The Artificial Horizon — Page 169, Lesson 206

Let’s start with the artificial horizon itself. This is a primary instrument — meaning it’s one of the six essential flight instruments you’ll rely on — and it gives you the aircraft’s attitude in both pitch and roll. In plain terms, attitude is the aircraft’s orientation relative to the earth: nose up or down, and banked left or right. The artificial horizon replaces the natural horizon when visibility is poor, so you can fly straight and level without looking outside. The display is built around two key parts. First, there’s a miniature aircraft shape, often called a ‘gull-wing’ because of its shape, painted or engraved centrally on the inside of the glass face. That’s fixed to the instrument case, and since the case is fixed to the aircraft, the gull-wing moves with the aircraft. Behind it is the horizon bar, which is linked to the gyro so that it stays gyro-stabilized parallel to the true horizon. So the gull-wing represents your aircraft, and the horizon bar represents the real horizon — and the difference between them shows your attitude. The artificial horizon can be suction-driven or electrically driven, and it goes by a few names: gyro horizon, attitude indicator, or simply AH. Now, the construction. The artificial horizon uses an earth gyro — that’s a gyroscope whose spin axis is maintained in, or tied to, the vertical by earth’s gravity. Because the spin axis is vertical, the plane of rotor rotation is horizontal. That gives you the stable lateral and longitudinal references you need — lateral being side-to-side, longitudinal being fore-and-aft. Let me walk you through the axes, because they’re central to understanding this. Figure 13.1 shows three axes: XX, YY, and ZZ. The spin axis is XX, and because the gyro is tied to the vertical, XX remains earth vertical when the aircraft is straight and level. That means YY — the lateral axis — is earth horizontal. So in level flight, XX points straight up and down, and YY points side-to-side, parallel to the ground. Now let’s look at pitch. In Figure 13.2, you see a nose-up attitude of 10 degrees. When the aircraft pitches up, the case rotates together with the attached outer gimbal ring about the lateral axis YY. As that happens, a guide pin protruding from the stabilized inner gimbal forces the horizon bar arm down. So the horizon bar moves below the gull-wing, producing the nose-up indication. The gimbal rings and the pitch-indication linkage are shown in greater detail in Figure 13.5. For pitch-down, the opposite happens — the horizon bar moves above the gull-wing. And you can select the angle of pitch using the pitch markers shown on the instrument face. Now roll. Here, the rigidity of the vertical gyro provides the stable attitude reference. As the aircraft rolls about the longitudinal axis — that’s 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. For a more accurate reading of the bank angle, there’s a pointer attached to the outer gimbal that shows bank angle on a scale painted on the face of the instrument. So to tie it together: the gull-wing is your aircraft, the horizon bar is the true horizon, and the gyro keeps that bar stable. Pitch moves the bar up or down relative to the gull-wing; roll rotates the gull-wing relative to the bar. That’s the artificial horizon.