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.

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.