Hydraulic systems are among the most important technologies used in modern aircraft to operate various components that require high force and precise movement. Although hydraulic fluid may appear to be a simple substance, its ability to efficiently transmit pressure makes it an essential element in aircraft operation, particularly for large components that would be difficult to move using direct mechanical force. The basic principle of a hydraulic system is to use pressurized fluid to transfer force from its source to hydraulic actuators, which convert pressure into mechanical motion. When pumps force the fluid through pipes and valves at high pressure, it reaches hydraulic cylinders or motors, generating sufficient force to move large aircraft components smoothly and with precise control. This process is based on Pascal’s principle, which states that pressure applied to a confined fluid is transmitted equally in all directions. Hydraulic systems are used in aircraft for many important applications, including operating landing gear, moving large control surfaces, operating brakes, and controlling certain wing components and other aircraft mechanisms. Their importance lies in their ability to generate large forces using components of relatively suitable size and weight, making them highly suitable for many aircraft applications compared with conventional mechanical systems. Hydraulic systems also provide rapid response and precise control of movement, which are essential during takeoff, landing, and various flight maneuvers. To ensure safety and reliability, these systems are generally designed with multiple levels of protection and redundancy. Large aircraft may have more than one independent hydraulic circuit to reduce the impact of a potential failure in any part of the system. The hydraulic fluid itself is also an important part of the system. It must have suitable properties for operation under different flight conditions, including resistance to temperature variations, chemical stability, and the ability to transmit pressure efficiently. Therefore, hydraulic fluids and their components are subject to strict requirements to ensure continuous performance and reliability over long operating periods. With the development of the aviation industry, some modern aircraft have begun to use advanced electrical systems alongside hydraulic systems, or to replace certain hydraulic applications with electromechanical systems. Nevertheless, hydraulics continues to play an important role in aircraft because of its high force capability and reliability in applications that require the movement of large loads. The use of fluids in aircraft is not simply related to the material itself, but to its ability to transfer force efficiently, rapidly, and accurately. For this reason, hydraulic systems have become an essential part of aircraft engineering, helping make the control of heavy and complex components safer and more effective. They demonstrate how a simple physical principle can play a crucial role in modern aviation technology. Assist. Lecturer Al-Boraq Hayder Mohsin