What type of hydraulic fluid is approved for use in the Challenger 300/350 aircraft?

The approved hydraulic fluid for the Challenger 300/350 aircraft is Skydrol LD-4. This specific fluid is designed for use in aviation systems because it offers excellent performance characteristics, including a wide temperature operating range and good fire resistance. Skydrol LD-4 is a phosphate ester-based fluid, which plays a critical role in the operational safety and efficiency of hydraulic systems in aircraft.

Other options do not meet the stringent requirements for aviation hydraulic systems. Mineral oil and conventional brake fluid are not suitable due to their lower thermal stability and compatibility issues with components that are routinely used in aircraft systems. Jet A fuel is entirely inappropriate in this context, as it is used for propulsion rather than hydraulic applications. Thus, Skydrol LD-4 is the clear choice for ensuring the safety and reliable operation of hydraulic systems in the Challenger 300/350.