Abstract:
Hydraulic contamination is a major contributor to flight control anomalies, binding, and leakage of hydraulic components. Typically, filters are rated using Beta Ratio/Efficiency at certain micron sizes per standard lab tests such as ISO 16889 or Mil-F-8815D (Ref. 1, 2). These are all steady flow tests at 100°F. Unfortunately, during normal aircraft operations, hydraulic filters are exposed to a wide range of operating conditions such as high oil temperatures, vibration due to pressure pulsations and flight maneuvers, cyclic flow and thermal excursions. The current hydraulic filters used on Army helicopters tend to shed particles during cyclic flow and vibration which can cause close clearance spools to lock-up in servo controls. This may result in un-commanded flight control inputs. This can also result in low Mean Time Between Failures (MTBF) on pumps and actuators. The US Army has developed new hydraulic filter specifications which require dynamic test procedures using more realistic conditions. This has resulted in the development of more robust media filters, which have been shown during on-aircraft demonstrations to increase fluid cleanliness and decrease maintenance costs.