FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsMedium
A technician is servicing a hydraulic system and must identify the fluid type before adding fluid. The system uses synthetic phosphate ester fluid, which is purple in color. Which seal material must be used with this fluid to prevent seal deterioration?
- ANeoprene seals, the same as used with petroleum-based MIL-H-5606
- BNatural rubber seals compatible with any hydraulic fluid
- CButyl or EPR (ethylene propylene rubber) seals compatible with Skydrol
- DNitrile seals compatible with petroleum-based fluids
Show answer & explanationAnswer & explanation
Correct answer: C. Butyl or EPR (ethylene propylene rubber) seals compatible with Skydrol
Skydrol and other phosphate ester fluids are incompatible with nitrile and neoprene seals, which they will swell and degrade; these systems require Butyl or EPR seals. Mixing seal types with the wrong fluid causes rapid seal failure and leaks.
Why the other options are wrong
- A. Neoprene is used with mineral-based fluid, not phosphate ester fluid.
- B. Natural rubber is not compatible with either major hydraulic fluid type.
- D. Nitrile seals swell and deteriorate in phosphate ester fluid.
Hydraulic Fluid/Seal Compatibility
Different hydraulic fluid types require specific seal elastomers to prevent chemical degradation.
- MIL-H-5606 (red, petroleum-based) uses neoprene/nitrile seals
- Skydrol/phosphate ester (purple) uses Butyl or EPR seals
- Never mix fluid types or seal materials
Memory trick: Purple Skydrol pairs with EPR, Red mineral oil pairs with neoprene.