Remote Indicating Magnetic Compass — Page 211, Lesson 250

Right, let's get into the Remote Indicating Magnetic Compass. This is a really important step in your instrumentation training, because it's the system that actually solves the problems of the older, simpler instruments. To understand why we need this system, I first want to walk you through the limitations of the two instruments that came before it. First, the Direct Reading Compass. It has three major limitations. The first is turning and acceleration errors — the compass simply cannot be read accurately during a turn. The second is that the magnetic sensing element, which is the magnets on the vertical card, is contained within the instrument itself. That means it has to be situated close to the pilot so the card can be seen. The problem is, the cockpit area is close to sources of deviation, such as electric lights, electric motors, and ferrous metal — that's iron and steel. So you get errors just from the environment around it. The third limitation is that the instrument is self-contained. It is not possible to take a magnetic heading and input it into other equipment. It just sits there on the panel; it can't feed anything else. Now, the Directional Gyro Indicator attempts to solve some of these problems by using an air driven or an electromechanical gyro. With that, turning and acceleration errors are eliminated, and an output can be taken to other equipment. That's a big improvement. However, there is no magnetic input. So if the gyro drifts with time — and gyros do drift — there is no automatic correction. The only correction is for the pilot to manually synchronize it to the direct reading compass at regular intervals. So you've traded one set of problems for another. What is required, then, is a system which combines the best of both. The short-term rigidity of the gyro overcomes turning and acceleration errors. That needs to be combined with the longer-term monitoring of the earth's magnetic field, so that if the gyro starts to drift, a servo system slaves it to alignment with a magnetic input. Such a system is a gyro-magnetic compass. Now, this gyro-magnetic compass is known by several names, and they all mean the same thing. It can be referred to as the Gyro-magnetic Compass, the Remote Indicating Compass, or the Slaved Gyro Compass. Don't let the different names confuse you — they're all the same system. Let's look at the basic system description. At its simplest, the system comprises these elements. First, the Magnetic Detector Unit — this is also often known as a flux valve or a flux detector. Second, the Heading Indicator — this is what most people refer to as 'the compass'. Third, the Precession Amplifier — this may also be known as a slaving amplifier. Fourth, the Precession Motor — this may also be known as a slaving or synchronizing motor. And fifth, the Horizontal Gyro. In simple systems, the horizontal gyro is directly connected to the compass card of the heading indicator via a bevel gear and a drive shaft. That's the assumption we'll use in the description that follows. So the gyro physically drives the compass card through that mechanical connection. Let me show you what that drive shaft arrangement looks like. And here's the magnetic detector unit — the flux valve — positioned in a part of the aircraft least affected by on-board electrical interference. And here are the flux valve components — the circular plate is screwed to the underside of the wing, with the black hemisphere protruding out. So to pull it together: the gyro gives you short-term stability, the magnetic detector gives you long-term accuracy, and the precession amplifier and motor form the servo system that slaves the gyro to the magnetic input. That's the heart of the gyro-magnetic compass.