Flight Controls — Page 186, Lesson 244

We're starting a brand-new chapter now — Chapter 9, Powered Flying Controls. This is where the flight control system stops being purely mechanical and starts using hydraulic and electric power to move the surfaces. Let me set the scene for you. First, the introduction. On some modern aircraft, the flying controls are subjected to heavy loads. That happens for two distinct reasons: either because you're moving large control surfaces, or because you're operating the controls at high speeds. Think about it — a big airliner's aileron or elevator is a huge surface, and at cruise speed the air loads on it are enormous. The maximum control loads that the system must be designed to handle are specified in a regulation called CS-25. That's the Certification Specification for Large Aeroplanes, and it's the standard that sets the legal limit for how much load the controls have to withstand. Now, because those air loads are so heavy, the stick forces — the physical effort you'd need to push the control column — would be far too great for a pilot to handle directly. So to reduce those stick forces, we use hydraulic or electric power. Here's the key point: the majority of powered flying controls are hydraulically operated. And depending on the degree of assistance required, the system will be either powered or power assisted. I'll come back to that distinction in a moment, but for now, hold onto the idea that there are two levels of assistance. Let's move into the core of the chapter: Power Operated Controls. I want to walk you through the essential components of a simple power operated control system. There are three of them, and you need to know all three by name. First, a hydraulic actuator. That's the muscle — the hydraulic cylinder that actually moves the control surface. It converts hydraulic pressure into mechanical movement. Second, a servo or control valve. This is the brain of the hydraulic circuit. It's the valve that directs hydraulic fluid to one side of the actuator or the other, depending on what the pilot has commanded. The pilot moves the control column, that movement positions the servo valve, and the valve then ports fluid to the actuator to move the surface. Third, an artificial feel unit. This is critical, and I want you to understand why it exists. In a purely mechanical system, the pilot feels the air loads directly through the control column — the stick force tells you how hard the air is pushing on the surface. But in a powered system, the hydraulic power does all the work, so the pilot would feel nothing. That's dangerous — you'd have no sense of how much load the surface is under. So the artificial feel unit generates a force that simulates those air loads, giving the pilot a realistic feel in the controls. Now, here's the crucial part. These three components must also incorporate two additional features. First, some form of control 'follow up' or 'feed back'. That's the mechanism that ensures the control surface movement is proportional to the amount of selection made. In other words, if you move the control column halfway, the surface must move halfway — not all the way, not a quarter. The follow-up feeds back the actual surface position so the system knows when to stop. Without it, the surface would just run to its full travel every time you touched the controls. Second, some form of feel which is proportional to the air loads on the control surfaces. Notice the distinction here: the artificial feel unit gives you a feel, but that feel must be proportional to the actual air loads. So as the airspeed increases and the loads on the surface grow, the feel in the controls must grow correspondingly. That's what makes the aircraft controllable and gives you the sensory feedback you need as a pilot. So let me tie this together. The three essential components are the hydraulic actuator, the servo or control valve, and the artificial feel unit. And the system as a whole must have follow-up or feedback for proportional control, plus feel proportional to air loads. That's the fundamental architecture of a power operated control system. Now, about that powered versus power assisted distinction I mentioned. I want to be careful here because the excerpt introduces it but doesn't fully expand it yet — that comes later in the chapter. But the principle is this: a powered control system is one where the pilot's input is essentially a signal, and the hydraulic power does all the work of moving the surface. A power assisted system is one where the pilot's physical effort is supplemented by hydraulic power — the pilot still contributes some force, and the hydraulics multiply it. The degree of assistance determines which one you have. We'll develop that as we go through the chapter. Let me also point you to the figure on screen — Figure 9.1, System requirements. That's the diagram that shows how these components fit together in the system. And you'll notice the chapter outline ahead of us: we're going to cover Power Operated Controls, then Artificial Feel Units, then the Artificial Feel System, Feel Trim System, Fly by Wire systems, their advantages and disadvantages, Redundancy, and Protection against Jamming of Controls. So this introduction is laying the foundation for all of that. One more thing before we move on — the control position indicators from the previous chapter. On modern jet transports, the position of the controls is shown on the electronic systems displays. But some older aeroplanes still use what are called "baby aeroplane" mechanical indicators — that's the nickname for the little miniature control surface symbols that move on a mechanical gauge, like a tiny model aeroplane showing you where the surfaces are. That's just context for where we've come from; now we're getting into how those surfaces are actually powered. So, to summarise where we are: heavy air loads on large surfaces or at high speeds create excessive stick forces. CS-25 sets the maximum control loads. We reduce those forces with hydraulic or electric power, mostly hydraulic. The system needs three components — the hydraulic actuator, the servo or control valve, and the artificial feel unit — plus follow-up for proportional movement and feel proportional to air loads. And the system is either powered or power assisted depending on the degree of assistance needed. That's the foundation. Let's keep going into the details of how these components actually work together.