Skydrol is a hydraulic fluid that is a formulated package of chemical additives dissolved into a fire-resistant phosphate ester base stock. The additives serve different functions such as inhibiting corrosion, boosting viscosity, and preventing erosion damage to servo valves. But the most important additive related to maintenance is the one that controls acidity in the fluid. This is the only additive that is consumed while the aircraft is flying, and it is the additive that defines fluid life. Once the acid control additive is gone, the fluid has reached the end of its useful life and must be replaced. Care must be taken by maintenance personnel to prevent, detect, and, if necessary, remedy contamination that may occur within the hydraulic system.
Change Intervals You will notice that nowhere in an aircraft maintenance manual is there a stated time interval for changing hydraulic fluid. That is because it is impossible to predict with certainty how long the fluid will last in a given aircraft. Every time a component is removed from an aircraft's hydraulic system, some fluid loss occurs, which must be replaced with fresh fluid. Topping up the system replaces the additive that determines fluid life. Aircraft that have fewer component removals get less fluid replenishment. If the component removal rate is high, and thus fluid replenishment is high, that aircraft may never need to have its hydraulic fluid changed because it is already being changed out gradually as a result of other maintenance operations.Maintaining Key Fluid Properties Instead of a time-based change interval for Skydrol, airframe manufacturers have established a list of key physical properties that must be maintained in the fluid in order for the hydraulic system to function properly and reliably. Acid control is just one of those properties, and it is measured in a laboratory test called acid number. Contamination is another key property that must be measured; contamination from moisture, particulates, chlorine, and other foreign substances. Generally it is recommended that hydraulic systems should be sampled every "C" check to keep track of these properties.The Problem with Acidity The maximum acid number allowed by most airframe manufacturers is 1.5 mg KOH/gram. New fluid starts out at 0.1 mg KOH/gram or less, and for most of the fluid's useful life it remains at or below 0.5 mg KOH/gram. But a curious thing happens as the acid number approaches the allowable limit - its rate of change accelerates. The time it takes to rise from 0.5 to 1.0 is very long, but the time it takes to move from 1.0 to 1.5 is much less. And it keeps rising faster and faster after it goes beyond the 1.5 limit, resembling an exponential curve. Acidity is harmful because it can eventually cause corrosion on hydraulic parts. Steel is the metal most susceptible to corrosion, but it tends to occur only if the acid number reaches a level of 8 or 9 mg KOH/gram.Catching Small Problems before They Become Big Problems High acid numbers in aircraft hydraulic systems do not occur often, but when they do, the effort required to correct the problem can be extensive. Aircraft hydraulic systems are not designed to be drained. The only component that can be drained is the reservoir, which usually holds only 25 percent of the total volume of the system. Therefore, in order to change out high-acid fluid, it is not a simple drain-and-replace operation, but rather a series of steps: 1) drain reservoir, 2) refill reservoir, 3) pump and operate system flight controls to mix new and old fluid, and 4) repeat. This sequence may have to be repeated several times depending on how high the acid number was allowed to reach. Taking action early, before the acid number reaches 1.5, makes maintenance much easier. One reservoir drain and replace operation is often all that is required if the problem is caught early. Solutia's chemical analysis experts can help find the point when a well-timed reservoir change prevents a small problem from becoming a much larger problem.Contamination Fluid properties other than acidity are measures of how much external contamination the hydraulic system has received. Some of this contamination is accidental and preventable. Chlorine, for example, appears when chlorinated solvents are used in the shop to clean hydraulic parts. If the solvent has not dried completely, it can find its way into the hydraulic fluid. Total chlorine should not exceed 200 ppm in a hydraulic system. Moisture is another accidental contaminant that is preventable. Skydrol will absorb moisture from the air. If it is allowed to come in contact with a constant supply of fresh air, it can become unsuitable for use after several days. For this reason, Skydrol containers should be kept closed when not in use. They should be stored in dry conditions, not left to sit out in the rain and weather. Although filters are installed on aircraft to keep particulates under control, particulate contamination is unavoidable. Any time an actuator rod is extended, it has a film of hydraulic fluid on its surface exposed to dust in the air. When the actuator is retracted, the dirt it accumulated while outside is deposited into the system fluid. Aircraft filters are designed to collect this dirt, and they do a very good job of doing so. But they do not last forever. Eventually the filters will need to be changed, and when they do, one of the symptoms can be a shedding of the particles it has been collecting. This will show up as a high particle count in the laboratory test of fluid. Airframe manufacturers state various allowable limits for particle counts in used fluid samples, which differ from the limits allowed in new fluid. Contamination by foreign substances is the most difficult to correct as there is zero tolerance for substances such as red oil, jet fuel, and engine oil, which from time to time can find their way into Skydrol hydraulic systems. No level of cross-contamination between Skydrol and hydrocarbon oils is safe because Skydrol systems use EPDM seals, which are destroyed by hydrocarbon oils, and hydrocarbon oil systems use other seal materials that are destroyed by Skydrol. It is important to recognize that Skydrol hydraulic systems require care and maintenance. A test once per year for key properties is a good way to be sure that these properties are maintained, and to alert the operator to easily preventable problems.
