Skydrol & Phosphate-Ester Seal Compatibility: Guide and Checker
A seal that works in mineral hydraulic oil is not automatically suitable for Skydrol. Treat a fluid change as a review of the complete sealing system, not a like-for-like rubber substitution.
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Check the fluid before the seal
Record the product name and grade from the reservoir service record or fluid supplier. “Synthetic” and “fire resistant” are not chemical specifications. Water-glycol, synthetic hydrocarbons and phosphate esters need different compatibility checks. Unknown or mixed fluid requires identification before material selection.
Compatibility screening tool
The result below screens a family only; it cannot approve an aircraft part, compound or operating temperature.
Inspect every component in contact with fluid
A PTFE ring may use a separate elastomer energizer. Verify that energizer, back-up rings, wipers, hoses, coatings and approved assembly lubricant as well as the visible seal. A compatible face material does not approve the complete assembly.
What to send to the supplier
- Exact fluid name, operating and peak temperature, and pressure cycle.
- Seal location, motion, groove dimensions and current part or compound number.
- Equipment approval requirements and any evidence of contamination or fluid mixing.
- Written confirmation covering the entire seal assembly, not just a generic polymer name.
If the old seal is swollen or sticky
Stop treating repeated swelling as a size problem. Preserve the failed parts and fluid identification, inspect other wetted components and investigate chemical attack. Follow the equipment maker’s fluid replacement and cleaning procedure; a new compatible ring alone does not remove contamination.
Aircraft and industrial applications
Aircraft maintenance requires approved parts and the applicable maintenance instructions. This checker is not a substitution authorization. Industrial phosphate-ester fluids also require their own supplier data; do not transfer a Skydrol family screen to every fire-resistant fluid.
To connect this inspection with the next repair decision, see Hydraulic Seal Material Compatibility: Match the Seal to the Fluid and Temperature.
Technical references
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