Degradation monitoring of aviation hydraulic fluids using non-dispersive infrared sensor systems

In commercial aircrafts, flats, slats, tail plane fins and landing gears, i.e. all kinds of safety-critical mechanical subsystems are powered by hydraulic actuators. The mechanical power produced by these actuators stems from pressurized hydraulic fluids, which for reasons of passenger safety need to be fire-resistant. Phosphate-esters fluids, which are widely used for this purpose, are hygroscopic in nature and therefore susceptible to thermal degradation as flight operations are carried out. The paper demonstrates that the processes of water take up and thermally stimulated hydrolysis can be monitored with the help of non-dispersive infrared systems. In addition, such systems yield valuable information about the degree of acid scavenger depletion. With the information provided by such sensor systems innovative aircraft maintenance schemes can be implemented with a high cost cutting potential.