Which fluid is commonly used in aircraft hydraulic systems?

The correct answer is based on the specific requirements and characteristics of fluids used in aircraft hydraulic systems. Phosphate ester-based fluids or petroleum-based fluids are preferred due to their high thermal stability, which means they can maintain their performance over a wide range of temperatures. These fluids have excellent lubricating properties and can withstand the high pressures typically found in hydraulic systems without breaking down or losing effectiveness.

Additionally, phosphate ester-based fluids are less flammable compared to other options, which is critical in aviation safety. Their chemical properties also allow them to provide effective protection against corrosion and wear within hydraulic systems.

In contrast, water-based liquids are not suitable for hydraulic systems in aviation due to their susceptibility to freezing and evaporation, which can lead to system failures. Alcohol-based solutions do not provide the necessary lubrication and thermal stability needed in hydraulic applications. Mineral oils, while they can be used in some systems, do not possess the same level of fire resistance and stability as phosphate ester-based fluids, making them less suitable for modern aircraft hydraulic applications.