With the flow
Innovative laboratory procedure for testing hydraulic fluids
During flight operations, various components are hydraulically activated, such as landing gear, stabilizer, elevators, rudder and brakes. In large airliners, several hydraulic systems are in operation. Each has its own supply of hydraulic fluid and generates the necessary pressure with the help of pumps powered either by the engines or electrically. The fluid is subject to routine inspections every 24 months. For the inspection, a sample is taken from each of the redundant systems of the aircraft and tested for potential impurities in the laboratory. Particular attention is paid to abrasion in the form of miniscule particles which can cause a blockage of the systems to be activated.
When the upper limits for the number of particles, as defined by the aircraft manufacturer, are exceeded, the hydraulic fluid for the system in question must be replaced. This operation takes several hours and is very costly. Since 2014, Lufthansa Technik's aircraft maintenance technicians have identified an increasing number of particles in hydraulic fluid. At the end of 2015, almost every customer aircraft from the Airbus A320 series had at least one system in which the upper limit was exceeded. Replacing the hydraulic fluid initially resolved the problem, but the cause was not identified. Lufthansa Technik's Laboratory Services division in Frankfurt, in collaboration with Frankfurt System Engineering and Innovation Management therefore launched an InnoBoost project to look at this issue in April 2016.
The aim of the project was to classify the particles in the hydraulic fluid, in order to gain an understanding of the causes of the increased particle levels. Based on these findings, methods and maintenance approaches could then be derived with the aim of reducing the need for replacing hydraulic fluids. At the same time, the sampling and measurement procedures were analyzed to determine potential for optimization. Mobile devices from various manufacturers were tested and compared with the laboratory-based measurement process. A special Hydraulic Fluid Sample Kit which avoids contamination of the samples has been in use since 2011. In the end, the test results provided decisive indications of the cause of the significantly higher particle numbers.
By combining various analytical procedures, Lufthansa Technik managed to characterize the finely distributed microparticles. For the most part, these are so-called "soft particles", introduced to the hydraulic system, for example, when mounting grease is used, which disrupt the automatic particle counting process. Thanks to the combined know-how of System Engineering and Materials Technology, the measurement process was modified. The "soft particles", completely harmless from an engineering point of view, are masked in such a way that they are no longer registered in the counting process and therefore do not influence the measurement result. The use of the modified process, which complies with prescribed norms and regulations, resulted in a significant reduction in the number of measurements that exceeded limits.
The precise source of the soft microparticles has not yet been identified. The large number of loadson the hydraulic system, such as hydraulic motors and hydraulic cyclinders, as well as the various workshops and suppliers involved, makes the search more difficult. Alternatively, clarification is needed as to whether lubricants are to be prohibited from hydraulic systems or an appropriate upper limit for such contamination in hydraulic fluid needs to be established. This is a question for the aircraft manufacturers.