Aviation Hydraulic Fluid Types: Selection Guide for MRO Buyers

Aviation hydraulic fluid types are commonly distinguished by their base chemistry: mineral-oil, synthetic-hydrocarbon and phosphate-ester fluids are three important families. The correct type for an aircraft hydraulic system is determined by its specified fluid, not by the fact that two products serve the same general purpose. For MRO buyers, the first step is to identify the exact part number or fluid requirement in the maintenance documentation.

Key Takeaways

• Base-fluid chemistry is a useful starting point, but it does not establish interchangeability.
• A landing gear shock-strut fluid and a system hydraulic fluid may have different specified applications.
• Product part number, aircraft application and required documentation make an RFQ more precise.
• Fire resistance is not a universal property of synthetic hydraulic fluids.

The Main Aviation Hydraulic Fluid Types

Hydraulic fluid transmits force through aircraft systems and also helps lubricate moving components. Classifying fluids by chemistry helps buyers narrow a search, but each family contains products with distinct formulations and intended uses.

• Mineral-oil fluids: Petroleum-derived fluids used in specified aircraft hydraulic applications. Source-listed examples include AeroShell ASF41, described as mineral-based, and Castrol Aero 40, described as a mineral fluid for landing gear shock struts.
• Synthetic-hydrocarbon fluids: Fluids formulated with synthetic base stocks. The source-listed NYCO Hydraunycoil-FH42 uses polyalphaolefins (PAO) and esters and is described for aircraft hydraulic applications.
• Phosphate-ester fluids: A separate aviation hydraulic-fluid family commonly associated with fire-resistant system-fluid applications. None of the products cited in this article is identified in the provided product information as a phosphate-ester fluid.

These labels describe chemistry, not a universal hierarchy of quality. A buyer should also distinguish a fluid intended for a particular component, such as a shock strut, from one requested for a broader hydraulic-system application.

How the Listed Products Compare

The following examples illustrate why “aircraft hydraulic fluid” is too broad a purchasing description. They differ in stated chemistry, application or both; the table is not a substitution guide.

For example, a request for shock-strut servicing should identify the shock-strut fluid specified for that aircraft and component. The fact that ASF41 and Castrol Aero 40 are both described as mineral-based does not establish that one can replace the other. Likewise, Hydraunycoil-FH42’s synthetic formulation does not make it a generic substitute for every synthetic or fire-resistant aircraft fluid.

What Matters When Selecting a Fluid

A practical selection process moves from the aircraft requirement to the purchasable product, rather than starting with a broad chemistry label. Engineers can identify the required fluid; procurement can then match that requirement to the quoted item and its documentation.

1. Identify the exact application. Record the aircraft or equipment model, the system and the component being serviced. “Landing gear” alone may not distinguish a hydraulic circuit from a shock strut.
2. Capture the required identifier. Use the specified product part number or fluid designation where available. Include any applicable revision or other identifying detail recorded in the purchasing requirement.
3. Check what is being compared. If reviewing an alternative, compare its chemistry and stated application as preliminary information—not as proof that it is interchangeable with the requested fluid.
4. Define the procurement requirements. State the quantity, packaging preference and any documents your receiving or quality process requires, so the quote addresses the complete request.

Fire behavior is another reason to use precise terminology. A product described as having fire-resistant properties is not, on that description alone, equivalent to a phosphate-ester fluid or to another product described the same way. The relevant question is whether the identified product meets the requirement for the specific installation.

Writing a Clear Hydraulic Fluid RFQ

A useful RFQ tells a supplier exactly what to quote and gives the buyer enough detail to evaluate the response. Separate the aircraft context from the requested part number: the context explains the need, while the part number identifies the item.

For example: “Please quote part number CASTROL-AERO-40-YELLOW, Castrol Aero 40 hydraulic fluid, for an aircraft landing gear shock-strut maintenance requirement. Quantity: [required quantity]. Please identify the offered packaging and provide the documentation specified in our purchase order.” This wording does not assume that another mineral fluid will meet the same requirement.

If a maintenance team has supplied only a general description, such as “synthetic hydraulic fluid for an actuator,” procurement should obtain the specified identifier before requesting substitutes. The aviation chemicals product category can help buyers locate fluid listings, but a category match is not a final product match.

Compatibility and Documentation

Fluid selection affects more than the liquid itself. A hydraulic installation includes seals, hoses, coatings and other wetted materials, and its service history may matter when a different fluid is proposed. Identifying the fluid already required by the installation helps the maintenance team assess any proposed change.

Approval and airworthiness caution: Before use, verify the required approvals and fluid identification against the aircraft maintenance documentation and follow the fluid manufacturer’s current data sheet. Do not mix fluids or approve a substitution solely because their chemistry, application descriptions or marketing terms appear similar.

For purchasing, keep the requirement, supplier quotation and received-product identification aligned. If a quote names a different product or part number, treat it as a proposed alternative for technical review rather than an automatic match.

Sourcing from CraftJetAero

Buyers can request a quote from CraftJetAero by part number. Include the quantity, packaging requirement and any documentation requirements in the inquiry so the requested item is clear.

Frequently Asked Questions

What are the main aviation hydraulic fluid types?

Mineral-oil, synthetic-hydrocarbon and phosphate-ester fluids are three important chemistry families. A family name alone does not identify the correct product for an aircraft.

Are all mineral aircraft hydraulic fluids interchangeable?

No. Mineral-based products can have different formulations and stated applications; the required fluid must be identified for the particular aircraft and component.

Is a synthetic hydraulic fluid the same as a phosphate-ester fluid?

No. “Synthetic” is broad terminology, while phosphate ester identifies a particular chemistry family. NYCO Hydraunycoil-FH42 is described as a PAO-and-ester synthetic fluid, not as a phosphate-ester fluid.

What is the difference between shock-strut fluid and system hydraulic fluid?

Shock-strut fluid is identified for use in a landing gear shock strut, while a system hydraulic fluid is identified for a specified hydraulic-system application. The component and its documented fluid requirement determine what to purchase.

What should I include when requesting an aviation hydraulic fluid quote?

Provide the requested part number, quantity, packaging preference and required purchasing documentation. Add the aircraft or component context when it helps clarify the requirement.