Pitch angle correct — Page 177, Lesson 215

I want to walk you through the electric artificial horizon now, and I'll start with the concept of pitch angle correction, because that's where this passage opens. We've been talking about how the artificial horizon can suffer from errors during turns and acceleration. The tilts we're dealing with here are small — on the order of 2°. That's the magnitude of the error we're trying to correct. Now, because the gyro axis is tilted by that amount, the horizon bar itself has to be modified — set — so that it still indicates correctly in level flight. In other words, we deliberately offset the horizon bar by the same amount as the gyro tilt, so that when the aircraft is truly level, the bar reads level. Now, this compensation isn't perfect. Small residual errors remain, particularly if the speed and rate of turn are not the ones for which the compensation was designed. But here's the key point: those residual errors are very much smaller than they would be if no compensation had been made at all. So it's a big improvement, even if it's not perfect. Next, let's talk about rigidity. This is the heart of why the electric horizon is better than the air-driven one. In suction horizons — the air-driven ones — rotor speeds can reach up to 15,000 rpm. That high speed gives high gyroscopic inertia. But in electric horizons, the rotor spins at 22,500 rpm — that's typical — giving even greater rigidity. Why does rigidity matter? Because of inertia. With high inertia, precession rates are low. That means the gyro is harder to disturb, so it holds its attitude better. But there's a trade-off: because precession is slow, once a horizon topples — once it's knocked over — it will take a significant period of time to re-erect itself, unless a rapid erection device is fitted. So rigidity is a double-edged sword: great accuracy, but slow recovery from a topple. Now, serviceability checks. Before flight, you check that the horizon bar takes up a laterally level position — that is, level side to side — with the correct pitch indication for the aircraft type. And you confirm that this indication is maintained when taxiing. If a caging device is fitted — that's a mechanism that locks the gyro — the instrument should be uncaged at least five minutes before take-off. Why? To ensure the rotor axis has had time to reach alignment with the true vertical. So you uncage early, give it time to erect. In flight, the artificial horizon should give an immediate and correct indication of any change in pitch or roll attitude. That's the in-flight check — you're verifying it responds instantly and accurately. Now let's move to the electric artificial horizon itself. The main advantage over the air-driven horizon is greater rigidity, due to the faster spin rate — we just covered that. And this greater rigidity results in increased accuracy because errors are reduced. The basic principle is the same as the air-driven horizon: the vertical gyro is still tied to earth's gravity. But instead of the pendulous vanes of the air-driven horizon, the electric horizon uses mercury levelling switches and torque motors. Let me explain that control system carefully, because it's the core of this instrument. The gravity-operated control system consists of mercury levelling switches, which are fixed to the base of the rotor, and electric torque motors. Here's how it works: if a levelling switch is not level, the mercury liquid ball moves from its central position. That movement closes the circuit, which drives its torque motor. The torque motor then provides the force which is precessed to return the gyro axis to the vertical. There are two levelling switches — one to sense pitch, one to sense roll. They activate the pitch and roll torque motors respectively, and those motors precess the gyro back to the vertical as soon as it starts to wander. Now, here's where the 90° precession rule comes in — and this is important. Because of that rule, the torque motor on the side of the inner gimbal corrects wander in the rolling plane. It applies torque round the lateral axis to produce rotation about the longitudinal axis. Likewise, the pitch torque motor is on the outer — the longitudinal — gimbal, so that the precession is about the lateral axis to correct for pitch. So the placement of each motor is dictated by the 90° rule: you apply torque on one axis to get precession on the perpendicular axis. And finally, this control system — like the suction horizon's — is designed to compensate for turning errors. It does this by maintaining the rotor axis slightly tilted away from the true vertical, and having the horizon bar compensated by a similar amount. So the same compensation philosophy carries over from the air-driven instrument. Let me show you the control system layout. That's the electric horizon control system in schematic form. You can see the two levelling switches and the torque motors and how they're arranged on the gimbals. So to tie it all together: the electric horizon gives you more rigidity through a faster spin rate, which means fewer errors. It uses mercury switches and torque motors instead of pendulous vanes to keep the gyro erect. And it compensates for turning errors the same way the suction horizon does — by tilting the rotor slightly and offsetting the horizon bar to match.