Final answer:
Quick disconnect fluid lines are used in aircraft hydraulic systems where maintenance requires regular disassembly or flexibility, such as on landing gear or systems prone to wear and need regular inspection.
Step-by-step explanation:
Quick disconnect fluid lines are typically used in aircraft (a/c) hydraulic systems in areas where regular disassembly is necessary for maintenance or where flexibility is needed for system operation. For example, quick disconnects can often be found on landing gear systems, where they allow for easier removal and servicing of components. Additionally, in engine systems and other areas susceptible to frequent wear and tear or systems that require routine inspections and testing, quick disconnects provide a convenient means of disconnecting and reconnecting hydraulic lines without the need for tools and without leakage of hydraulic fluids.