Final answer:
Before using the hydraulic selector switch, the engine speed must be at idle to prevent potential damage to the hydraulic system. This is a standard procedure in aviation and other industries that employ hydraulic systems in their machinery.
Step-by-step explanation:
Before using the hydraulic selector switch, the engine speed must be at idle. Hydraulic systems are used in various types of machinery, including aircraft, to operate components like landing gear, flaps, and brakes. It is critical that the engine speed is at idle to ensure the hydraulic system is not subject to excessive flow rate or pressure that could occur at higher engine speeds, potentially leading to malfunctions or damage to the system.
In aviation, for example, pilots are often instructed to ensure that the engine RPM is set to idle before engaging the hydraulic systems to avoid abrupt and potentially damaging movements of aerodynamic surfaces or landing gear. It's essential to follow the specific manufacturer's guidelines or the procedures outlined in the aircraft's flight manual when operating hydraulic systems.