The Turn and Slip Indicator — Page 192, Lesson 229

I want to walk you through the Turn and Slip Indicator, and we're going to start with the Slip Indicator itself. The whole point of this instrument is to tell you whether your turn is properly balanced — meaning there's no side slip and no skid. A balanced turn implies that your angle of bank is correct for your true airspeed, the TAS, and for the rate of turn you're making. Now, you could calculate the correct bank angle precisely, or you could use a rule of thumb. Here's the one they give us: for a rate 1 turn, take one tenth of the TAS and add seven. That gives you approximately the required bank angle. So for a rate 1 turn at a TAS of 150 knots, you take 15 and add 7, giving you 22 degrees of bank. This rule gives reasonable accuracy for rate 1 turns with a TAS between 100 and 250 knots. But here's the thing — during instrument flight, the fewer calculations you have to make, the better. That's exactly why the slip indicator exists. It gives you a direct indication of the state of balance of the turn, so you don't have to do the math in your head. Let's look at the construction. Early types of slip indicator used a simple metal pendulum suspended in the instrument case, with its oscillations controlled by a piston-in-cylinder damping device. The modern version is usually a ball-in-tube inclinometer. That's the exact name — a ball-in-tube inclinometer. It comprises a solid ball in a curved tube containing liquid, and that liquid damps out the unwanted oscillations. The heavy ball behaves like a pendulum, with the centre of curvature of the tube acting as the effective point of suspension. So even though it's a ball in a tube, think of it as a pendulum whose pivot is the centre of that tube's curvature. Now the operating principles. Consider 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, acting upwards towards the centre of curvature of the tube. If the wings are level, the ball will lie just between the two vertical lines etched on the tube. Those etched lines are your reference marks — when the ball sits between them, you know the wings are level and the turn is balanced. Let me show you what this looks like. That's the ball-in-tube in level flight. And here's the aircraft in level flight with lift balancing weight. So the key idea to hold onto: the ball is your balance indicator. When it's centred between the lines, your turn is coordinated. When it's not, you're either slipping or skidding — and we'll get into exactly how that shows up on the ball in a moment.