MIL-H-5606 refers to which type of hydraulic fluid commonly used in aircraft, sometimes called 'red oil'?

• A. Phosphate-ester-based synthetic
• B. Mineral-base hydraulic fluid
• C. Water glycol
• D. Fully synthetic silicone oil

Answer: (B)

MIL-H-5606 denotes a mineral-oil based hydraulic fluid used in aircraft systems, and it’s the fluid commonly referred to as red oil. This specification identifies the fluid type by its hydrocarbon base, its lubricating properties, and its material compatibility, which is why it’s distinct from other families.

Phosphate-ester fluids (often called fire-resistant fluids) are a different category and are typically purple; they have different chemical properties and compatibility, not described by MIL-H-5606. Water-glycol fluids are another distinct type that isn’t the MIL-H-5606 family, prone to different issues like water carryover and freezing. Fully synthetic silicone oil is also a separate fluid type with its own characteristics and uses. So the mineral-base fluid described by MIL-H-5606 is the best match for “red oil.”