Which condition will illuminate the fault light on the IDG DISC switchlight?

• A. CSD high oil temperature or low oil pressure
• B. CSD low oil temperature or high oil pressure
• C. Battery voltage fault
• D. Cabin pressurization fault

Answer: (A)

The light is designed to warn about lubrication problems in the Constant Speed Drive that powers the IDG. When the CSD oil gets too hot or the oil pressure drops too low, the drive can overheat or fail to be adequately lubricated, risking damage to the IDG and the electrical system it supports. Illuminating the DISC switchlight tells the crew to disengage the CSD/IDG to protect the gear and prevent further issues. Other conditions listed don’t indicate a CSD lubrication fault: a battery voltage fault and cabin pressurization fault involve different systems, and a low or high oil temperature in isolation isn’t the specific fault mode that triggers this light.