Flight Control Systems — Page 156, Lesson 204

Let’s pick up right where we left off — we’ve set the cable tension, and now I want to walk you through what happens next, because tension isn’t a set-and-forget thing. First, temperature compensation. When you check the tension of a control cable, you have to make allowance for the temperature, and use the correct tension figure for the ambient temperature. Here’s why: temperature changes affect the length of the cables and the length of the airframe structure, but they’re made of different materials, so they expand at different rates. For a normal aluminium alloy airframe with steel control cables, an increase in temperature causes the aluminium alloy to expand more than the steel cables. That means the structure grows longer than the cable does, and the cable gets pulled tighter — so tension increases with temperature. On some aircraft, a temperature compensator is fitted in the control system, and it automatically maintains the correct tension when temperature changes. So you don’t have to keep adjusting it manually. Now, safety and locking. Once the tension is correctly set, the turnbuckle must be checked to safety. That means enough thread must be engaged between the end fittings and the central barrel of the turnbuckle to take the load that will be placed on the cable. To verify this, inspection holes are provided in the turnbuckle. To be “in safety,” the inspection hole must be completely blocked by the thread of the end fitting — and you verify that by attempting to pass a hardened pin through the inspection hole. If the pin won’t go through, the thread is covering the hole, and you’re in safety. Some turnbuckles don’t have inspection holes, and for those you check safety by seeing that not more than three threads of the end fitting are visible outside the barrel. Once the tension is set and the turnbuckle is in safety, it must be locked to prevent any change of tension during operation. The reason is vibration — vibration could cause the barrel of the turnbuckle to rotate, which would allow the cable tension to decrease. So the turnbuckle must be locked to prevent any rotation of the barrel relative to the end fittings. The most common method is locking wire, but many other approved systems are in use, like locking clips and locking plates. Finally, range of control movement. The movement of each control surface to either side of its neutral position is laid down so it can achieve the required control over the full range of operating conditions. And note this: the movement is not necessarily the same on each side of neutral. For example, an elevator usually has a greater deflection upward than downward. The limit of movement of the control surface is determined by a mechanical stop — and that’s where we’ll go next.