Which oil’s contamination is indicated by a CAD caution on the main transmission oil?

• A. Tail rotor oil
• B. Hydraulic oil
• C. Main Transmission oil
• D. Engine oil

Answer: (C)

Cross-system oil contamination is what this CAD caution is signaling. Seeing a CAD caution on the main transmission oil means something different from what the main lubricant should contain has mixed in. The contaminant in this case is hydraulic oil, because hydraulic fluid should remain in the hydraulic system. Its presence in the main transmission oil points to a leak path or seal failure allowing hydraulic fluid to enter the gearbox, which can alter lubrication properties and risk gear damage. Tail rotor oil would indicate contamination from the tail gearbox, engine oil would point to engine-to-transmission leakage, but the warning on the main transmission oil specifically flags hydraulic oil as the contaminant.