Directional Gyro Indicator (DGI) — Page 30, Lesson 31

I want to walk you through the Directional Gyro Indicator, or DGI, and some important effects that affect its accuracy in flight. Let's start with how a change in aircraft latitude affects a compensated DGI. You already know that the apparent drift rate caused by the Earth's rotation depends on the sine of the latitude. Let me put that in plain terms: if you have a gyro that isn't corrected, it will appear to drift at a rate that changes depending on how far north or south you are from the equator. The formula is 15 × sin(latitude) degrees per hour. Now, imagine an aircraft tracking due north, starting from the equator. At the equator, the initial apparent drift rate of an uncorrected gyro is zero. As the flight progresses northward, the DGI reading decreases. By the time the aircraft reaches 30° north, the DGI reading is decreasing at a rate of 7½ degrees per hour. At 60° north, it's decreasing at 13 degrees per hour. If you continue all the way to the pole, the apparent drift rate reaches 15 × sin(latitude) degrees per hour — which at the pole, where sin 90° equals 1, gives you 15 degrees per hour. The key point is that when flying due north or south from the equator at a constant ground speed, the apparent drift rate increases from zero up to that full 15 degrees per hour at the pole. But the rate of increase of that drift rate is not constant — it changes depending on whether you are north or south of the latitude at which the DGI was compensated. Let's move on to errors caused by unstable rotor rpm. The rate of precession of a gyro depends on its rotor rpm. In a suction-driven DGI, you don't have precise control over that rpm. That means the latitude nut compensation — the mechanical correction applied to counteract apparent drift — is only approximate. Here's what happens in practice. At high altitude, if you have inadequate suction, the rotor rpm will be lower than the design value. That lower rpm reduces the gyroscopic rigidity of the rotor. Because the rotor is less rigid, the latitude nut produces too high a precession rate, which over-corrects the apparent drift. On the other hand, if the rpm exceeds the design figure — which is less likely to happen — the rigidity would increase, and the latitude nut would produce a lower rate of precession, so it would under-correct the apparent drift. Now let's talk about transport wander. At any latitude other than the equator, the meridians — the lines that define local north — are not parallel to each other. If you align your gyro to one meridian, then fly east to west, the new meridian at your destination will be inclined relative to the old one. That inclination is called transport wander. Let me give you a calculation example to tie this together. Suppose an aircraft is stationary at 60° north. We want to calculate the hourly wander rate for an uncompensated gyro. The solution is: apparent wander equals negative 15 times the sine of 60 degrees, and that's decreasing, so it's minus 12.99 degrees per hour. The negative sign tells you the reading is decreasing. Let me show you the diagrams that illustrate these concepts. So to summarise what we've covered: apparent drift varies with the sine of latitude, rotor rpm variations cause the latitude nut compensation to over-correct or under-correct, and transport wander occurs because meridians converge as you move east or west away from the equator.