Directional Gyro Indicator (DGI) — Page 153, Lesson 184

I want to walk you through the Directional Gyro Indicator — the DGI — which is often just called the Direction Indicator, or DI. This is the instrument that gives you a stable heading reference in azimuth, meaning in the horizontal plane, so you can hold accurate headings and fly precise turns. The first thing to understand is what the DI is not. There is no magnetic element in it. So it is not north-seeking. It cannot find north on its own. That means you must initially synchronize it with the magnetic compass — you set it to match the compass — and then you must check that synchronization at regular intervals, because the gyro wanders. We call that drift, and it comes in two forms: real wander and apparent wander. So the DGI does not replace the compass. Its job is complementary. It gives you a stable, dead-beat indication — dead-beat meaning the needle doesn't oscillate or swing around — and that stability complements the compass's ability to seek north. And here's a key advantage: because the DGI has no magnetic element, it does not suffer from the compass turning errors and acceleration errors that come from the vertical component of the earth's magnetic field. Those are the errors that make a magnetic compass unreliable during turns and speed changes. The DI is immune to them. Now let's get into the principle and construction. The DI employs what we call a tied gyro. Let me define that precisely: a tied gyro is a gyro having freedom of movement in three planes mutually at right angles, but with the rotor axis maintained in the yawing plane of the aircraft. In plain terms — the rotor is free to move in all three dimensions, but its spin axis is held in the yawing plane. That means in level flight the rotor axis is horizontal. And because of gyroscopic rigidity — the gyro's resistance to being disturbed — that horizontal axis provides the datum, the reference line, from which heading is measured. Let's look at the physical construction. The rotor is mounted in the inner gimbal, and that inner gimbal is mounted on bearings inside the outer gimbal. The inner gimbal has restricted freedom to turn. The outer gimbal, on the other hand, can rotate through 360 degrees about the aircraft's vertical axis, on bearings in the case. So the outer gimbal spins freely around the vertical, and that's what lets the instrument read any heading around the compass rose. I should mention one thing that ties into this construction — gimbal lock. For most unsophisticated aircraft, gimbal lock is a problem and it results in toppling. That's when the gyro's axes align in a way that it loses its freedom and the instrument tumbles. It's a real operational limitation you'll need to be aware of when flying. So to summarize where we are: the DI is a tied gyro, horizontal in level flight, giving you a rigid datum for heading. It's not north-seeking, so you sync it to the compass and recheck it regularly because of wander. And it's free of the magnetic compass's turning and acceleration errors. That's the foundation of the DGI.