What is the minimum hydraulic reservoir temperature for engine start?

The correct answer is based on the operational guidelines for the hydraulic system in the aircraft. The minimum hydraulic reservoir temperature for engine start is set to ensure that the hydraulic fluid remains within optimal viscosity limits during colder temperatures. If the temperature is too low, the hydraulic fluid may become too viscous, potentially leading to difficulties in starting the engine or affecting the performance of hydraulic components.

A minimum temperature of -18°C has been established because, at this temperature, the hydraulic fluid can still flow properly and perform its necessary functions without risking damage to the hydraulic pumps or the systems they serve. This ensures that the hydraulic pressure can build adequately during engine start, allowing for effective operation of control surfaces and systems reliant on hydraulic power.

Other temperature options suggested are not recommended because they would put the system at risk of having hydraulic fluid that is too thick to function optimally. Maintaining the hydraulic reservoir temperature above -18°C is critical for ensuring reliability and safety during the critical phase of engine start.