Which statement is true about hydraulic fluids mineral and synthetic?

• A. They are the two most common hydraulic fluids
• B. They are colorless
• C. They are expensive
• D. They are used only in aircraft

Answer: (A)

In aircraft hydraulic systems, the two most common fluid types are mineral-based fluids and synthetic ester fluids. Mineral-based fluids are petroleum-derived and have been widely used because they’re typically cheaper and offer good lubricity and seal compatibility, though they’re flammable and don’t perform as well at very high temperatures. Synthetic ester fluids, like phosphate esters, provide excellent fire resistance and better high-temperature performance, but can be more expensive and may have compatibility considerations with certain materials. Because these two categories cover the vast majority of fluids used in aviation hydraulics, they are the true common choices. The other statements aren’t reliable: hydraulic fluids aren’t universally colorless, their cost varies and they aren’t always expensive, and these fluids are used in many industries beyond aircraft.