HPU Preventative Maintenance Techniques: Hydraulic Fluid Care

HPU Preventative Maintenance

Hydraulic power units are a large, necessary investment for working on aircraft. Many maintenance repair and overhaul (MRO) providers and fixed-base operations (FBO) are able to trouble-shoot a variety of issues on hydraulic power units. While these technicians have the expertise, they find that repairs on this type of equipment can be time-consuming and costly. Equipment manufacturers have found that the majority of HPU issues arise from improper maintenance.

HPU preventative maintenance goes a long way to extending useful life and reducing equipment issues. This overview will provide you with what you need to know to keep your HPU running smoothly for years to come.

HPU Hydraulic Fluid Care

Hydraulic fluid should be inspected at regular intervals, typically every C check (20 – 24 months) to ensure normal color and smell. If the color varies, the fluid appears to have visible contaminants, or the smell seems strange, the fluid may have deteriorated. If either of these are observed, the oil may have gone through contamination, oxidation, or thermal degradation.

To check if the hydraulic fluid is in normal working condition, perform a sample analysis.


The most common type of contamination is dirt and debris. Not only do these contaminants cause excess wear, they can also increase the oxidation rate.


Oxidation is by far the most common cause of degradation. Oxidation occurs from increased temperature, pressure, and speed of hydraulic machines. When this happens, hydraulic fluid takes on a sludge-like viscosity. Temperatures over 203 Fahrenheit (95 Celsius) can cause oxidation. Check for any points where the fluid can come into contact with air.

Thermal Degradation

Thermal degradation refers to the separation of carbon bonds when extreme heat is added to hydraulic fluid’s maximum temperature. This process occurs when temperatures are over 572 degrees Fahrenheit (300 degrees Celsius).

To reduce the occurrence of thermal degradation, it is recommended to allow enough time for proper equipment start-up and shut-down. The fluid performs best when heating or cooling is gradual. During machine shut-down, it is possible for fluid to become trapped in the heater and experience excess heat exposure.

Tronair Hydraulic Power Unit, 57 series takes Skydrol or Hyjet (Exxon) hydraulic fluid.

This article is intended as a means to perform hydraulic power unit preventative maintenance. If you find an issue that needs intensive repair or have any hydraulic power unit repair questions, contact the experts at e2b calibration. Contact Us