First, let's talk about control surface movement limits — Page 156, Lesson 206

I want to walk you through the mechanical limits and safety features of a flight control system. We're looking at the stops that define how far a control surface can travel, and then the systems that protect the aircraft when it's parked or about to take off. First, let's talk about control surface movement limits. A control surface like an elevator has a greater deflection upward than downward. The limit of that movement is determined by a mechanical stop. A stop which limits the movement of the control surface itself is called a primary stop. So the primary stop physically blocks the surface from travelling any further. Then there's a second type. A stop which limits the movement of the control column or the rudder pedals is called a secondary stop. Here's the key relationship: when the primary stop is closed — meaning the control surface has reached its limit — there will be a small clearance at the secondary stop. So the secondary stop is a backup that catches the cockpit controls, but it's designed so it doesn't quite touch when the surface is at its full travel. Now let's move to control system friction. The friction in the control system determines the force required to move the controls when the aircraft is stationary. In flight, the stick forces increase because of the air loads acting on the control surfaces. If the friction loads are too high, the feel of the controls with changing airspeed will be distorted — you won't get that accurate feedback of what the aircraft is doing. The friction is measured by attaching a spring balance to the control and moving it through its full travel. Excessive friction may be due to over-tensioned cables or a lack of oil on the bearings. Next, backlash. Control systems should be free of backlash. Backlash is free or ineffective movement of the cockpit control when the direction of movement is reversed. In other words, you move the stick one way, then reverse — and there's a little dead zone before the control actually responds. It may indicate worn or incorrect components in the control system. Now, control locks. When an aircraft is parked in the open, strong or gusty winds could blow the controls about against their stops with enough force to cause mechanical damage. To prevent this, control locks are fitted. They may be external or internal, and they may be fitted to the control surface or to the cockpit control. If they're fitted to the cockpit control, they may be arranged so that it's impossible to open the throttle until the control locks are removed — a nice interlock. There's an important note here about servo-operated control surfaces. With those, movement of the cockpit controls is possible even with external control locks in position. Similarly, with a spring tab assisted control, some movement of the cockpit control would be possible with external locks fitted, but the control would feel very stiff. Then we have duplicate inspection of controls. Because of the vital importance of the control system, a procedure for duplication of inspection is laid down in maintenance regulations. It requires that if the control system is disturbed in any way, the system shall be inspected separately by two qualified persons before the aircraft is permitted to fly. In some circumstances, the second of these persons may be the pilot. Duplicate inspection procedures are given at Appendix A. Finally, the take-off configuration warning. This is armed when the aircraft is on the ground and the forward thrust levers are advanced for take-off. An intermittent take-off warning sounds if some or all of the required configuration is not correct. Let me show you the elevator control system and the turnbuckle that's part of it. That's the full picture of these control system safeguards.