FAA AMT (Aviation Maintenance Technician) Airframe Practice Exam

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Which seal/material is used with phosphate ester base hydraulic fluids?

Silicone rubber
Nitrile rubber
Polyurethane
Butyl rubber

The correct answer is: Butyl rubber

Phosphate ester base hydraulic fluids can be very aggressive and can cause deterioration in certain types of seals and materials. Butyl rubber is known for its excellent resistance to phosphate ester hydraulic fluids, making it the correct choice in this scenario. Silicone rubber, nitrile rubber, and polyurethane are not suitable for use with phosphate ester base hydraulic fluids and can degrade when in contact with these fluids.