Abstract
Passenger safety requires that in commercial airplanes hydraulic actuators be powered by fire-resistant hydraulic fluids. As a downside, such fluids are hygroscopic which means that these tend to accumulate humidity from the environment and that the dissolved humidity tends to produce acidity which can corrode all kinds of metallic components inside a hydraulic system. As such damage in safety-critical subsystems is hard to localize and expensive to repair, sensor technologies are required which allow the state of water contamination and fluid degradation to be routinely checked and necessary maintenance actions to be scheduled in a way that causes minimum flight interruptions. The paper reviews progress that has been made in developing such sensor systems and in commissioning these into practical flight operation. Sensor technologies that proved optimally adapted to this purpose are multi-channel non-dispersive (NDIR) systems working in the mid-infrared range. Additional options concern optical absorption sensors working in the near-infrared and visible ranges as well as fluorescence sensors.
Highlights
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
Absorbed water vapor together with Joule heat, which is generated upon hydraulic actuation, tends to break down the base fluids into smaller molecular fragments and into several kinds of acidic phosphates which are problematic for two reasons: firstly, such fragments support autocatalytic reactions, which speed up the degradation process, and secondly, these tend to corrode all kinds of metallic components inside a hydraulic system, generating metallic particle debris
Section 3fluids we present optical absorption data, obtained on a range of widely used hydraulic fluids and of fluid contamination and fluid degradation have been identified which can be used to advantage spanning over a wide range of photon energies
Summary
Flats, slats, tail plane fins and landing gears, i.e., all kinds of safety-critical mechanical subsystems, are powered by hydraulic actuators. A keya range problem pressurized reservoirfluid and by sending tapped fluid toorspecialized of inherent in this approach is that this maintenance-related information typically becomes available relevant fluid properties can be determined Depending on these results, either water is withdrawn with the a delay of one week or the more, when the serviced aircraft should normally beinherent back in from hydraulic fluid and/or fluidi.e., is partially or completely exchanged. Airliners have asked for in-plane fluid monitoring systems which allow relevant maintenance is associated with an interruption of the normal flight schedule and considerable cost to fluid properties to bethis measured withunfavorable a daily or weekly frequency fluidAirliners, from the the airline operating plane such situations urgentlywithout need totapping be avoided.
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