If phosphate ester base hydraulic fluid is used in an aircraft system that requires mineral base hydraulic fluid, what will be the likely effect on the system?

Using phosphate ester base hydraulic fluid in a system designed for mineral base hydraulic fluid can lead to significant compatibility issues. Phosphate esters are chemically different from mineral oils, and they do not blend, which can cause the formation of undesirable contaminants. This immiscibility results in fluid separation, leading to contamination within the system.

Additionally, the chemical properties of phosphate esters can react adversely with the materials—especially seals—specifically designed for use with mineral oils. This can result in the degradation of seals, leading to leaks and failure of the hydraulic system components. This situation not only compromises the integrity and operational reliability of the system but could also pose safety risks, especially in aviation applications where system integrity is critical.

Therefore, the compatibility issues, potential contamination, and risk of seal failure make this answer the most accurate reflection of what would likely occur in such a scenario.