What are the two most common types of hydraulic fluid used?

• A. Mineral and synthetic
• B. Mineral
• C. Synthetic
• D. Water-based

Answer: (A)

In aviation hydraulics, the two most common fluid types are mineral-based fluids and synthetic fluids. Mineral-based fluids come from petroleum and are widely used because they are generally economical and work well with a broad range of seals and components. Synthetic fluids, often phosphate esters, offer superior fire resistance and better performance at higher temperatures, which makes them desirable for systems with greater heat or stricter safety requirements. The trade-offs are that synthetic fluids can be more expensive and may require compatible materials and seals, while mineral fluids are typically less costly but do not offer the same level of fire resistance or high-temperature stability.