The Artificial Horizon Introduction . . . . . . . . . . . . . . . . . . . .… — Page 169, Lesson 204

We're starting a brand-new topic: the Artificial Horizon. This is one of the most important flight instruments you'll ever use, so let's get into it properly. The artificial horizon is your primary reference for the aircraft's attitude — its pitch and bank relative to the real horizon. When you're in cloud or at night, you can't see the actual horizon, so this instrument gives you a synthetic one. It's a gyroscopic instrument, and the whole chapter is built around its construction, its indications, its limitations, and the two main ways it's powered. Let me walk you through the structure of what we're covering. We start with the introduction and construction, then we look at the indications it gives you. After that, we get into the limitations — and this is critical, because every gyro instrument has errors you must understand. Then we cover the control systems, and we split into the two types: the air-driven artificial horizon and the electric artificial horizon. For each, we look at specific errors — acceleration errors and turning errors — plus rigidity, serviceability checks, the fast erection system, the adjustable aeroplane datum, and finally the vertical gyro unit. Let's begin with the construction. The artificial horizon uses a gyroscope that's mounted so its spin axis is vertical — that's why it's sometimes called a vertical gyro. The gyro maintains this vertical reference in space due to rigidity, which is the gyroscopic property that resists any force trying to change the direction of the spin axis. The instrument displays the aircraft's attitude relative to this fixed vertical reference. Now, the indications. The artificial horizon shows you two things simultaneously: pitch, which is nose up or nose down relative to the horizon, and bank, which is the angle of the wings relative to the horizon. The display typically has a miniature aeroplane symbol fixed to the instrument case, and a horizon bar that moves with the gyro. When you pitch up, the horizon bar moves down relative to the miniature aeroplane; when you bank, the horizon bar tilts to show the angle of bank. But here's where we get into the limitations. A gyro that's free to move in space would eventually drift due to the Earth's rotation — this is called apparent drift. To correct for this, the instrument has an erection mechanism that applies a torque to keep the gyro spin axis vertical. This is what we call the control system. The figure I have here, Figure 12.13, illustrates graphically the effect of compensating a gyro for the apparent drift — it shows how the erection system corrects the gyro back to vertical. Now, the air-driven artificial horizon. This type uses a stream of air, drawn by a vacuum pump, to spin the gyro. The air is directed onto buckets on the gyro rotor to spin it at high speed. The erection system in the air-driven type uses gravity — small pendulous vanes that sense when the gyro is not vertical and direct air to correct it. But the air-driven type has specific errors. The acceleration error occurs during acceleration or deceleration in straight and level flight. When the aircraft accelerates, the pendulous vanes are deflected by the inertial force, not by gravity, so they sense a false vertical and the horizon bar shows a false pitch attitude. Similarly, turning errors occur during turns — the centrifugal force deflects the vanes and causes the horizon to show an incorrect bank angle. These are inherent limitations of the air-driven design. Rigidity is the gyroscopic property that keeps the spin axis fixed in space, and it's what makes the whole instrument work. But rigidity also means the gyro resists the erection forces, so the erection system must be powerful enough to overcome this. Serviceability checks are essential. Before flight, you check that the instrument is functioning correctly — the horizon bar should be level, the instrument should erect properly, and the flag or indication should show that the instrument is serviceable. Now, the electric artificial horizon. Instead of air, this uses an electric motor to spin the gyro. The electric control system is different — it uses an electrolytic level sensor, which is a small glass tube filled with conductive liquid and three electrodes. When the gyro is vertical, the liquid is centered and the electrical resistance is equal on both sides. When the gyro tilts, the liquid shifts, changing the resistance, and this signal drives torque motors to erect the gyro back to vertical. The electric horizon also has acceleration errors, but they're different in character from the air-driven type. Because the electrolytic sensor responds to the resultant of gravity and inertial forces, acceleration can cause a false erection. However, the electric system is generally more accurate and has a faster response. The fast erection system is a feature of the electric horizon. It's used to erect the gyro quickly after the instrument is switched on, or after the aircraft has been in an unusual attitude. It applies a strong torque to bring the gyro vertical in a short time. The adjustable aeroplane datum is a feature that lets you adjust the miniature aeroplane symbol relative to the horizon bar. This is used to compensate for the aircraft's attitude when it's parked or when you want to set a specific pitch reference. Finally, the vertical gyro unit. This is the complete gyroscopic assembly — the gyro, the erection system, and the sensors — packaged as a unit. It's the heart of the artificial horizon, and it's also used as the reference for other systems like the autopilot. So, to summarize: the artificial horizon is a vertical gyro instrument that shows pitch and bank. It comes in air-driven and electric versions, each with its own erection system and its own characteristic errors. The air-driven type uses pendulous vanes and has acceleration and turning errors. The electric type uses an electrolytic sensor and has a fast erection system. Both have serviceability checks you must perform, and both are built around the vertical gyro unit. That's the full scope of this chapter. We'll go through each of these in detail as we work through the material.