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Controlling the Acid in Phosphate Ester-Based Hydraulic Fluid*

June 01, 2013
Jean Van Rensselar
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Acid control additives (epoxides) were developed in the late 1960s. These additives react even at normal ambient temperatures to neutralize the damaging phosphoric acid derivatives. This keeps the aircraft’s hydraulic system free of strong acids until the acid control additives are nearly depleted. The issue is that the allowable amount of acid control additive is limited because of crucially important seal swell and flammability considerations.

Even though all commercial aviation hydraulic fluids are comprised of phosphate esters and they all contain essentially the same amount of additive, they are not equally stable.** This is because of differences in the resistance of the base oil constituents to hydrolysis. Also, supplementary additives can be used to slow down the rate of hydrolysis.

Water levels that exceed the capacity of the acid control additive are not uncommon. For example, most aircraft manufacturers allow operation with up to 0.8 percent water in the hydraulic system. This is based on the assumption that strong acidity will not occur before the next fluid maintenance due to routine makeup with new fluid and the typically low hydrolysis rate at the moderate temperatures aircraft hydraulic systems are exposed to.

To protect against damage from high acidity, aircraft manufacturers typically recommend a maximum acid number of 1.5, beyond which the fluid should be replaced. Serious damage would start occurring at acid numbers above 5, so the acid number limit of 1.5 provides a significant safety cushion against aircraft system damage.

In addition to the highly damaging acid, most aircraft hydraulic fluids also contain weak acids (i.e., carboxylic). In fact, acidity measured in aircraft hydraulic systems with acid numbers below 1.5 is usually caused by weak acids that are not neutralized by acid control additives (they can also be a byproduct of the additives themselves). These weak acids are fairly innocuous.

*With the exception of MIL-PRF-5606, military hydraulic fluids do not degrade under normal operating conditions. In fact, they can operate indefinitely if contaminants can be removed.

**Most small private aircraft use MIL-PRF-5605 and many small commercial aircraft use MIL-PRF-87257.
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