The Artificial Horizon — Page 177, Lesson 208

Let’s start with the big picture. The artificial horizon is the instrument that tells you the aircraft’s attitude — nose up, nose down, bank left or right — relative to the real horizon. It’s a gyroscopic instrument, and the whole display is built around a gyro that stays fixed in space while the aircraft moves around it. Now, the first thing I want you to take away is the limitation of the instrument. The amount the case can move relative to the gyro is controlled by fixed stops. In older designs, the typical limits are ±60° in pitch and 110° each way in roll. In modern instruments, there is complete freedom in roll and up to 85° plus or minus in pitch. If you exceed those limits, the gyro “topples” — and that gives you violent, erratic movements of the horizon bar. And here’s the critical part: unless a fast erection system is incorporated, you will not get accurate indications until the gyro has re-erected itself, and that takes a period of 10 to 15 minutes. So if you ever see the horizon bar thrashing around, you know the gyro has toppled, and you’re looking at an unreliable instrument for a long time. Next, the control systems. The rotor assembly is made very slightly bottom-heavy. Why? To keep down the time taken for initial erection when the gyro is first started up. But that slight bottom-heaviness alone isn’t enough — a complex control system is required to maintain the rotor axis vertical in flight. There are two families of artificial horizons here. A suction or air driven artificial horizon exhausts air through four slots which are normally half covered by four pendulous vanes. An electric artificial horizon uses levelling or mercury switches and torque motors instead. Let’s go into the air driven one in detail, because that’s where the real mechanism lives. In the air driven artificial horizon, an engine-driven suction pump — or a venturi tube in some light aircraft — is used to create a suction of about 4 inches of mercury in the instrument case. Replacement air, drawn in by that suction via a filter, is ducted through the outer and inner gimbals to enter the rotor case as a jet. That jet spins the rotor at up to 15,000 rpm. After driving the rotor, the air passes into the instrument case through slots at the base of the rotor housing. Now the control system of the air driven artificial horizon — this is the heart of how it stays erect. It consists of four slots and four pendulous, hanging vanes at the base of the rotor housing. The vanes hang down so that when the rotor axis is vertical, each slot is half covered by its vane. That means four equal jets of air emerge from the slots — fore and aft, and left and right. Because the four jets are of equal strength but in opposite directions, no force is exerted on the gyro, and therefore no precession occurs — the gyro rotor remains vertical. So the whole trick is balance. When the aircraft is level, the vanes hang symmetrically, the jets are equal, and nothing disturbs the gyro. The moment the aircraft pitches or banks, the case moves relative to the gyro, the vanes no longer cover the slots evenly, the jets become unequal, and that imbalance creates a force that precesses the gyro back toward vertical. That’s the self-erecting mechanism in a nutshell. That figure shows the equilibrium state — the four equal jets, fore and aft, left and right, with the rotor axis vertical. And the earlier figures show you what the display looks like in a nose-up and a nose-down attitude. So to tie it together: you have a gyro spinning at up to 15,000 rpm, driven by air at about 4 inches of mercury suction. You have a control system of four slots and four pendulous vanes that keeps the rotor axis vertical by balancing air jets. And you have hard limits — ±60° pitch and 110° roll in older designs, full roll and ±85° pitch in modern ones — beyond which the gyro topples and takes 10 to 15 minutes to re-erect. That’s the artificial horizon, end to end.