Gridded Charts — Page 471, Lesson 465

Let’s pick this up right where the magnetic compass gives out. I want you to picture flying at very high latitude, near the pole. The magnetic compass becomes useless there — the horizontal component of the Earth’s magnetic field is too weak, and in some regions the rate of magnetic variation changes too fast to be usable. So we stop steering by magnetic reference and we steer by gyro instead. Now, a gyro is not perfect. It has three errors we must know cold: Real Drift, also called Random Wander; Earth Rate; and Transport Wander. Let me take each one. Real Drift. In an INS-quality gyro, Real Drift is on the order of one-hundredth of a degree per hour. That’s tiny. But here’s the practical point: if an INS is fitted, you normally wouldn’t need Grid at all, because most INS are polar-programmed and can keep operating in True up to very high latitudes. However, if no INS is fitted, on a typical modern airline-quality Slaved Gyro Compass, the drift rate is still only about one-tenth of a degree per hour when operating in DG mode — that’s Directional Gyro mode. At that level, Real Drift is low enough to be insignificant. Earth Rate. This one is a bit more subtle. The gyro is aligned with True North at some particular meridian. The aircraft then rotates with the Earth, along with that meridian — but the gyro stays aligned to the point in space where it was originally aligned. So the gyro appears to drift relative to True North. The correction factor is 15 × sine latitude, in degrees per hour. Many modern Slaved Gyro Compasses have a Latitude Correction control — a knob the crew turns to set the local latitude, and the correction is then fed to the gyro. The crew keeps resetting the latitude as it changes. Transport Wander. This is just another name for meridian convergence. Again, the gyro is aligned with True North at some particular meridian. Then, as the aircraft travels either east or west, the gyro stays aligned to the original direction in space — but the direction of the meridians, and therefore the local direction of True North, changes beneath you. That mismatch is Transport Wander. So the picture is: at high latitudes the magnetic compass fails, and we switch to gyro steering. The gyro has three errors — Real Drift, Earth Rate, and Transport Wander — and we correct or accept each one. That sets us up for the Grid system itself, which is how we actually steer by that gyro in polar regions.