The Turn Co-ordinator — Page 200, Lesson 236

Let’s start with the Turn Co-ordinator, because it’s a really neat piece of kit and it builds directly on something you already know — the Turn and Bank indicator. I want you to think of the Turn Co-ordinator as an interesting development of that older instrument. The primary difference between the two is in two things: the setting of the precession axis of the rate gyroscope, and the method of display. Now, let’s unpack that. Inside the Turn Co-ordinator we have a rate gyroscope — that’s a gyroscope that measures how fast the aircraft is turning. But here’s the key: this gyroscope is spring restrained, and it’s mounted so that its axis is at about 30 degrees with respect to the aircraft’s longitudinal axis. The longitudinal axis is the line running nose to tail through the aircraft. By tilting the gyro’s axis to that 30-degree angle, the instrument becomes sensitive not just to turning, but also to banking of the aircraft. That’s the clever bit — it senses both. Here’s how it works in practice. A turn is normally initiated by banking the aircraft. When you bank, the gyroscope will precess — that’s the gyroscopic effect where the applied force produces motion at 90 degrees to the force. This precession moves the aircraft symbol on the instrument face, and it indicates the direction of bank. That lets you, the pilot, anticipate the resulting turn before it fully develops. So you’re not just reacting to a turn that’s already happening — you’re seeing the bank that will produce it. Once you’re in the turn, you control it at the required rate by aligning the aircraft symbol with the graduations on the instrument dial. The graduations are the marked divisions on the face. The rate of turn will depend on the instrument in use — it could be a rate one turn, which is 3 degrees per second, or any other rate depending on the design of the instrument. So the dial is telling you how fast you’re turning, and you fly the symbol to the graduation that matches the rate you want. Now, there’s one more thing you must always keep in mind: the ball still has to remain central for a balanced rate of turn. That ball is the inclinometer — the same one you’d see on the Turn and Bank indicator. If the ball is off-centre, you’re in a skid or a slip, and the turn isn’t balanced. So the Turn Co-ordinator gives you the rate of turn, but the ball tells you about coordination. One last point, and it’s an important one for avoiding confusion. The indicator scale carries the annotation “No Pitch Information.” Why? Because the presentation of the Turn Co-ordinator looks similar to the gyro horizon — the artificial horizon instrument. If you saw that aircraft symbol and those graduations, you might be tempted to use it for pitch control, like you would with the gyro horizon. But this instrument gives you no pitch information at all. That annotation is there specifically to stop you from making that mistake. So, to pull it together: the Turn Co-ordinator is a spring-restrained rate gyroscope mounted at about 30 degrees to the longitudinal axis, making it sensitive to both bank and turn. It shows you the direction of bank so you can anticipate the turn, you fly the symbol to the graduations to set your rate of turn — rate one being 3 degrees per second — you keep the ball central for a balanced turn, and you remember it gives no pitch information.