Hydraulic fluid is a specialized aviation lubricant liquid that conveys power from one part of the aircraft system to another, enabling movement and action through various hydraulic components. It transmits power to flight controls, landing gear, brakes and gear retraction, while simultaneously lubricating moving parts and dissipating heat. In daily aircraft hydraulic system operations this single working medium sustains hydraulic pumps, accumulators and safety systems, guaranteeing the longevity and precise handling of hydraulically powered components.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is hydraulic fluid in aircraft?
Hydraulic fluid is the medium by which power is transferred in hydraulic machinery. Examples of equipment that might use hydraulic fluids are aircraft flight control systems. Hydraulic fluid is a specialized aviation lubricant liquid and the lifeblood of hydraulic systems. It is a non-compressible fluid, the medium via which the hydraulic system transmits its energy. In aircraft, hydraulic fluid is used to move mechanical components. Fluid under pressure drives machinery and distributed pressure allows controlled movement. The same fluid readily absorbs heat and is also used to dissipate heat within the system.
Do planes use hydraulics?
Yes, planes use hydraulics because of the immense pressures on the control surfaces during flight. Most planes rely on hydraulics for braking, for moving flaps, and for the precise extension and retraction of the landing-gear assembly. Almost all aircraft make use of some hydraulically powered components. In larger, more intricate aeroplanes, the use of such components is much more common. Light general-aviation aircraft are generally limited to providing hydraulic pressure to activate the wheel brakes, whereas larger aircraft carry a primary engine-driven hydraulic pump plus an auxiliary electric pump. Newer large commercial and military aircraft often replace pure hydraulic or cable/hydraulic hybrid control systems with the electro-hydraulic servovalve concept, giving lighter, more reliable circuits that still use hydraulic fluid as the working medium. Military aircraft hydraulic fluid, like the high-viscosity-index fluid specified by MIL-PRF-5606, is formulated to maintain performance in missile and fighter systems that experience wide temperature swings and high mechanical stress. Whether civil, commercial, or combat, the robust hydraulic power system remains fundamental to safe, precise flight.
What do hydraulic systems do in an airplane?
Hydraulic systems provide the necessary force for nearly every large-scale movement on the aircraft. Hydraulic systems provide motive power for flight control actuation, enabling precise and reliable control of the aircraft. Flight control surfaces are hydraulically actuated; hydraulic systems drive ailerons, elevators and rudder. Hydraulic systems provide motive power for flap and slat retraction/extension, allowing lift adjustment in every phase of flight. Hydraulic systems provide motive power for landing gear retraction/extension, while the robust hydraulic power system is fundamental for precise extension, retraction and locking of the landing gear assembly. Brakes rely on hydraulic fluid to apply immense pressure to wheel brakes, enabling safe deceleration and stopping after landing. Hydraulic systems provide motive power for nosewheel steering, giving the pilot accurate directional control on the ground. Hydraulic systems provide motive power for ancillaries like airstairs and cargo doors. Hydraulic systems provide motive power for emergency electrical generation, guaranteeing continued power if engine-driven generators fail.
What is aircraft hydraulic fluid made of?
Aircraft fluid is made up of different materials depending on the type. Mineral-based fluids like MIL PRF 5606, AeroShell Fluid 41, and Red Oil start with mineral oil that is derived from crude oil. This base stock goes through an industrial process to remove impurities, receives corrosion inhibitors, anti-wear additives, and oxidation-corrosion inhibitors, and is then filtered to a very high level of cleanliness. The result is a petroleum product that is relatively cheap to produce and very suitable for operating intricate machinery, including brakes. Fire-resistant Skydrol relies on a synthetic foundation. Skydrol is phosphate ester-based and made of fire-resistant phosphate ester base stock containing diester, triester, and tetraester phosphate compounds, which include oil additives like corrosion and erosion inhibitors. The synthetic formulation enables higher performance and stability at higher temperatures while reducing aircraft weight. MIL H 83282 is synthetic but uses a polyalphaolefin base stock to achieve similar fire-resistance benefits. Intermixing of these different chemistries is avoided, because even small contamination forces a complete drain-and-flush of the hydraulic system.
What are the properties of hydraulic fluid in aircraft?
The properties of hydraulic fluid in aircraft are listed below.
- Hydraulic fluids should have good anti corrosion properties
- Hydraulic fluids should have good lubricating capability
- Viscosity is one of the most important properties of hydraulic fluids
- Incompressibility is property of hydraulic fluids
- Hydraulic fluids should be fire resistant
- Demulsibility is property of hydraulic fluids
- Load carrying capacity is property of hydraulic fluids
- Thermal capacity/conductivity of hydraulic fluids is their ability to absorb and release heat and is a property
- Foam resistance is property of hydraulic fluidsGood low temperature fluidity, anti wear, oxidation corrosion inhibition and shear stability are desirable properties of hydraulic fluids
- Hydraulic fluids should have low density
- Hydraulic fluids are formulated to withstand both high and low temperatures
- Mineral based hydraulic oils must be compatible with seal materials
- Hydraulic fluids must meet extreme requirements of pressure
What are the types of aviation hydraulic fluids?
The types of aviation hydraulic fluids are listed below.
- Phosphate Ester-Based Hydraulic Fluids (Skydrol and Hyjet)
- Mineral-Based Hydraulic Fluids
- Synthetic Fire-Resistant Hydraulic Fluids
- Water-glycol Hydraulic Fluids
- Polyalphaolefin-Based Hydraulic Fluids
- Water-Based Fire-Resistant Hydraulic Fluids
- Types IV and V Hydraulic Fluids
- Synthetic Hydrocarbon-Based Hydraulic Fluids
Hydraulic fluids fall mainly into mineral-based and fire-resistant phosphate-ester types. The three principal categories of hydraulic fluids are minerals, polyalphaolefins, and phosphate ester-based fluids. Mineral Base Fluid is a type of hydraulic fluid and typically uses seal materials like Buna-N or fluorocarbon elastomers. MIL-H-5606 is the oldest one, and it is referred to as MIL-PRF-5606. Skydrol is purple, and it is manufactured by Solutia (now part of Eastman Chemical Company). Skydrol was formerly manufactured by Monsanto, and Skydrol lines include Skydrol LD-4 and SKYDROL 500B-4. MIL-H-83282 is used in military aircraft and hydrofoils, and MIL-H-46170 is a rust-inhibited version of MIL-H-83282. Synthetic hydrocarbon fluids provide superior performance in extreme temperatures.
What is a common hydraulic fluid used in aircraft hydraulic systems?
Phosphate ester-based fluid is the common hydraulic fluid in aircraft hydraulic systems. Skydrol, a purple fire-resistant phosphate ester fluid, dominates commercial aviation, while specification MIL-H-19457 phosphate ester fluid serves aircraft elevators, ballast valve operating systems, and replenishment-at-sea systems, giving operators precise control.
Military aircraft and hydrofoils rely on MIL-H-83282 or MIL-PRF-5606 synthetic hydrocarbon fluids, and military specifications sometimes allow MIL-H-5606 red mineral fluid, yet these do not displace phosphate esters in civil transports.
The best hydraulic fluid for aircraft is the type approved for the specific airframe - phosphate ester-based fluids for airliners, MIL-H-83282 for military jets, and high-cleanliness mineral fluids like AeroShell Fluid 41 for older or light aircraft - because each formulation balances fire resistance, low-temperature performance, and compatibility with onboard systems.
How can the proper hydraulic fluid for an airplane be determined?
Proper hydraulic fluid for an airplane is determined by consulting the aircraft manufacturer's service manual, the aircraft Type Certificate Data Sheet, and the aircraft parts manual. Suitable hydraulic fluid must be identified by appropriate markings. Each hydraulic system must be designed to use any suitable hydraulic fluid specified by the airplane manufacturer. Oil selection is based on aircraft maintenance manual instructions and must be based on OEM approvals.
Viscosity grades for hydraulic fluids are determined using the ISO viscosity grading scale. The most popular ISO viscosity grades are ISO VG 32 and ISO VG 46. System performance satisfaction vital characteristics to look for include Viscosity Indexing and Oxidative Resistance. A high viscosity index promotes more feasible run times at varying temperature settings and is optimal for cold starting a machine.
Another key decision maker in the selection of a hydraulic fluid, specifically phosphate ester technology, is the density of the fluid. A less dense fluid significantly contributes to reduced costs through weight, and ultimately fuel, savings. The condition of a hydraulic system can be determined by analyzing the operating fluid. Contaminated or aged oil turns dark brown, black, or milky. Using a non-specified fluid voids warranties and creates compliance issues.
What are the safety considerations for hydraulic fluid in aviation?
Safety considerations for hydraulic fluids include that they must be stored in a clean, dry, properly labeled, airtight container, kept in a safe, secure area away from heat and potential ignition sources, and apart from incompatible materials like oxidizers. It must be kept free from contamination by water, dirt, or other substances, and any spill must be cleaned immediately with soap and water and disposed of following approved procedures. Skydrol and other phosphate-ester fluids soften plastic materials and paints, and therefore, handlers must wear gloves impervious to Skydrol, long-sleeve shirts, pants, and eye protection to prevent skin and eye contact that causes severe irritation, chemical burns, or systemic toxicity.
Before any maintenance, hydraulic system pressure must be released safely through lockable or monitored isolation valves to prevent high-pressure fluid injection injuries that penetrate skin, cause tissue necrosis, infection, or require immediate amputation. High-pressure jets cut skin and clothing, and improperly secured hoses or fittings burst, whipping and causing lacerations or crushing injury and therefore, hoses and fittings must be inspected for wear and replaced before failure, and components must be depressurized before disconnecting, removing, or refitting. Pressure relief valves must be functional to regulate system pressure and prevent over-pressurization, and system pressure must be checked regularly to guarantee it stays within safe operating limits, avoiding large drops that generate excess heat and degrade oil faster.
Hydraulic fluid is flammable and ignites if it contacts welders, electrical junctions, or other ignition sources, causing fires or explosions, mist or fine spray ignites at flame contact and explodes, so ignition sources must be prevented from contacting spilled, sprayed, or misted fluid. Hot fluid causes burns, so fluid must be allowed to cool before replacing connections, and temperature must be monitored to avoid overheating. Low-pressure conditions create the diesel effect, forming harmful nitrogen oxides, while correct viscosity and adequately sized pump suction prevent micro-combustions. Leaks must be detected with cardboard instead of bare hands to avoid slips and falls on spilled fluid, and system integrity must be verified before returning the aircraft to service, guaranteeing all safety devices are re-installed and functional.
How is hydraulic fluid pressure indicated in transport aircraft?
Hydraulic fluid pressure in a transport aircraft is indicated by the main pointer of the triple pressure gauge. That gauge is normally mounted behind the throttle, on the left shelf of the instrument panel. Two smaller pointers on the same instrument show left and right brake pressure.
The flight deck receives continuous electrical signals from system sensors. A pressure transducer in the high-pressure manifold sends a direct pressure value to the displays. A reservoir-pressure switch guards the lower limit: if manifold pressure falls below 600 PSI, the switch energizes the red HYDRAULIC PRESSURE failure warning light. The same low-pressure event triggers a Master Warning: a yellow triangle appears on the left side of the primary flight display and an amber box on the compass rose of compatible glass-cockpit systems. An aural tone and the master caution light accompany the visual cues, giving the crew multiple, simultaneous indications.
