The Artificial Horizon — Page 169, Lesson 206

I want to walk you through the artificial horizon now — the instrument that gives you your attitude in both pitch and roll. This is a primary instrument, which means it’s one of the six essential flight instruments, and its job is to replace the natural horizon when you can’t see it, like in poor visibility or cloud. Let me start with the basics. The artificial horizon, or AH, shows you the aircraft’s attitude — that’s your pitch, nose up or down, and your roll, bank left or right. It’s also known by two other names: the gyro horizon and the attitude indicator. And it can be driven either by suction or electrically — those are the two common power sources you’ll see. Now, the display itself. The attitude display is made up of two key parts. First, there’s a miniature aircraft shape, often called a gull-wing because of its shape, and it’s a tail view. This is painted or engraved centrally on the inside of the glass face of the instrument. Because it’s on the glass, it’s fixed to the instrument case, and therefore fixed to the actual aircraft — it moves with you. Behind this miniature aircraft is the horizon bar. The horizon bar is linked to the gyro in such a way that it’s gyro-stabilized parallel to the true horizon. So the gull-wing moves with the aircraft, and the horizon bar stays level with the real horizon. Let me explain the construction. The artificial horizon uses what’s called an earth gyro. In an earth gyro, the spin axis is maintained in, or tied to, the vertical by earth’s gravity. That means the plane of the rotor rotation is horizontal. So the rotor spins in a horizontal plane, and that provides the stable lateral and longitudinal references you need — lateral being side to side, longitudinal being fore and aft. Let me bring in Figure 13.1, which shows the three axes of the gyro: XX, YY, and ZZ. Because the gyro is tied to the vertical, the axis XX — that’s the spin axis — will remain earth vertical. And therefore the axis YY will be earth horizontal when the aircraft is straight and level. So in level flight, XX points straight up and down, and YY is horizontal. Now let’s look at pitch. In Figure 13.2, we have a nose-up attitude of 10 degrees. Here’s what happens mechanically. When you pitch up, the case rotates together with the attached outer gimbal ring about the lateral axis YY. As this rotation occurs, a guide pin protruding from the stabilized inner gimbal forces the horizon bar arm down. So the horizon bar ends up below the gull-wing, and that produces the nose-up indication. The gull-wing stays put, and the bar drops below it — that’s your nose-up picture. For pitch-down, it’s the reverse situation, and the three views in Figure 13.3 relate to that. One important detail: the angle of pitch may be selected using the pitch markers shown on the instrument. So you can read your pitch angle off those markers. Now for roll. In roll, just as in pitch, the rigidity of the vertical gyro provides the stable attitude reference. When the aircraft rolls — that’s rotation about the longitudinal axis, which is ZZ in the diagrams — the instrument case and the gull-wing rotate about the stabilized gyro rotor and gimbal system. The gyroscopic rigidity of the spinning rotor holds the horizon bar in the rolling plane. So the amount and direction of bank are displayed by the gull-wing relative to the horizon bar. The gull-wing tilts with the aircraft, and the bar stays level. There’s also a more accurate indication of the amount of bank. That comes from a pointer attached to the outer gimbal, and it shows the bank angle on a scale painted on the face of the instrument. So you have the gull-wing relative to the bar for a quick picture, and the pointer on the scale for a precise bank angle reading. Let me just tie the whole thing together. The key principle here is gyroscopic rigidity — the spinning rotor resists changes to its axis, so it holds the horizon bar stable while the case and gull-wing move with the aircraft. That’s how you get a reliable attitude reference even when you can’t see the real horizon.