In today's ground lesson we'll be discussing the heading indicator. and explaining how it works.

The heading indicator, which is also known as a directional gyro, is one of the six primary flight instruments. These are often referred to as the pilot's six pack. Now, the heading indicator is one of the three gyroscopic instruments, and that means it uses the principles of a gyroscope to operate. We'll talk about that more in just a minute.

Now, when it comes to navigation and situational awareness, the heading indicator is one of the best tools you have available. But with that in mind, let's take a look at the Federal Aviation Regulations. Specifically, I want to look at the VFR Day Minimum Equipment Requirements. Now while we do need this instrument if we want to fly IFR, you can see here that we don't actually need this instrument if we want to fly under visual flight rules.

But check this out, we do need a magnetic direction indicator. And that's just a fancy name for the magnetic compass. So why don't we fly off of this thing instead of using the heading indicator? Well, the first reason is that when you use a magnetic compass, it seems to operate backwards. For example, let's say you're facing directly north on a 360 heading. If you want to make a turn to 330, you'll need to make a left turn. But notice, the compass seems to indicate that you'll need to make a right turn to intercept a 330 heading. Now, when you do make your turn, the compass will move in the opposite direction of your travel. And that's because your airplane is on the outside of that compass. But by using gyroscopic principles, we're able to put the airplane inside of the compass rose on a heading indicator. And that allows us to fix this problem. Then, in addition to this, the mag compass is subject to dip errors. Turning errors, acceleration errors, and several other things.

Now that we know why we use a heading indicator, let's talk about how it works for a little bit. As I mentioned before, the heading indicator uses gyroscopic principles. Let's talk about that for a minute. Have you ever tried to balance a bicycle while the bicycle is not moving forward? It seems like it's impossible, right? And if you pedal slowly, it's still pretty wobbly. But once you start gaining speed, the faster you go, the easier it is to balance. And that's because, as the wheels turn more quickly, they want to continue in the direction they're going. And that makes the wheels more resistant to outside forces. And that means they're more rigid and more resistant to wobbling.

This principle is known as rigidity in space. Now let's talk about how we use this in the heading indicator. First, let's zoom way in to take a look at the actual gyro inside the heading indicator. In order to create rigidity in space, we need to get this thing spinning somehow. Now on some airplanes, we use electric motors to do this. But, on most training aircraft, we use the suction from the vacuum system to create this movement. Let's take a look at this really basic schematic of the vacuum system in a training aircraft. Now, you don't have to memorise this thing or anything. I just want you to see that if you're not getting good suction on your vacuum system, the gyro inside your heading indicator may not be spinning the way it needs to in order to work properly. If that gyro's not spinning, you're not getting rigidity in space, which means your heading indicator isn't accurate. Okay, so now we have the gyro rotor spinning on its axis by using suction from the vacuum system. Then, we have two rings which surround the gyro rotor. These rings, which are called gimbals, allow the aircraft to move around the gyro rotor. Without affecting its position in space. These gimbals are then mounted to the inside surface of the heading indicator, and they're mounted so that when the aircraft makes a turn, the gyro is able to maintain its position in space. The gyro is then connected to a gear, which is also connected to smaller gears, which ultimately drive the heading indicator. Then, as the aircraft turns, the gyro maintains its position in space. While these gears ultimately turn the compass card to match the gyro while the aircraft is turning. Now, most heading indicators don't know which direction north is. And that's because most of these don't receive any kind of magnetic signal. It's actually up to the pilot to align the heading indicator with the magnetic compass. We call this slaving the heading indicator. And you can do this by pushing this little button on the heading indicator. By pushing this button, the compass card is then disconnected from the gears which run to the gyro. Then, by rotating this button, the compass card also rotates until it's aligned with the magnetic compass on the proper heading. So I've already aligned my heading indicator with my compass. Why doesn't it just stay that way? And the answer is because of something called precession. If we take a closer look, at the inside of this heading indicator, there's a lot of moving parts here. Any one of these parts that are able to move, like these gimbals or gears, are subject to friction. And this friction eventually causes the heading indicator to drift from its set position, and this drift is known as precession. The other problem is actually caused by the rotation of the Earth. Depending where you are on the globe, it rotates at about 15 degrees per hour. And what might have been north at the beginning of your flight, after an hour, might be slightly different. And that's why you want to resolve your heading indicator every time you do a run up and about every 10 to 15 minutes in flight. Just remember, when you're in the air, the airplane needs to be in straight and level flight in order to be as accurate as possible.

Now, on newer aircraft with digital flight instruments, the heading indicator often comes in the form of a horizontal situation indicator, or HSI. Now the way these look to the pilot is really similar, but the way these operate is quite a bit different. In these aircraft, a device called a magnetometer senses the Earth's lines of magnetic flux. It then relays this information to the aircraft's attitude and heading reference system. This is also called AHERS. This device then routes the heading to the Primary Flight Display, or PFD, so the pilot can use this information in flight. Because the magnetometer is constantly sensing and sending updated heading information, there is no longer a need to reslave the heading indicator on this device.