What is the difference between Skydrol and MIL-H-5606 in terms of fire resistance and handling?

• A. Skydrol and MIL-H-5606 are identical.
• B. Skydrol is phosphate ester with higher fire resistance but more corrosive; MIL-H-5606 is mineral-based with lower fire resistance but easier handling.
• C. MIL-H-5606 is more corrosive and harder to handle.
• D. Skydrol is mineral-based and easier to handle.

Answer: (B)

The key idea is the fluid chemistry and how it affects fire risk and material compatibility. Skydrol is a phosphate ester hydraulic fluid, which gives it a higher flash point and better fire resistance than many mineral-based fluids. That means leaks or spills are less likely to ignite in a fire situation.

But that same phosphate ester is highly corrosive to many materials used in aircraft systems—certain metals, paints, elastomers, and seals—so it can cause more wear on components and require more careful material compatibility, specialized cleaning, and protective handling.

MIL-H-5606, being mineral-based, is generally easier on metals and seals and easier to clean up, contributing to simpler handling and maintenance. However, its fire resistance is lower than Skydrol, so it poses a higher fire risk in a leak or spill.

So, Skydrol offers higher fire resistance at the cost of more aggressive handling and material compatibility requirements, while MIL-H-5606 is easier to handle but less fire-resistant.