Hydraulic Fluid Contamination

Contamination is a leading cause of hydraulic system failure. Common sources include wear particles from pumps, external ingress, or incorrect maintenance procedures. Symptoms of contaminated hydraulic fluid include sluggish actuator response, abnormal noises, and visible fluid discoloration. Effects can range from reduced efficiency to total system failure. Aircraft hydraulic systems use filters (typically removing particles larger than 25 microns) to maintain fluid purity, with filter placement both after the pump and sometimes in return lines.

Handling and Safety

Hydraulic fluids can irritate skin and eyes, so proper personal protective equipment is essential during maintenance. Always use the correct fluid type for the system and never mix fluids, even if they appear similar in color.

Maintenance Practices

Regular system checks, fluid sampling, and filter inspections are vital for early detection of contamination or degradation. Monitoring fluid temperature and color helps prevent overheating and ensures continued system reliability.