The Artificial Horizon — Page 177, Lesson 213

Right, let's pick this up with the artificial horizon. We've already got the instrument spinning up in our heads, so now I want to walk you through the two big families of errors that plague it: the roll error from acceleration, and the turning errors. First, the roll error. Picture the gyro spinning inside its housing. The housing has a weighted base, and that base has inertia. When the aircraft accelerates, that heavy base wants to lag behind—it resists the change in motion. Now, here's the key: that lagging force doesn't just push the base backwards. Because the gyro is spinning, that force gets precessed. The result is that the base of the rotor housing moves to starboard—to the right—and the gyro axis itself precesses out of the vertical. That tilts the whole rotor and gimbal assembly about the longitudinal axis, and the instrument shows a right wing down indication. So, acceleration gives you a false right-wing-down. Now, deceleration is the mirror image. It causes a nose-down, left wing low error—exactly the opposite of the acceleration error. But—and this is critical—all of this assumes the rotor is rotating anticlockwise when viewed from the top. That's the case for British air-driven artificial horizons. But most electric horizons, and some American air-driven ones, spin clockwise. And if the spin direction flips, the errors flip too. So you get the opposite indications. Always know which direction your rotor spins before you trust that attitude. Now let's move to turning errors. Whenever an aircraft turns, there's an acceleration towards the centre of the turn—that's the centripetal force. The pendulous vanes inside the horizon are now feeling a horizontal acceleration on top of gravity. That messes with both pitch and roll indications. During the turn, the centrifugal force acts on two things: the fore and aft pendulous vanes, which gives you what's called the 'erection' error, and the weighted base of the rotor housing, which gives you the 'pendulosity' error. These errors are complex, and they change as the turn progresses. The good news is they cancel out after a full 360° turn. The magnitude depends on speed, rate of turn, and the type of horizon. And for a chosen speed and rate of turn, you can compensate by tilting the top of the rotor axis slightly forward—that fixes the erection error—and slightly to the left—that fixes the pendulosity error. But in an uncorrected instrument, here's what you'll actually see. Assume the classic instrument: air-driven, gyro rotating anticlockwise from above. Turning through 90°, the instrument under-reads bank angle, and the pitch error indicates a climb. At 180°, the bank angle reads correctly, but the pitch error still indicates a climb. At 270°, it over-reads bank angle, and again the pitch error indicates a climb. And at 360°, the bank angle is correct again. So through the whole turn, the pitch error keeps telling you you're climbing, while the bank error shifts from under-reading to over-reading and back to correct. Let me show you what that nose-up and nose-down attitude actually looks like on the instrument face. And here's the equilibrium state, where the gyro is properly aligned. So the takeaway: acceleration and deceleration give you roll errors that depend on rotor spin direction, and turns give you both pitch and bank errors that cycle through the turn. Know your rotor direction, and know that a 360° turn brings you back to truth.