Aviation hydraulic fluids are hygroscopic and, as a result, their lifetime is highly unpredictable. The performance of the entire aircraft hydraulic system is affected by the condition of the hydraulic fluid and if degradation goes undetected, it may cause damages with serious consequences. These may be economic at best or catastrophic at worst. At present, assessing the condition of the hydraulic fluid in an aircraft is laborious, time-consuming and expensive. Therefore the fluid is typically tested less than once a year, with the risk of unscheduled maintenance if the fluid has exceeded its limits of usage. Consequential interruption of the airline service results in a huge economic cost.
This project proposed the development of an optimised maintenance concept based on an autonomous onboard system capable of monitoring the fluid condition and restoring it when required. This would increase the lifetime of the fluid yet prevent damage caused by degraded fluid. If external reconditioning or change of the fluid should prove to be unavoidable, this could be scheduled to coincide with regular service and maintenance operations, thanks to the predictive capability of the monitoring system. Fibre-optic sensors using luminescent indicators as well as alternative optical and electrochemical sensors were developed for fluid monitoring. Similarly, different water separation and elimination techniques were investigated and selected.
The chosen approach yielded a balanced-risk strategy in which established techniques were combined with cutting-edge research, the outcome of which results in concurrent individual deliverables of high intrinsic value, thereby enhancing the combined benefits expected from the project. The impact of this system would extend far beyond the consortium partners: the cost savings to airlines due to the optimised maintenance strategy would give European constructors such a competitive advantage that the entire industry would be strengthened.
Three identical monitoring prototypes have been manufactured, each destined to undergo a different set of tests. The control system unit (CSU) gathers the data from the sensors and that provides the connections to the test bench. The power supply and protection boards are considered as part of the CSU. Unit 1 was put through vibration tests and unit 2 was tested against endurance and fatigue. Unit 3 was intended for the functional testing, performed by Airbus France.
The design was deemed sufficiently robust to withstand possible vibrations during functional tests and therefore does not present any relevant risk to the Esther test bench. Furthermore, it was noted that when pushing the tests to the extreme nearly all the sensors and their parts, in particular the luminescent sensors and the conductivity and capacitance sensor successfully withstand the tests. Only the infra-red and particle counter sensor's electronics were damaged after prolonged vibration. Five thousand (5 000) pressure cycles and electronic cycles were performed on the unit. At regular intervals of one thousand (1 000) cycles, a check of the CSU was performed to validate that the system was behaving as expected. The unit has passed successfully the endurance test and its design is deemed fit to be integrated in Airbus Esther test bench for functional tests. Some issues related to leakage have manifested themselves due to the degradation of Orings which were incompatible with hydraulic fluid.
After the fatigue test, a product acceptance test was performed, except for the part of the PAT corresponding to conductivity and capacitance sensor unit. The results of the test were within tolerance and no further leakage or problems were detected. The results were similar to the results of the initial PAT and therefore the design was deemed to have passed sufficiently the fatigue test in order to undergo tests in the Esther test bench. During the functional tests, tests were performed at two different temperatures: 41 degrees Celsius and 56 degrees Celsius (measured at SSK unit outlet). Fluid samples were taken periodically, prior to any new contamination or relevant changes applied to the hydraulic fluid under test, and for every change of temperature. This was done using the AC sampling valve on the HP manifold. These samples were sent to be tested by conventional laboratory methods in order to contrast the performance of the SSK monitoring unit against the present State-of-the-Art. In general, tests were deemed a success, apart from