Aircraft Hydraulic Fluid

Aircraft · 5 min read · Apr 14, 2022
aircraft hydraulic fluid

Every modern aircraft uses at least a few hydraulic systems responsible for different stages of its successful operation.

While smaller aircraft such as less advanced fighter jets and light planes are using hydraulic systems mainly for moving and actuating flaps and brakes – and, sometimes, also the landing gear, bigger airplanes are hard to imagine without a hydraulic system responsible for the proper functionality of their thrust reversers, spoilers, flight controls, and even some other apparatus. 

Being more durable than a pneumatic system which was much more frequently used on airplanes in the past, the hydraulic system also has a great advantage of being more lightweight – which is crucial for the commercial viability of modern planes.

Yet, such advanced systems are quite complex and, despite their excellent durability, sometimes can require even more maintenance – that was especially true in the early days of such systems on airplanes.

The reason behind increased durability and reducing the need for maintenance is the technological evolution of such systems. However, the biggest improvements that helped achieve it are undoubtedly related to the evolution of hydraulic fluids, known to be the vital blood inside an aircraft’s heart and veins.

An exposed piece of hydraulic system of an aircraft engine.

What are aviation hydraulic fluids?

Aviation hydraulic fluid is a type of hydraulic fluid used both in civil as well as in commercial aviation – as well as to ensure the proper functionality of military aircraft and other related equipment.

The most common example of an aviation hydraulic fluid could be an airplane hydraulic fluid. Airplane hydraulic fluid is a type of special technical liquid which is used to distribute force through various mechanisms of the plane.

  • A liquid of a designated composition is used for such a purpose, while such composition may vary depending on the type of hydraulic fluid.

Such fluid is not just any liquid – yet, if in theory, it could be any because fluids are practically incompressible, hydraulic fluids used in most modern airplanes must ensure proper functionality of the system’s operation, which is dependable on hydraulic fluids.

For an aircraft’s proper,  safe, and efficient operation, such systems must be fully functional in various conditions, including extreme cold and heat. In addition, hydraulic fluids should ensure that such parts should not be expected to wear out too fast and should be prone to corrosion.

What properties should airplane hydraulic fluid have?

As modern aircraft should equally efficiently operate in unforeseen situations, such as landing at some emergency airfield and in harsh climate conditions, hydraulic fluids, which are nowadays used in virtually any plane, must meet certain conditions. Such conditions are mainly related to the desired properties of such liquids, such as flammability characteristics. We will list most such properties.

An ideal hydraulic fluid should be a good lubricant

Every hydraulic system used on modern aircraft is an extremely complex structure consisting of many vital parts, such as tubes, seals, gaskets, hoses, or other hydraulic components.

Some of them are moving segments that come into contact with one another, so the extreme friction here usually should be avoided in order for such parts to function properly and not to wear out too soon. That is why every modern hydraulic fluid must not only act as a lubricant – but be specifically good at performing such tasks.

The viscosity of such fluid is extremely low

Low viscosity hydraulic fluid is now used on virtually every aircraft. Such circumstance is closely related to the previously mentioned required feature of the liquids in question. The low viscosity of hydraulic fluid aims to minimize friction in pipelines while ensuring the high-speed operation of motors and pumps.

Chemical stability and resistance to foaming are also essential

While the first feature historically was always at the basis of the development of every new or improving already developed and more or less widely used aviation hydraulic fluid, the second one is among relatively new requirements for a good hydraulic liquid.

While air entrainment is not usually something dangerous in hydraulic systems of, for example, ground transportation vehicles, even here, it can lead to undesirable effects such as fluid leaks. And as it poses a much higher danger to the safe operation of an airplane, resistance to foaming is crucial for an aircraft’s hydraulics.

Resistance to corrosion is also required for the safety of the operation

No commercially viable aircraft can enjoy the luxury of such a steel piping in its hydraulic system as, for example, marine vessels. That’s why any rust here should be avoided by using corrosion-resistant fluids.

Non-flammable? Usually, yes – but not always

Most early airplane hydraulic fluids were extremely flammable – technology dating seven, six, or even five decades ago did not allow to make such liquids easy and relatively cheap to produce if they were to meet both non-flammability and all the above requirements.

That is why, for example, MIL-H-5606 and MIL-H-6083 (rust inhibited version of MIL-H-5606 which means it has higher corrosion resistance) type fluid, which is highly flammable, was among the most widely used hydraulic liquids in aviation as late as in the seventies.

Later on, as technology (especially phosphate ester development) advanced, much less flammable, and finally – completely fire-resistant alternatives were developed for effective production, distribution, application, and use, including Type IV fluids. For example, MIL-H-83282, MIL-H-87257, MIL-H-83282 (Polyalphaolefin-based fluid), MIL-H-46170 (rust-inhibited version of MIL-H-83282). Recently, phosphate ester technology gave us a Skydrol, which can’t be called even extremely fire resistant – it is a completely fire resistant hydraulic fluid.

However, as both other airplane systems and standalone components, together with the environment in which an aircraft is operating in, became much more fire-resistant themselves, flammable hydraulic fluids are still very popular within the aviation industry.

The Pratt & Whitney J58 jet engine and some of its hydraulic systems.

Do not forget about the requirement for a wide operating temperature range of hydraulic systems

As it was already mentioned, airplanes are rarely made for operating in specific lukewarm climate conditions. From the shallow temperature far north, especially in the cruising altitudes of jet airplanes with low air density, to the extreme heat, which can become a real problem for water and vegetable-based oil lubricants.

Which will boil at temperatures in which almost any aircraft hydraulic system operates – the best aircraft hydraulic fluids must neither become extreme viscosiosus nor noticeable move towards its boiling point in heat up to 275° F – like phosphate ester hydraulic fluid.

That is why, for example, MIL -H-83282 and MIL-H-46170 type of hydraulic fluid is not used in aircraft which tend to operate or be stored in shallow temperatures. Just because its lower operating limit is officially only -40° F – yet, it is still quite popular in some Navy aircraft.

Can you mix different types of hydraulic fluid in the same aircraft?

Different types of hydraulic fluid, for example, phosphate ester-based and mineral ones, should not be mixed in the same system. All modern aircraft operate primarily with one type of such fluid in their systems. Except for some military aircraft, even different fluids with similar compositions can not be mixed to avoid failure in their hydraulic systems.

Generally speaking, when it comes to fluids of different compositions, mixing petroleum-based and phosphate ester hydraulic fluid would result in almost immediate failure of the systems as neither interchange seals are specified for a particular fluid. However, if such a mixing occurred immediately, a hydraulic system flush or simpler drain – depending on the system – according to the precise manufacturer’s specifications can help avoid costly repairs.

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Jet pilot @NASA

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