Good Quality Hydraulic Fluid for Safe Operations

An A320 family aircraft was on descent passing through 14 000 ft. After a brief extension and retraction of the speed brakes by the flight crew, the aircraft suddenly banked to the right and the F/CTL SPOILER FAULT ECAM alert triggered. The pilot flying disconnected the autopilot and manually flew the aircraft. The flight crew observed that spoiler 3 of the wing on the right side was still fully extended. They performed the ECAM actions and the pilot flying continued to manually fly the aircraft and safely landed without further incident. Spoiler 3 remained fully extended with the aircraft on the ground. Maintenance crews found that the servo valve of spoiler 3 was stuck in the extended position. It was replaced and sent to the equipment supplier for analysis and repair.

Analysis of the faulty servo valve removed from spoiler 3 revealed that the high acidity of the hydraulic fluid led to corrosion of the internal parts and a crack was found. This caused the servo valve to malfunction, which resulted in the spoiler 3 to remain deployed after the speed brakes retraction command.

Poor quality hydraulic fluid can cause damage to the hydraulic circuit and the hydraulic components in systems such as flight controls, braking, and steering. A regular check of the hydraulic fluid quality can help to prevent damage.

The Maintenance Planning Document (MPD) task for A300, A310, A320  family, A330, A340, A350 and A380 aircraft requests sampling and analysis of the hydraulic fluid from each hydraulic circuit. This is to check the quality of the fluid, and these checks should be performed at regular intervals shown in the table below.

The hydraulic fluid sample must be sent to an approved laboratory for analysis in accordance with NSA307110 standard recommendations. This documentation can be found in the Airbus Process and Material Specification (PMS) available in Airnav^X.

An A220 MPD AMP task requests a check of the Differential Pressure Indicator (DPI) of the pressure filter, case drain filter, and return filter every 1 200 FH. Hydraulic fluid sampling should be done if contamination is identified during this check. The sample should then be sent to an approved laboratory for analysis.

The laboratory analysis checks the physical and chemical characteristics of the hydraulic fluid:

Particle contamination

Contamination of the hydraulic fluid by particles may cause erosion and damage to components such as pumps, valves, and ultimately cause components to jam or fail.

If the particle contamination is above the tolerance value provided in the AMM/MP, then the hydraulic fluid must be flushed and replaced or cleaned. For A220 aircraft, the hydraulic fluid must be replaced.

Viscosity & density

The hydraulic fluid must also have the correct viscosity and density characteristics to ensure the appropriate level of performance and responsiveness for the hydraulic system.

If the viscosity or density is not in the range provided in the AMM/MP/AMP, the hydraulic fluid must be replaced within the permitted time frame.

Electrical conductivity

If the electrical conductivity of the fluid is too low, it may lead to electrical discharging effects and arcing within the hydraulic system. This may further degrade the hydraulic fluid quality and cause damage to hydraulic components.

If the electrical conductivity is below the value provided in the AMM/MP/AMP, the hydraulic fluid must be replaced within the permitted time frame.

Acidity

High acidity of the hydraulic fluid leads to corrosion and erosion of the components it is in contact with. The Total Acid Number (TAN) is used to measure the hydraulic fluid acidity. The TAN is the number of milligrams of potassium hydroxide (KOH) needed to neutralize the acid in one gram of hydraulic fluid. The AMM/MP will define the amount of hydraulic fluid that must be replaced depending on the measured TAN value in the laboratory analysis report for the sample taken. This can range from replacing the quantity of hydraulic fluid in the reservoir (between 10 and 15% of the total fluid quantity) (fig.1) to full replacement (flush) of the fluid. For A220 aircraft, the hydraulic fluid must be replaced.

Chlorine content

The use of chlorinated solvents to clean components of the hydraulic system introduces chrorine in the hydraulic fluid. Chlorine creates acid when combined with any water present in the fluid and this will increase the level of acid in the fluid.

If the quantity of chlorine is greater than the tolerance value provided in the AMM/MP, the hydraulic fluid must be either cleaned or flushed and replaced within the permitted time frame.

Water content

Water content can lead to an increase of acid in the fluid and can modify the physical characteristics of the hydraulic fluid, which may reduce the performance of the system. It also can contribute to corrosion of hydraulic circuit components.

If the quantity of water is greater than the tolerance value provided in the AMM/MP/AMP, the hydraulic fluid must either be cleaned or changed within the permitted time frame. Additional inspections and tests may be required depending on the level of water contamination.

In 2021, Airbus launched an improvement of the AMM/MP procedure for analysis of the hydraulic fluid of A300/A310/A320 family/A330/A340/A350/A380 aircraft. The AMM/MP of A320 family, A350, and A380 aircraft was updated accordingly in 2021. The AMM of A330/A340 aircraft will be updated by mid 2022.

The AMM/MP improvement modifies the acidity thresholds and their associated fluid replacement procedure. This is to increase the hydraulic fluid lifetime and reduce the risk of reaching high TAN values, which may result in system malfunctions.

An acid scavenger is an additive in hydraulic fluid to reduce the likelihood of acid level increases in the fluid. This additive is progressively consumed over time and the level of acid in the fluid will begin to rise. When most of the additive is consumed, the acid level can rapidly increase (fig.2).

The improved procedure now requests corrective actions to reduce the acidity of the fluid from a TAN threshold of 1.0 mg KOH/g instead of the previous threshold of 1.5 mg KOH/g.

This lower limit for acid levels of 1.0 mg KOH/g was determined using in-service data analysis. This showed that when the results from the analysis of hydraulic fluid samples had a TAN value between 1.0 and 1.5 mg KOH/g, the very next sample taken was likely to show a significant increase of acid levels.

Performing corrective actions from 1.0 mg KOH/g will further prevent reaching high acidity values and this will improve the reliability of hydraulic components.

As an example, (fig.3) shows that the 1.5 mg KOH/g TAN threshold would require a full hydraulic fluid replacement at the third check. This would allow the TAN value to exceed 2.0 mg KOH/g for one year and would be likely to reach high TAN values above 4.5 mg KOH/g before the fluid replacement is due.

The 1.0 mg KOH/g threshold requires that only a volume of fluid 2x the hydraulic reservoir contents is replaced at the second and fourth checks. This will maintain the TAN value below 1.3 mg KOH/g during the entire period.

Precautions taken during servicing and maintenance operations can reduce the risk of contamination of the hydraulic fluid and reduce the likelihood of increased levels of acid in the fluid.

Hydraulic fluid handling and storage

New fluid containers must be correctly blanked to prevent water ingress from humidity in the ambient air. They should be stored in dedicated areas and away from used fluids. Operators should refer to the recommendations provided by the fluid supplier for fluid handling and storage.

Hydraulic Ground Cart

Airbus recommends periodic monitoring of fluids that are contained in hydraulic ground cart reservoirs. This is to prevent contamination during refilling of the hydraulic system when using a cart.

Maintenance operations

Any maintenance operation (especially hydraulic component replacement) can have a risk of hydraulic fluid contamination. Hydraulic lines and equipment must be blanked and stored in clean areas. Aircraft hydraulic servicing ports must be blanked and hydraulic equipment must be suitably protected against contamination.