Gyroscopic Instruments – 3 Essential Instruments and How They Work

Gyroscopic instruments are devices that use a spinning gyro to indicate your attitude, heading, rate of turn, and rate of roll.

You could find yourself forty minutes into a cross-country flight, when you glance down at the panel. You see that your heading indicator reads ten degrees off your magnetic compass.

If you’re familiar with how a gyro works, you would know that this is because of drift. It happens, and it’s no cause for alarm.

Now, let’s talk about the mechanisms behind these gyroscopic flight instruments. We’ll also take a look at why they drift and how they fail.

Key Takeaways

• The gyroscopic instruments are the attitude indicator, the heading indicator, and the turn coordinator.
• A spinning rotor’s rigidity in space gives every gyroscopic instrument its fixed reference datum.
• Most light-aircraft gyros run on engine-driven suction, while the turn coordinator runs electrically.
• Warning flags indicate only lost power or suction, not an inaccurate instrument.

What Are Gyroscopic Instruments?

The 3 main gyroscopic instruments found on aircraft are the attitude indicator, the heading indicator, and the turn coordinator. You’ll find them on the “six pack” in front of you. All three use gyroscopic axes to provide a datum from which the aircraft’s orientation can be measured. More advanced systems can also be found on complex aircraft.

Gyroscopes have limits, but overall, they provide a reliable solution for airplane navigational and attitude instruments.

Don’t worry if you’ve never heard of a gyroscope or are familiar with the term but don’t understand it.

You are in good company.

We will go through how a gyroscope works, its main components, and how these components are utilized in common flight instruments.

Here’s everything you need to know about gyroscopic flight instruments.

What Is a Gyroscope?

A gyroscope is a spinning wheel rotating around an axis. The wheel’s inertia, a function of its speed and mass, gives the gyroscope a specific property called ‘rigidity in space’. This rigidity means that, provided it is undisturbed, the gyroscope axis will stay pointed in a given direction.

This same principle is what allows the instrument to hold a steady reference in an aircraft gyroscope.

Don’t believe us?

Here’s a quick video showing the concept of a gyroscope in its most basic form.

Here’s what we’ll do.

First, let’s break down the common features of most gyroscopes. Once you are familiar with the parts of a gyroscope, we’ll discuss exactly how they work.

All gyroscopic instruments contain the following gyroscope parts.

The Spin Axis

A gyroscopic axis is vital to all gyroscopes.

Remember how we just said that a gyroscope is, essentially, a spinning wheel.

Well, for a wheel to spin, it must have something to spin around. Here’s where the gyroscope spin axis comes in. You can consider the axis a little like the axle on a bicycle wheel. It goes straight through the center of the wheel at a 90° angle to the direction of the wheel’s rotation.

However, gyroscopes can get confusing when it comes to talking about axes.

Why?

Because a single gyroscope will actually have three axes:

• The spin axis (think of it like an axle, as just discussed).
• The input axis.
• The output axis.

Fortunately, when discussing aeronautical gyroscopic instruments, we always reference the spin axis and where it is aligned, making understanding gyroscopes that much easier.

The Rotor

Let’s be honest.

An axle without a wheel is just a stick.

Gyroscopes need a wheel to spin in order to establish that all-important rigidity (which we’ll tell you all about below).

For now, all you need to know is that there is a wheel, and it needs to fulfill two criteria related to ‘rigidity’:

1. The greater the wheel’s mass, the greater the rigidity.
2. The faster the wheel spins, the higher the rigidity.

For the more scientifically minded among you, spinning wheels, mass, and speed may start to ring a bell in the back of your mind. Combined, these two forces make up something called ‘angular momentum’, which in turn is a product of inertia.

For those who aren’t scientifically minded, here’s what you need to know in simple terms…

Inertia is a function of mass and speed. The higher the inertia of an object (or a spinning wheel in a gyroscope), the more resistant it is to changes in direction.

In even simpler terms, imagine someone rolling a bicycle wheel towards you. Not such a big deal, right? Now imagine someone rolling a tractor wheel towards you at speed. The tractor wheel is both heavier and faster, leading to an increase in inertia. The bicycle wheel is easy to stop. The tractor wheel will resist your efforts, and it is all down to inertia.

It’s the same with gyroscopes. Those with heavier wheels and faster rotation are more resistant to directional change (which, in gyroscopic instruments, is something we really want).

You may also hear this wheel referred to as a gyroscope rotor. Throughout this article, ‘wheel’ and ‘rotor’ will be used interchangeably, but they mean exactly the same thing.

What Is a Gimbal vs a Gyro?

Axes and spinning wheels are all good and well, but we need something to hold them. And that something is called a ‘gimbal’. Imagine holding a bicycle wheel on an axis between two hands.

Congratulations, you have just become a gimbal.

In aircraft gyroscopic instruments, we need something slightly more precise (and smaller) than our hands. So clever designers develop highly engineered mounts that will hold the axis. These gimbals allow free movement of the gyroscopic axis. They also allow us to orient the gyro axis in a useful direction. A gyroscope gimbal will keep the rotor supported and its axis fixed in space.

Power for the Gyroscopic Rotor

One thing that all gyroscopes need to work is a spinning motion. And, obviously, that doesn’t happen on its own.

There are a few ways that the gyro wheel can be spun up to maintain that all-important rigidity.

Most systems on light aircraft will use suction to drive the rotor. Low pressure is created by an engine-driven pump that sits outside the instrument. The result is that higher-pressure ambient air is pushed in from the outside. This air is directed onto the rotor, causing it to spin really fast. If this tube becomes blocked or the pump fails, the rotor may slow to the point that it doesn’t maintain rigidity.

And what happens then?

The gyro may tumble. This will lead to a malfunction of gyroscopic instruments. The sole purpose of gyroscopes used in instruments is to create a datum. The instrument becomes useless if this datum can no longer be relied upon!

To summarize, a gyroscope will have the following components:

• A rotating wheel.
• An axis.
• A gimbal.
• A means of spinning the rotor.

Remember these terms because we’ll be referencing them often as we explain how gyroscopic instruments work. Speaking of which…

How Do Gyroscopic Instruments Work?

You’ll often read complex descriptions of gyros, precession, angular momentum, and axes, and many other advanced terms.

But there is a simpler way to understand how gyroscopic instruments work. It goes something like this.

All gyroscopic instruments rely on using the gyroscopic axis as a datum. Now, that datum is only possible because the spinning rotor maintains its rigidity in space.

Wait, what’s a datum?

A ‘datum’ is a term you’ll find used often in aviation. It simply means a set point from which you can measure something. In the case of gyroscopic instruments, we tend to use the datum created by a gyroscopic axis to measure aspects of the aircraft’s attitude.

Want it in really simple terms? Here’s how all gyroscopic aircraft instruments work in principle:

1. We point the gyroscopic axis in a relevant direction, creating a fixed datum.
2. When the aircraft moves from that datum, we measure the amount of movement or change.
3. The change is displayed on the flight instruments as data that the pilot can use to monitor deviation.

Simple, right?

However, truly understanding how gyroscopic instruments work requires a little more in-depth knowledge.

What Is Rigidity in Space?

Gyroscopes have a unique property called ‘rigidity in space’.

It can actually be considered a literal term. (NASA uses gyroscopes for navigation on board their spacecraft). Gyroscopic-powered instruments are great because their gimbals allow them to be pointed literally in any direction. Or, to put it another way, at any point in space.

Above or below the horizon, or in any given direction, in three dimensions. Ideally, we want the gyroscope to stay pointed in the direction we aim it. The more resistant to movement the gyro is, the better.

This resistance is called rigidity, which comes from gyroscopic inertia. And rigid gyros are a good thing.

However, this behavior presents us with particular problems here on Earth.

Why?

Because let’s say we point the gyro axis towards the sun and want to use that as our datum. As you’ll no doubt know, the Earth’s rotation means that the sun appears to ‘move’ across the sky. Provided the gyro is kept spinning and not disturbed, its axis will stay pointed towards the sun!

The gyro axis and the sun stay ‘fixed’, but the Earth’s surface doesn’t! So, the gyro axis may also appear to move (or, in extreme cases, ‘tumble’).

The above is an interesting concept, and there are a few things that will affect how much influence this has on gyroscopic instruments:

• The rigidity of the gyroscope.
• The latitude of the gyroscope.
• The point in space to which the gyroscopic axis is oriented.

Just a minute. Latitude and where we point the gyro affect how well it works?

Yes. And here’s why.

Imagine a gyroscope located on the north pole with its axis pointing vertically straight up. Because the axis matches the orientation of the Earth’s axis (straight up), the gyro axis will stay where we put it.

However, imagine aligning the gyroscope spin axis horizontally while we remain standing on the north pole.

Now the axis will stay aligned towards a fixed point in space on the horizon. The gyro, to us, will appear to spin horizontally, at the same rate as the Earth’s rotation (15° per hour).

There’s a name for this apparent movement.

We call it precession.

Here are a few simple rules to remember regarding precession and the apparent movement of gyroscopes:

• A gyroscope with its spin axis aligned horizontally to a given meridian (a line running north-south) will appear to move 360° over 24 hours at the pole.
• A gyroscope with its spin axis aligned horizontally to a given meridian (a line running north-south) will show no apparent movement at the equator.

Why is the above relevant in aviation?

Because we use north as a datum, and we use a heading indicator to tell us where this datum is. The heading indicator is a gyroscopic instrument. The further north you go, the more often you need to realign it due to precession!

What Is Gyroscopic Precession?

Gyroscopic precession is when a force acts on a spinning gyro. The reaction appears 90 degrees further around the wheel, in the direction of rotation.

With apparent drift, the gyro holds still while the Earth rotates beneath it, so it looks like it’s wandering. In fact, you can even predict it. Multiply 15 by the sine of your latitude, in degrees per hour.

What Are the Two Types of Gyros?

The attitude gyro drives your attitude indicator and heading indicator, since it stays rigid in space even as the aircraft moves around.

Your turn coordinator uses the rate gyro. It precesses in proportion to your rate of turn and rate of roll, which is the principle behind rate instruments.

How Are Gyroscopic Instruments Powered?

Gyroscopic instruments run on one of two power sources. These could be from vacuum systems or the aircraft’s electrical system.

In a typical training airplane, the attitude and heading indicators run on suction. The turn coordinator, meanwhile, runs on electrical power.

The benefit of splitting them is that a single failure won’t wipe out every gyro at once.

Vacuum-Driven Gyros

A vacuum-driven gyro is spun by a stream of air pulled through the instrument.

Outside air enters through a filter and passes into the instrument case. It then strikes the rotor vanes to spin it, and then exits to the engine-driven pump.

Your suction gauge is the only direct read you get on the system’s health. Typical values run around 4.5 to 5.5 inches of mercury. That said, check your POH for the figures that apply to your aircraft.

How Does an Electrical Gyro Work?

An electrical gyro uses a small electric motor that spins the rotor directly, which draws power from the aircraft bus.

Even if the vacuum pump quits, your electric turn coordinator would keep working.

What Happens If the Vacuum System Fails?

There’s a pattern to how your instruments behave during a vacuum failure. The attitude and heading indicators go sluggish, then drift, then tumble. The electric turn coordinator could still keep reading normally at the same time.

Many vacuum attitude indicators have no flag at all, so the gyro just spins and feeds you wrong information.

But even where an “OFF” flag exists, it only monitors power or suction reaching the instrument. An instrument can be powered and unflagged but still be wrong.

What Are the 3 Gyroscopic Instruments in the Six Pack?

In most airplanes, you’ll find the following gyroscopic instruments. These combine with pitot-static instruments to make up the ‘six pack’ of primary flight instruments:

• The Artificial Horizon
• The Heading Indicator
• The Turn Coordinator

Here’s a brief description of what these instruments are used for, where the gyroscopic axis is aligned, and how they work.

Instrument Name	What It Shows	Gyroscopic Axis Alignment
Artificial Horizon	Attitude and Bank	Vertical to the Center of the Earth
Heading Indicator	Heading	Aircraft Yaw Axis
Turn Coordinator	Rate of Turn and Rate of Roll*	Canted ~30° From the Lateral Axis

*It is important to note that the turn coordinator does not show bank angle!

Here’s a detailed description of all of the above gyroscopic instruments.

The Attitude Indicator (Artificial Horizon)

As the name suggests, the artificial horizon is a pictorial display of the aircraft’s attitude in relation to the Earth’s horizon.

It is a disc split into two segments. The ‘blue’ represents the sky above the horizon, and the ‘brown’ represents the Earth beneath the horizon.

It is used to indicate the aircraft’s attitude relative to the horizon with a small airplane symbol with a dot in the center showing whether the aircraft is nose up or nose down. It also displays the bank angle relative to the horizon.

The gyro axis is kept in a vertical state by weights at the bottom of the gimbal, so it always points towards the direction of gravity (the center of the Earth).

The airplane symbol is actually fixed, and it is the disc that moves. This disc is attached to a gyroscopic gimbal. The gyroscope wheel is aligned with the horizon. Because of its rigidity, it doesn’t move, regardless of the aircraft’s attitude.

Essentially, the aircraft symbol (and the aircraft) fly ‘around’ the fixed axis of the artificial horizon gyroscope.

Some important points to note:

• Bearing friction still precesses the rotor, so the instrument self-erects using gravity-driven pendulous vanes.
• It also reads slightly nose-up during rapid acceleration and nose-down during rapid deceleration.
• Extreme maneuvers may cause the gyro to “tumble,” usually past roughly 100 to 110 degrees of bank or 60 to 70 degrees of pitch.
• Friction in the gimbal may cause the gyro rotor to slow, meaning that a bank angle or pitch attitude will be displayed, even if you are wings level.

Stay especially mindful of those last two attitude indicator errors. Pair a gyro tumble with gimbal friction, and the instrument can show the wrong attitude until it re-erects itself.

The Heading Indicator

The heading indicator is another great example of a gyroscopic instrument. Some people also call it a directional gyro.

Unlike the artificial horizon, whose axis is aligned to the horizon, the heading indicator measures the deviation from a different datum.

Which is?

Magnetic north.

At the start of the flight, the gyro axis will be aligned ‘level’ to the horizon. The aircraft yaw axis, also called the vertical axis, is pointed at the center of the earth (provided you are level). Effectively, we define the axis around which the airplane will turn. Remember, the gyroscopic axis will stay where we point it.

The easiest way to visualize this is to imagine the airplane moving around the gimbal and its aligned axis.

We rotate a movable card in the instrument to calibrate to magnetic north on a compass. Then we can read how much the aircraft has turned away or towards it. The result in degrees is the aircraft heading.

But remember, because the Earth keeps spinning on its own from underneath you, your heading indicator drifts by roughly 15 multiplied by the sine of your latitude (in degrees per hour). You’ll have to regularly realign it throughout the flight.

How much does it change?

As we saw above, the precession of heading indicators is at its highest at the poles and lowest at the equator. There is a linear scale that we can apply depending on how close we are to either. We call this linear scale Sine.

Sine has its highest value at 90°. Happily, 90° latitude is the North Pole. Sine has its lowest value at 0°. This 0° latitude is at the equator. Suppose the maximum rate of change is 15° per hour at the pole and 0° at the equator. In that case, we can work this into a clever formula to determine the rate of change of a heading indicator due to precession.

It looks like this.

Heading indicator precession = (Sine of your latitude) X 15

Don’t want to sit with a calculator? Don’t worry. We’ve done the hard work using the above equation, so you don’t have to. Here’s a table detailing the rate of precession at each latitude.

Aircraft Latitude	Rate of Precession of Heading Indicator (per hour)
0°	0°
5°	1.3°
10°	2.5°
15°	3.8°
20°	5.1°
25°	6.3°
30°	7.5°
35°	8.5°
40°	9.6°
45°	10.6°
50°	11.5°
55°	12.3°
60°	12.9°
65°	13.5°
70°	14°
75°	14.5°
80°	14.8°
85°	14.9°
90°	15°

The heading indicator drift shows you the importance of realignment intervals on longer flights.

Some Important Points To Note:

• The higher your latitude, the more often you must align your heading indicator. This error is called ‘apparent drift’.
• The heading indicator holds whatever heading you set against the magnetic compass, then drifts on its own.

The Turn Coordinator

The turn coordinator is a little bit different than your standard gyroscopic instrument. The artificial horizon and heading indicator aim to maintain a consistent datum and axis alignment.

However, in the case of the turn coordinator, its axis alignment is short-term and dynamic.

It displays a rate of turn, rate of roll, and slip. One of the most handy references here is the standard rate turn. You’re turning at a rate of 3° per second, so a full 360° takes two minutes.

If the aircraft is banked (a fancy airplane term for one wing higher than the other) but maintains a constant heading, the turn coordinator will display a zero rate of turn.

The gimbal is canted about 30° up from the aircraft’s lateral axis, so it also senses roll rate.

It measures a rate of change from this axis created by yaw. When the aircraft moves left or right, the gyro axis exerts pressure on a calibrated spring that causes the airplane symbol to show a turn in the direction of yaw.

Some Important Points To Note:

• The gyro axis of the turn coordinator is fixed relative to the aircraft, not the horizon.
• There are two parts to this instrument. The rate of turn indicator (gyro-driven) and the slip indicator (the ‘ball in the tube’, which isn’t a gyroscope instrument). This slip indicator is also called the inclinometer.
• In recent years, the turn and slip indicator has been deemed to be a relatively obsolete gyroscopic instrument. AC 91-75 instead talks about how you can substitute an approved attitude indicator for the rate-of-turn indicator required by 14 CFR 91.205(d)(3). According to the FAA, precisely measured standard rate turns matter little in today’s ATC system. But note that the slip/skid ball is still required under 91.205(d)(4).
• So, don’t be surprised if you fly an aircraft with two artificial horizons installed in one panel.

Do Airplanes Still Use Gyroscopes?

Because of their amazing properties and their ability to stay aligned to a given point, occasionally, you’ll find gyroscopic instruments used elsewhere on aircraft.

Want an example?

How about huge airliners like the Airbus A380?

Big aircraft often fly over long distances without navigational aids or radio beacons.

However, they don’t just rely on ‘dead reckoning’. In fact, they use gyroscopes to ensure very accurate navigation.

Most commercial airliners use an inertial navigation system that relies heavily on gyroscopes and accelerometers. While these gyroscopes aren’t quite the same as the gyros you’ll find on light aircraft, they work similarly.

Ring laser gyros are used to fix the position of the aircraft. They are aligned to both the horizon and true north. Then, using some clever computers, the precession is monitored. By working out the precession, the aircraft computers can accurately determine the aircraft’s position.

A modern ring laser gyro has no spinning wheel at all. Instead, it detects rotation using beams of light, which is what makes it so accurate and reliable in an inertial navigation system.

When the aircraft accelerates, decelerates, or changes direction, accelerometers measure what has changed based on a fixed datum provided by the gyros. This change is measured and plotted on a visual display in the flight deck and fed to other aircraft computers.

In many modern glass-cockpit aircraft, an AHRS (Attitude and Heading Reference System) handles this job. It usually uses a solid-state MEMS gyroscope in place of a traditional spinning-wheel gyro.

Frequently Asked Questions

Is the Attitude Indicator the Same as an Artificial Horizon?

Yes. They’re the same instrument under different names. You’ll find attitude indicator, artificial horizon, and gyro horizon all used across FAA publications.

Why Does My Heading Indicator Keep Drifting?

Apparent drift comes from the Earth rotating beneath a gyro that’s holding its position, while real precession comes from bearing friction inside the instrument itself.

Reset the heading indicator against your magnetic compass roughly every 15 minutes to stay accurate.

Can You Legally Fly IFR Without a Turn Coordinator?

Usually, no. 14 CFR 91.205(d)(3) requires a “gyroscopic rate-of-turn indicator.” There’s a narrow exception for airplanes carrying “a third attitude instrument system” usable through 360 degrees of pitch and roll.

AC 91-75 allows that substitution, yet 14 CFR 91.205(d)(4) still separately requires a “slip-skid indicator.”

What Happens if a Gyroscopic Instrument Fails in Flight?

The failure splits along power sources. Lose suction, and your attitude and heading indicators go first, while the electric turn coordinator keeps reading normally.

Don’t over-trust a red flag, though, since it only indicates a loss of power or suction. If the gyro is no longer accurate, the flag won’t always appear.

Conclusion

The compass isn’t necessarily broken when it disagrees with your heading indicator. The Earth kept turning from below, as it always does, and now your gyro has drifted.

The solution, however, will take a change in habit. Have a set schedule to reset the heading indicator. Also, keep in mind which instruments will malfunction during a vacuum failure.

Gyroscopic instruments are a beautiful result of physics, but they only do their job if you truly understand how they work.