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Introduction to Hydraulic Fluids in Aerospace

Hydraulic systems are found in both commercial and military aircraft, converting pressurised fluids to power a variety of components. To reduce maintenance costs, selecting the right hydraulic fluid is essential, and the following introduction provides an insight into what considerations should be taken when choosing the right fluid.

What is a hydraulic fluid?

There are various types of hydraulic fluids (sometimes referred to as hydraulic oil) currently in operation around the world. Generally, a hydraulic fluid should exhibit the following properties:

  • Low freezing point;
  • Good anti-corrosion properties;
  • Flammability characteristics;
  • Seal compatibility;
  • Good temperature stability at both high and low temperatures;
  • Long service life.

Aircraft wheels on runway

Aerospace hydraulic fluid applications

On-board commercial and military aircraft, hydraulic fluids power all sorts of key equipment including:

  • Flaps and slats;
  • Access doors and ramps;
  • Landing gears;
  • Brakes;
  • Flight control systems;
  • Missiles and ordnance.

Aeroplane wing against sky

What to consider when choosing your hydraulic fluid

Depending on the application requirements, you may need to explore more than one type of hydraulic fluid. Below is a general breakdown on the different types available.

Hydraulic Fluids by Base Oil Type

In the same way that turbine engine oil technology has evolved over time, hydraulic fluids are no different. Mineral hydraulic fluids, while still common place in military applications, have been replaced by phosphate ester based fluids for commercial aviation due to their superior flammability properties. A small range of synthetic hydrocarbon based fluids are also in operation within military aviation to cope with some of the flammability issues identified in mineral oils.

Type IV or Type V Phosphate Ester Fluids

Within the phosphate ester range of hydraulic fluids, the products available are classified further by Type IV or Type V. Type V fluids generally offer better stability at higher temperatures and longer fluid life, meaning a possible reduction in maintenance costs. Both types of phosphate ester fluid are fully compatible with each other, regardless of brand, but it should be noted that phosphate ester and mineral or synthetic fluids are fully incompatible and any product changes should be carried out in accordance with service manuals.

Weight and Fuel Savings in Phosphate Ester Fluids

Another key decision maker in the selection of a hydraulic fluid (specifically phosphate ester technology) is the density of the fluid. To reduce operational costs, aircraft operators demand lighter aircraft, and a less dense fluid can significantly contribute to reduced costs through weight, and ultimately fuel, savings.


It is vital that any oil selection is based on the OEM approvals and aircraft maintenance manual instructions. We have put together a full cross reference guide for hydraulic fluid specifications and approvals which you can download.

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