Hydrolysis of phosphate‐based aviation hydraulic fluids

High-temperature in-service failure of phosphate ester-based aviation fluids is primarily due to acid-catalysed hydrolysis. The reaction is controlled with epoxides, but careful attention to the other additives is needed to avoid including species which catalyse hydrolysis. From an airline's point of view, water concentration is the critical factor, since it is the only variable that it can control. From a system designer's point of view, minimising fluid temperature should be a goal. Plots relating fluid life to temperature and water level for typical commercial fluids are included.