Hydraulic Fluid Quality
Several cases of uncommanded spoiler
extension were reported to Airbus in recent years. Investigations showed that a
high acidity level of the hydraulic fluid was a contributor to these events.
Analysis of the faulty servo valve removed from spoilers 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 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.
To take a sample of the hydraulic fluid, it is important to apply the steps in the AMM/MP procedure. Avoid contaminating the sample to prevent an erroneous contamination measurement. Flush the sampling valve by discarding the first 200 ml of hydraulic fluid to remove any particles. Then collect the fluid sample in a clean and dry chlorine-free bottle.
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 AirnavX.
The laboratory analysis checks the physical and chemical characteristics of the hydraulic fluid, Particle contamination, Viscosity & density, Water content, Chlorine content, Electrical conductivity and 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) to full replacement (flush) of the fluid. For A220 aircraft, the hydraulic fluid must be
