What Is a Hydro-Mechanical Flight Control System?

Flight control systems are often categorized according to how they operate. Electrical flight control systems, for instance, operate electrically. They feature an electronic interface, such as a computer, that pilots use to control the aircraft. Also known as fly-by-wire (FBW) systems, electrical flight control systems are commonly found in commercial aircraft.

Mechanical flight control systems, on the other hand, operate mechanically. They are considered the most basic type of flight control systems. Mechanical flight control systems involve manual operation. The pilot may have to engage mechanical components like pushrods, cables or pulleys to control the aircraft. In addition to electrical; and mechanical flight control systems, however, there are hydro-mechanical flight control systems.

Overview of Hydro-Mechanical Flight Control System

A hydro-mechanical flight control system is a type of flight control system that operates both mechanically and hydraulically. Like all flight control systems, hydro-mechanical flight control systems are used to control various parts of the aircraft. While other types of flight control systems typically rely on a single method of operation, though, hydro-mechanical flight control systems operate mechanically and hydraulically, hence their name.

The 2 Elements of a Hydro-Mechanical Flight Control System

Hydro-mechanical flight control systems are characterized by their use of hydraulic and mechanical elements. They leverage these two types of control elements to provide pilots with a more intuitive way to control the aircraft.

Mechanical elements consist of pushrods, cables, pulleys and similar components. While they vary in design, mechanical elements such as these typically involve a direct connection between the pilot and the part of the aircraft he or she is controlling. The pilot may engage a pushrod, for example, to control the ailerons or rudders.

Hydraulic elements also require some type of input from the pilot, but they are different from mechanical elements by their use of pressurized fluid. All hydraulic elements contain or at least use pressurized fluid. A pump will typically pressurize the fluid, which in turn, makes the aircraft easier to control. Hydro-mechanical flight control systems combine the properties of mechanical and hydraulic elements.

How Hydro-Mechanical Flight Control Systems Work

Hydro-mechanical flight control systems still feature mechanical elements, but these components are connected to a hydraulic valve that provides pressurized fluid. A pilot may move the yoke, for example, resulting in pressurized fluid flowing through a line.

The pressurized fluid will make the yoke easier for the pilot to move. It’s the equivalent of power steering in an automobile. Many modern-day automobiles are equipped with a power steering system. As pressurized fluid flows through the steering rack, drivers can turn the steering wheel more easily. This same principle applies to hydro-mechanical flight control systems in airplanes.

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