Flight Control Systems — Page 156, Lesson 202

I want to walk you through the heart of how a manually operated flight control system behaves, because this sets the foundation for everything else in the chapter. Let's start with the most important word here: reversible. In a manually operated control system, the primary flying controls are reversible. What does that mean? It means the relationship between the cockpit control and the control surface works in both directions. If you apply a force to the cockpit control — say you push the control column forward — that force moves the control surface. But it also works the other way: if a force is applied to the control surface, that force will cause the cockpit control to move. The practical consequence is that the air pressure acting on the control surfaces is felt by the pilot through the cockpit controls. That's your natural feel — you literally sense the aerodynamic load through your hands. Now contrast that with a fully power operated control system. That system is irreversible. If you apply a load to the control surface, it cannot move the cockpit control. The system has no natural feel, because the hydraulic or electric power does the work and the surface doesn't push back through the linkage. So here's the problem: because a power operated system is irreversible and has no natural feel, we have to introduce feel artificially. That's the job of the artificial feel unit. And it has a very specific requirement: it should increase the cockpit control load in proportion to two things — the control deflection, and the speed. So the further you deflect the control, the heavier it gets, and the faster you're flying, the heavier it gets. That gives the pilot a sense of how much control authority they're using. There's a bracketed note in the text about a manually operated trimming tab, and it's a subtle but important point. A manually operated trimming tab is irreversible. Once its position has been set by the trim wheel, it cannot be moved from that position by a load on the trimming tab. So even in a reversible system, the trim tab itself locks in place once you set it. And there's a note about power assisted controls. These are different from fully power operated. Power assisted controls still retain their natural feel, and if the loads at the surface are large enough, they are reversible. So power assisted is a middle ground — you get assistance, but you keep the feel. Now let's move to the practical side: control system checks. During servicing, and after any adjustments to the flying control system, various checks are required. In some situations the pilot may have to perform part of these checks. The main checks are for five things. First, cable tension. Second, safety and locking of controls. Third, range of movement of controls — that means freedom of movement and operation in the correct sense. Fourth, friction in the system. And fifth, backlash of the system. Let's focus on cable tension, because it's the first and it's critical. It's important to have the correct tension in the control cables. If the tension is too low, the cables will be loose, and that permits excessive cable movement. If the tension is too high, the controls will be too stiff to move. So you need a sweet spot. Cable tension is adjusted by means of turnbuckles, and measured with a tensiometer. Let me show you the turnbuckle. A typical turnbuckle consists of a central barrel, and two end fittings, to which are attached the ends of the cable. So you turn the barrel to draw the two end fittings together or apart, which tightens or loosens the cable. And to measure the tension, you use a tensiometer — a simple illustration of one is shown in Figure 7.4. So to tie it together: the reversibility of a manual system gives you natural feel; a power operated system loses that and needs an artificial feel unit that scales with deflection and speed; and when you're maintaining the system, the checks focus on cable tension, locking, range of movement, friction, and backlash — with tension set by turnbuckles and verified by a tensiometer.