FAA Aviation Mechanic General (AMG)Ground Operations and ServicingMedium
A mechanic is servicing the hydraulic reservoir on a transport aircraft that uses a phosphate ester-based fluid (Skydrol). What is the primary hazard of inadvertently adding a mineral-based hydraulic fluid (MIL-PRF-5606) to this system?
- AThe mixed fluid will turn from purple to red with no effect on system components
- BThe mixed fluid will become more fire-resistant than either fluid alone
- CThe fluid viscosity will decrease, improving low-temperature flow
- DThe seals designed for phosphate ester fluid will swell and deteriorate when exposed to mineral-based fluid
Show answer & explanationAnswer & explanation
Correct answer: D. The seals designed for phosphate ester fluid will swell and deteriorate when exposed to mineral-based fluid
Skydrol systems use seals compatible with phosphate ester fluid; introducing mineral-based fluid causes seal swelling and deterioration, leading to leaks and system failure. The two fluid types are chemically incompatible.
Why the other options are wrong
- A. A color change would occur, but seal damage—not just color—is the real hazard.
- B. Mixing incompatible fluids does not improve fire resistance; it degrades system components.
- C. Viscosity change is not the primary concern; seal damage is the critical hazard.
Hydraulic Fluid Compatibility
Aircraft hydraulic systems require specific fluid types; mixing incompatible types damages seals and can cause system failure.
- MIL-PRF-5606 is mineral-based (red)
- Skydrol is phosphate ester-based (purple), fire-resistant
- Mixing incompatible fluids swells/deteriorates seals
Memory trick: Purple Skydrol hates rubber not made for it.