Hydraulic systems are hidden away within an aircraft. They’re not normally visible to passengers or crew. Technicians need to open access panels to troubleshoot and maintain these systems.
Despite that lack of visibility, hydraulic systems and the aircraft hydraulic fluid that moves within them are crucial to modern aviation. The basic principle is the same as any other hydraulic system. Applying pressure in a closed system transmits that pressure across the system.
In aircraft, that pressure can be used to control a variety of critical systems. Brakes, flaps, and landing gear, among other systems, can be easily and reliably controlled with a well-maintained hydraulic system.
Airplane hydraulic fluid is indispensable in these systems. It’s the medium through which pressure is applied, multiplied, and distributed. Without hydraulic fluid, the safe and consistent operation of modern aircraft is not possible.
Keep reading to learn more about aircraft hydraulic fluid. Topics covered include the principles of hydraulic fluid and hydraulic systems, common types of aircraft hydraulic fluid, and more.
The Basics of Aircraft Hydraulic Fluid and Systems
Hydraulic Systems
In the mid-1600s, French physicist and mathematician Blaise Pascal developed a principle that’s still crucial in today’s world. It plays a formative role in the development and operation of modern aircraft, as well as in hydraulic systems in general.
NASA shares Pascal’s Law, also called Pascal’s Principle: “When there is an increase in pressure at any point in a confined fluid, there is an equal increase at every other point in the container.”
A closed system prevents the fluid from escaping that system. As long as the system’s components can contain the additional pressure applied, that pressure is transmitted across the system. Hydraulic systems use cylinders, pistons, tubes, hoses, and other components, like motors and pumps, to effectively transmit pressure.
More complex hydraulic systems use cylinders of different sizes. This allows for not only the transfer of pressure or force but its multiplication as well.
One key advantage of hydraulic systems for aircraft is the ability to use relatively small components, like hoses and tubes, to transmit a great deal of pressure or force. These components can easily fit into the limited space available within aircraft and effectively control heavy components like landing gear and flaps.
Hydraulic Fluid
Hydraulic fluids are distinguished in part by their resistance to being compressed. This differs from pneumatic systems, which use compressible fluid.
The lack of compression eliminates delays in initiating an action, such as extending landing gear, and the process beginning. This resistance to compression is also important because of the pressure changes aircraft experience while in operation.
In other words, this fluid is the medium by which energy is transferred. Without the right kind of hydraulic fluid, a hydraulic system simply won’t work.
A wide range of hydraulic fluids, often but certainly not always based on mineral oils, is available for many specific applications. The particular properties of these fluids lend themselves to the types of systems in which they’re used.
Consider Skydrol, one of the most widely used aircraft hydraulic fluid types. Skydrol will be explored in greater detail later on. For now, it’s important to understand that Skydrol is fire-resistant, chemically stable, and thermally stable. These qualities are especially important in the safe and efficient operation of aircraft.
Fire resistance is a critical safety measure for a vehicle traveling miles above the ground at hundreds of miles per hour. High chemical stability increases the useful life of Skydrol, making it a cost-effective choice for the aircraft that use it. Strong thermal stability allows this aircraft hydraulic fluid to maintain its other key properties even when regularly exposed to the high temperatures common in aircraft operation.
How Aircraft Use Hydraulic Systems and Hydraulic Fluid
Hydraulic systems can generate substantial force using relatively small components. Hydraulic fluid carries this force across the system, reliably applying it to achieve the desired effect.
In aircraft, hydraulic fluid and hydraulic systems control vital equipment and systems. It’s fair to say that modern aviation as it is currently understood would not be possible without hydraulic fluid and the systems that use it as a medium.
For aircraft specifically, the following systems rely on hydraulic systems to provide the necessary force and control.
Landing Gear
Landing gear needs to be sturdy enough to handle the considerable weight of an aircraft, as well as its passengers, crew, and cargo. On modern airliners, that can mean safely supporting a takeoff and landing weight of hundreds of thousands of pounds.
Retractable landing gear is crucial for more efficient flight. Storing landing gear creates a more aerodynamic profile for the aircraft. By necessity, the landing gear needs to extend and retract while the plane is in flight. Hydraulic systems provide the necessary force to reliably extend and retract landing gear in flight.
Brakes
Aircraft brakes are a key safety measure during taxiing, takeoff, and landing. They allow for improved control of the aircraft and help to avoid collisions and other seriously negative outcomes.
Hydraulic brakes are by far the most common system in use in aircraft. Their near-instantaneous action and the significant force generated through hydraulic fluid for aircraft provide consistent and precise control.
Flight Control Surfaces
Flight control surfaces — ailerons, rudders, elevators, and flaps, among others — provide the precise control pilots need to effectively control an aircraft while in flight.
These control surfaces themselves can weigh a considerable amount. In addition, they need to be used while the plane is in flight. That means a significant amount of drag is often acting against the control surfaces.
Hydraulic systems provide the necessary force to overcome those obstacles. The ability of pilots to make real-time adjustments to flight control surfaces depends on aircraft hydraulic fluid and hydraulic systems.
Thrust Reversers
Thrust reversers improve deceleration, allowing for shorter landing distances, better control, and the sparing of wear on brake systems. Diverting thrust requires a significant amount of power to stand up to the force generated by jet engines.
This is yet another vital application of hydraulic systems in aircraft. The force generated by compressing hydraulic fluid operates the thrust reversers, moving them into and helping them maintain the correct position.
Benefits of Aircraft Hydraulic Fluid and Hydraulic Systems
Hydraulic systems offer a number of advantages in general use and specifically for aviation.
One key benefit to consider is reliability. Reliable systems are incredibly important for aircraft, as they need to operate high above the ground and at fast speeds.
Despite the force they can produce, hydraulic systems operate on relatively simple principles. When designed, built, and maintained properly, they can offer increased reliability as compared to more complex systems.
For a similar reason, hydraulic systems are also generally cost-effective. Their reliance on basic principles of physics doesn’t preclude the need for maintenance or eliminate potential issues, of course. However, more streamlined maintenance and repair have positive impacts on overall cost.
Fast response time is another valuable benefit to keep in mind. Aircraft need to be precisely controlled for safe and efficient travel.
The resistance to compression offered by hydraulic fluids leads to real-time action. Flight control surfaces adjust in line with a pilot’s use of controls. Landing gear must extend and retract without delay. Brakes must work when they need to, not after valuable seconds have been wasted.
Common Types of Hydraulic Fluid Used in Aircraft
There are many types of hydraulic fluid used in aircraft. Skydrol, a brand of phosphate ester-based aircraft hydraulic fluid, is offered in a wide range of formulations designed to meet the specific needs of different aircraft and hydraulic systems.
However, there are three types of hydraulic fluid used in aircraft across the board. It can help to consider these as broad hydraulic fluid categories to differentiate them from the many formulations offered by producers.
Aircraft Hydraulic Fluids Based on Mineral Oils
Mineral oils are a common base for hydraulic fluids in general. Mineral oil itself is a byproduct of petroleum production.
Many aircraft, especially those used in general aviation and commercial flight, use this common preparation in their hydraulic systems.
Aircraft Hydraulic Fluids Based on Synthetic Hydrocarbons
Modern hydraulic fluids include those based on synthetic hydrocarbons. This version of hydraulic fluid is used in some types of military and high-performance aircraft. It offers improved performance at high pressures and temperatures.
Aircraft Hydraulic Fluids Based on Phosphate Esters
Phosphate ester-based hydraulic fluids offer improved fire resistance, among other benefits. Skydrol is just one example. This is another common category of hydraulic fluid.
Answering Common Questions About Hydraulic Fluid for Aircraft
Is Aircraft Hydraulic Fluid Flammable?
The answer depends on the specific hydraulic fluid in question. Reducing and eliminating flammability has been a major factor in the development of aircraft hydraulic fluid over time. Earlier fluids presented a fire risk while in flight if the system reached a critical temperature or was breached, creating major safety concerns.
Many modern hydraulic fluids, although not every specific type currently in use, are fire-resistant.
What Is the Color of Hydraulic Fluid in Aircraft?
The color of hydraulic fluid depends on the specific fluid in question. There is no requirement that all hydraulic fluids be dyed to match a specific color.
For example, Skydrol has a purple color, at least when it is first produced and added to an aircraft hydraulic system. However, that color can fade and change over time.
Do Aircraft Need to Use a Specific Type of Hydraulic Fluid? Can Hydraulic Fluids Be Mixed?
Airplane hydraulic fluids are designed to meet exacting requirements. Individual brands produce several types and grades of hydraulic fluid with specific additives. These precise formulations are taken into account in the design of aircraft hydraulic systems.
In other words, hydraulic fluids should never be mixed. Mixing fluids can lead to the degradation and eventual breakdown of hoses and other components of a hydraulic system. Differences in viscosity between fluids can also cause hydraulic systems to perform less efficiently and more unreliably.
The specific hydraulic fluid recommended by the aircraft manufacturer should always be used. As long as these guidelines are followed, selecting the right hydraulic fluid for a given aircraft is a simple process.
How Are Hydraulic Systems Maintained and Tested?
Hydraulic systems need an input to create and multiply the force they generate. While an aircraft is operational, this force comes from the plane itself. When aircraft undergo maintenance, an external source of power is needed.
This is where hydraulic power units (HPUs) serve a critical role. These pieces of equipment, also known as hydraulic mules, provide a power source. This enables technicians to test and maintain vital aircraft systems, including aircraft control surfaces and landing gear.
Pilot John International: Reliable Sales, Service, and Support for Aircraft Hydraulic Fluid Needs
Pilot John International (PJi) is proud to be the global source for all things aviation. That includes aircraft hydraulic fluid and extends to ground service equipment, aircraft parts, other consumables, maintenance, trade-ins, and much more.
With global shipping available to all customers, PJi serves as an essential link in the aviation supply chain. By carefully monitoring and maintaining inventory, it’s easier to get the right parts, GSE, and consumables to the right place at the right time.
Trade-ins, financing, and other options make it easier to acquire critical equipment and consumables when they’re needed. Flexible payment options support effective budgeting for clients without delaying the purchases that keep aircraft safe, functional, and airworthy.
At PJi, customer service is supported by automation instead of being replaced by it. Clients know they can expect support from a real person who will take the time to understand and address individual needs. That means getting answers to important questions and receiving helpful guidance that supports successful aircraft operations.
PJi stocks all major brands of aircraft hydraulic fluids in a variety of specific formulations. Looking for an aviation supplier that goes the extra mile for its customers? Buy aircraft hydraulic fluid today!
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