First, the pendulosity of the gyro — Page 184, Lesson 219

Let’s pick up right where we left off — we’ve just finished the artificial horizon, and I want to close out that chapter with a few refinements before we move into the turn and slip indicator. First, the pendulosity of the gyro. Pendulosity just means bottom-heaviness — the gyro is deliberately weighted at the bottom so gravity can erect it. The point here is that if you reduce that bottom-heaviness, you reduce the turning and acceleration errors. That’s the trade-off: less pendulosity means the gyro is less aggressively corrected by gravity, so it’s less disturbed during turns and acceleration. Now, a very important operational note about the fast erection knob. When you’re airborne, that knob can only be used successfully in level flight with no acceleration. Why? Because during acceleration or a turn, the liquid level switches inside the unit are off-centre. If you operate the fast-erection system then, it will align the rotor axis with a false vertical — you’d be erecting the gyro to a wrong reference. So the rule is: fast erection only in steady, level, unaccelerated flight. Next, the adjustable aeroplane datum. This is a refinement found on some American artificial horizons. The idea is this: if an aircraft is trimmed to fly straight and level but it has a pitch-up attitude — meaning the nose sits slightly high in level flight — you can adjust the little aeroplane symbol so it lies on the horizon. That makes the display look correct for your particular trim condition. But there’s a real risk here. If you adjust that datum in flight, you could end up with a misleading datum for flying approach procedures in IMC — instrument meteorological conditions. The Aeronautical Information Circular 14/1969 discusses this risk in depth, and it strongly recommends that in light aircraft the datum be set before flight and thereafter left well alone. And EASA goes further: they require that such movable datums be removed or otherwise rendered inoperative on aircraft having a maximum all-up weight in excess of 6000 pounds, which is 2727 kilograms. So on heavier aircraft, this adjustable feature simply isn’t allowed to work. Now let’s move to the vertical gyro unit. This unit performs the same functions as the gyro horizon — it establishes a stabilized reference about the pitch and roll axes of the aircraft. It’s sometimes called a remote vertical gyro, or occasionally a vertically axised data generation unit. The key difference is in how it delivers that information. Instead of providing attitude displays directly to a dial, it’s designed to operate an electrical transmission system to a steering computer, which is then usually displayed onto a combined attitude indicator and flight director display. So the vertical gyro is the remote sensing element that feeds the flight director system, rather than driving a standalone instrument face. That figure shows you the vertical gyro unit itself — the physical hardware that houses the gyro and its electrical transmission system. Now, that closes out the artificial horizon chapter. The next chapter is the turn and slip indicator, and I want to give you the roadmap of what’s coming. We’ll start with the rate of turn indicator, then the rate gyro itself, then its operation and constructional details. We’ll look at the effect of varying rotor speed, and the errors in the looping plane. Then we’ll cover the slip indicator — its construction and operating principles — and finally the turn and slip displays. That’s the full structure of Chapter 14. Let’s begin with the rate of turn indicator. The heart of it is the rate gyro. Unlike the artificial horizon’s gyro, which is free to move in two axes, a rate gyro is constrained — it measures the rate at which the aircraft turns about one axis. The rate of turn indicator uses this rate gyro to show the pilot how fast the aircraft is rotating about the vertical axis, which is the rate of turn. The operation is straightforward: when the aircraft turns, the gyro precesses against a spring, and that precession is translated into a needle deflection on the instrument face. The faster the turn, the greater the precession, the more the needle moves. Now, the constructional details matter here. The rate gyro is typically driven by a rotor, and the rotor speed is critical. If the rotor speed varies — say it slows down — the gyro’s rigidity changes, and that affects the accuracy of the rate indication. That’s the effect of varying rotor speed we’ll examine. There’s also a specific error called the errors in the looping plane. This is a known limitation of the rate gyro design — under certain manoeuvres, particularly in the looping plane, the gyro can produce erroneous indications. We’ll get into the specifics of that. Then we move to the slip indicator. This is the ball — the inclinometer that shows whether the aircraft is in coordinated flight. Its construction is simple: a curved glass tube containing a ball, usually in a damping fluid. The operating principle is that during a turn, the ball is acted on by gravity and centrifugal force. In a coordinated turn, the ball stays centred. If you’re slipping — that is, the aircraft is yawing toward the inside of the turn — the ball moves to one side. If you’re skidding — yawing toward the outside — the ball moves to the other side. Finally, we’ll look at the turn and slip displays — how the rate of turn needle and the slip ball are combined on a single instrument face, and how the pilot reads them together to maintain coordinated, standard-rate turns. That’s the full picture of Chapter 14. Let’s start working through it in detail.