FAA Aviation Mechanic Airframe (AMA)Hydraulic and Pneumatic SystemsEasy
A technician checks the sight gauge on a hydraulic reservoir and observes fluid that is a deep purple color rather than the reddish tint expected. This coloring most likely indicates the system is serviced with which type of hydraulic fluid?
- AVegetable-base fluid
- BPetroleum/mineral-base MIL-PRF-5606 fluid
- CSynthetic hydrocarbon-base fluid
- DPhosphate ester-base (fire-resistant) fluid
Show answer & explanationAnswer & explanation
Correct answer: D. Phosphate ester-base (fire-resistant) fluid
Fire-resistant phosphate ester-base hydraulic fluids (such as Skydrol) are dyed purple/violet, while mineral-base MIL-PRF-5606 fluid is red. Mixing these fluid types damages seals because they use incompatible seal materials, so color observation is an important quick check before servicing.
Why the other options are wrong
- A. Vegetable-base hydraulic fluids are essentially obsolete in modern aircraft and not commonly color-coded purple.
- B. MIL-PRF-5606 mineral-base fluid is dyed red, not purple.
- C. Synthetic hydrocarbon fluids are not the standard purple-dyed phosphate ester type.
Hydraulic Fluid Color Coding
Aircraft hydraulic fluids are dyed distinct colors to help identify fluid type and prevent accidental mixing of incompatible fluids.
- MIL-PRF-5606 mineral-base fluid is red
- Phosphate ester fire-resistant fluid (e.g., Skydrol) is purple
- Never mix fluid types—incompatible seals and chemistry
Memory trick: Purple = phosphate; Red = regular (mineral) fluid