The Turn and Slip Indicator — Page 192, Lesson 229

Let’s start with the big picture. In instrument flying, we want turns to be properly balanced — that means no side slip and no skid. A balanced turn happens when the angle of bank is exactly right for your true airspeed, or TAS, and for the rate of turn you’re making. Now, you could calculate that correct bank angle. There’s a handy rule of thumb: for a rate 1 turn, take one tenth of your TAS and add seven. So if your TAS is 150 knots, one tenth is 15, add 7, and you get 22 degrees of bank. That rule is reasonably accurate for rate 1 turns when TAS is between 100 and 250 knots. But here’s the thing — during instrument flight, the fewer calculations you have to do, the better. That’s exactly why we have the slip indicator. It gives you a direct indication of whether the turn is balanced, without any mental arithmetic. Let’s look at the construction. Early slip indicators used a simple metal pendulum suspended inside the instrument case, and its oscillations were controlled by a piston-in-cylinder damping device. The modern version is usually a ‘ball-in-tube inclinometer’. That’s a solid ball inside a curved tube that contains liquid, and that liquid damps out the unwanted oscillations. You can see it sketched in Figure 14.4 and Figure 14.6. The heavy ball behaves like a pendulum, and the centre of curvature of the tube acts as the effective point of suspension. Now the operating principle. Imagine the aircraft in level flight, with lift L balancing weight W. The weight W of the ball in the tube acts downwards, and it’s exactly balanced by the equal and opposite reaction of the base of the tube on the ball, which acts upwards toward the centre of curvature of the tube. So when the wings are level, the ball sits just between the two vertical lines etched on the tube — that’s the neutral, balanced position shown in Figure 14.4. So the key idea: the ball’s position tells you the state of balance of the turn. When it’s centred between the lines, the turn is balanced. When it moves off-centre, you’re either slipping or skidding — and we’ll get into exactly which direction means what next.