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 type of hydraulic fluid is essential for the aviation industry. Here, we delve into the characteristics, key differences, and crucial considerations for these vital fluids.

What is a hydraulic fluid?

Hydraulic fluid, or hydraulic oil, is a non-compressible fluid used to transfer power within hydraulic systems. It allows machinery to perform various tasks like lifting, braking, or moving components. These fluids are essential for the operation of machinery ranging from heavy industrial equipment to advanced aerospace systems. Generally, a hydraulic fluid should exhibit the following properties to perform effectively:

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

Aerospace hydraulic fluid applications

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

Flaps and Slats: Hydraulic systems extend and retract these wing components, crucial for increasing lift during takeoff and drag during landing.
Access Doors and Ramps: From cargo doors to passenger ramps, hydraulics ensure smooth, controlled, and secure opening and closing.
Landing Gears: The robust hydraulic power system is fundamental for the precise extension, retraction, and locking of the landing gear assembly.
Brakes: Aircraft braking systems rely on hydraulic fluid to apply immense pressure to the wheel brakes, enabling safe deceleration and stopping after landing.
Flight Control Systems: Many primary and secondary flight control surfaces (like ailerons, elevators, and rudder) are moved by hydraulic actuators, providing pilots with precise control over the aircraft's flight path.
Missiles and Ordnance: In military aircraft, hydraulics are essential for the deployment, positioning, and firing mechanisms of various weapons systems.

Optimising Fuel Efficiency with Hydraulic Fluid Density

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.

Hydraulic Fluid Specifications

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

Common Hydraulic Fluid Types in Aerospace

In the same way that turbine engine oil technology has evolved over time, hydraulic fluids are no different. There are various types of hydraulic fluids currently in operation around the world. These fluids are categorised by their base oil, with each type designed for specific performance needs in commercial and military aviation.

Mineral-Based Hydraulic Fluids

Mineral-based hydraulic fluids are among the most common and historically significant hydraulic fluid types in aviation. These fluids remain prevalent in certain military applications and are widely used in general aviation aircraft and some commercial flight operations. They are relatively cost-effective to produce and perform well in various small and larger aircraft systems.

Synthetic Hydrocarbon Hydraulic Fluids

Primarily used in military and high-performance aircraft, these hydraulic fluid types offer significantly improved performance at high pressures and temperatures. Key advantages include reducing friction and heat generated within the hydraulic system, which enhances overall efficiency.

Phosphate Ester Hydraulic Fluids

Phosphate ester hydraulic fluids are the mandatory choice for modern commercial aviation due to their exceptional fire resistance. This hydraulic fluid type is crucial for passenger safety and represents a non-negotiable advancement in airplane hydraulic fluid technology.

Within this category, Type IV and Type V fluids denote performance levels:

Type IV Fluids: A well-established and reliable option, providing excellent fire resistance for commercial aircraft.

Type V Fluids: The next generation, offering enhanced stability at higher temperatures and extended fluid life. This can lead to reduced maintenance costs and often features lower density for improved fuel efficiency.

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.

FAQs about Airplane Hydraulic Fluids

Do aircraft need to use a specific type of hydraulic fluid?

Absolutely, aircraft must use specific hydraulic fluid types as dictated by the aircraft manufacturer (OEM) and maintenance manuals to ensure system compatibility, safety, and optimal performance.

Can hydraulic fluids be mixed?

Generally, mixing different hydraulic fluid types, especially those with different base chemistries (like mineral and phosphate ester), is highly discouraged due to severe incompatibility issues that can lead to system failure.