The Heading Indicator or HI is also known as the Direction Indicator or DI, or sometimes the Directional Gyro. This instrument is used in almost all airplanes to show heading information. The Heading Indicator is situated on the instrument panel and is directly in front of the pilot in most planes.
This is because, along with the altimeter and airspeed indicator, it is one of the most important instruments needed to ensure an accurate and safe flight.
The Heading Indicator basically complements the magnetic compass, which is actually the primary indicator of direction, as you might expect.
However, for a number of reasons magnetic compasses are not that accurate when flying a plane, the Heading Indicator is used both for additional accuracy and to make things very much easier for the pilot.
During my many years of piloting a large number of aircraft of all types, I have used these little instruments frequently. I can confirm that they are extremely useful. I have also flown aircraft that do not have one, as is the case if you fly some old aircraft or a number of homebuilt types.
So I have found out from experience how much more difficult it is to be forced to use the magnetic compass alone, as of course early pilots used to do. It is certainly not impossible, but it is much harder.
This article will look at all aspects of how the Heading Indicator works and why it is used, and also how to fly in an aircraft that does not have one, in case you should ever have to do so.
Bottom Line Up Front
The magnetic compass is the primary indicator of heading information in an aircraft, and knowing your heading is essential in order to find your route. However, the magnetic compass is inaccurate due to acceleration and turning errors.
For this reason, virtually all modern aircraft use a Heading Indicator, which is a gyroscopic instrument, so it is not dependent on the earth’s magnetic field in the way that a compass is.
This Heading Indicator is then manually aligned to the compass, in order to accurately work out the aircraft’s heading. Heading Indicators are easy to use, but they do have some of their own errors and idiosyncrasies, so it is necessary for pilots to understand them properly in order to be able to use them to full advantage.
What is the Aircraft’s Heading?
Let’s start right from the beginning, and define the term ‘heading’? Pilots need to know in which direction they should fly in order to reach their destination. They need to follow a particular magnetic track, which can often be found on a map or by following features on the ground.
However, the wind continuously pushes the aircraft off track. This means that the pilot needs to calculate the magnetic direction in which he or she needs to fly in order to actually follow the correct track along the ground.
This direction is called the aircraft’s ‘heading’, and headings are usually worked out relative to magnetic north. The heading needed for each flight, or each leg of a longer flight, is worked out in advance by a pilot before he or she flies a particular route.
Using the Magnetic Compass to Find the Heading
There is a magnetic compass in every aircraft, and this is the primary instrument for calculating the required aircraft heading. However, magnetic compasses are not accurate in flight at all times. The compass suffers from acceleration and turning errors.
This means that in steady flight it is fine, but as soon as you turn, accelerate, or decelerate, the compass will no longer indicate the correct heading. For example, in the Northern Hemisphere, as you accelerate, your compass will show a turn to the North.
And as you decelerate, your compass will show a turn to the South. When the speed stabilizes, the compass returns to normal. Similar errors occur when you turn the aircraft, and things are different if you are in the Southern hemisphere.
It is possible to use the compass to find your heading if you have to. I have done so fairly frequently, and flown a route quite successfully, without getting lost!
But early pilots found that flying this way was difficult and inaccurate, and also it really added to pilot workload. Clearly another instrument was needed which did not suffer from these errors. And so the Heading Indicator was invented.
What is a Heading Indicator
The Heading Indicator is a gyroscopic instrument. Its workings are quite complicated, but explained very simply, it works by sensing the aircraft’s yawing plane, which is the plane defined by the horizontal and longitudinal axis of the aircraft.
Since it is not reliant on the earth’s magnetic field in any way, the Heading Indicator does not suffer from the magnetic acceleration and turning areas of the magnetic compass.
The Heading Indicator is connected to a compass card, which is the part of the instrument visible to the pilot on the instrument panel. The compass card moves in relation to the aircraft heading, and then displays the compass direction in five-degree increments. Thus you can read it quite easily, just as though it is a compass.
Indeed, some novice pilots who have not had the benefit of studying an article like this often think it is simply another style of magnetic compass! I believe I felt this way in my very dearly flying days.
But it is not a compass, and it works very differently. Firstly, it has to be set up before the flight so that it is aligned to the magnetic compass and will read correctly. It then needs to be realigned at frequent intervals during the flight, for various reasons which we will now look at in more detail.
Heading Indicator Errors
The Heading Indicator may not suffer from the same errors as the magnetic compass, but it does have some of its own. Mechanical factors, mainly friction, cause it to drift away from its original setting, because of gyroscopic precession.
This is known as mechanical drift. Another error is apparent drift, caused by the fact that Heading Indicator is a gyro that is orientated in space, so it is affected by the earth’s rotation. This fact may cause it to change by 15 degrees for every hour of use.
Please don’t worry if you don’t understand the above. It is unnecessary for a light aircraft pilot to follow these explanations in any detail. The light aircraft pilot simply needs to know that these errors occur, no more than that.
And because they occur, the Heading Indicator has to be realigned to the magnetic compass at regular intervals during the flight.
How to Align to the Heading Indicator to the Magnetic Compass
A small knob on the Heading Indicator allows it to be re-aligned to the compass during flight, correcting for both mechanical and apparent drift. This should be done every ten to fifteen minutes during flight, and more frequently than this in windy or stormy conditions or if doing maneuvers such as aerobatics.
To manually align the heading indicator you need to do the following…
- Fly steadily, straight and level, towards a reference point in front of you. According to the text books it is preferable if you can do this in still air, although of course in real life that is not always possible.
- While keeping the aircraft nose steadily on the reference point, wait until the magnetic compass is steady. Then carefully read the compass heading.
- Carefully maintaining your heading, adjust the Heading Indicator’s knob so that it is indicating the same heading as the compass.
- Check that your compass heading is still the same, and that it still agrees with the Heading Indicator. If this is not the case, repeat the whole procedure.
This might sound complicated, but trust me, it really is not. After a little flying experience, setting the Heading Indicator at frequent intervals becomes second nature and just takes a few seconds.
See also: Best Kneeboards for Pilots Guide
Important Points when Using the Heading Indicator in Flight
- Check the power source of the Heading Indicator prior to flight
- Check that the instrument is working correctly and turning when it should; you can do this during taxying.
- Remember to align the Heading Indicator at the start of the flight
- Realign it frequently, at least every 10-15 minutes
- It should be noted that the Heading Indicator numbers have the final zero omitted. For example, 5 represents 050 degrees, while 22 indicates 210 degrees. Don’t let this confuse you.
- If for any reason your Heading Indicator stops working – and it is possible – don’t panic. It is perfectly possible to find your heading using the magnetic compass.
You should now be in a position to fly accurately using the Heading Indicator.
Frequently Asked Questions
Question: I Need to Fly a Very Old Aircraft that does not have a Heading Indicator. Do I have to Use the Compass Instead, and will this Give Me Any Problems?
Answer: It should give you no serious problems. Many pilots talk as though it is nearly impossible to use the compass to find your heading, but I have done it often, and this is not the case. Theoretically, you need to know which direction the compass will move when the plane is accelerating and turning in order to correct for these errors.
In other words, should you roll on your heading early, or late, and do you need to under-correct or over-correct? To be honest, I have never bothered with all the details of this. While you are flying straight and level, the compass will accurately indicate your heading.
If you turn, or speed up or slow down, simply pick the heading, then wait for the compass to settle down, then correct it. This may be slightly slower than doing it the ‘correct’ way, but trust me, it works, though the purists really don’t like it! But it saves you remembering the theory of compass errors at a perhaps high workload phase of flight.
Question: If I Forget to set the Heading Indicator, how Inaccurate will it be?
Answer: It will be reasonably accurate for a few minutes or so. But if you leave it for too long it could be very inaccurate. So you need to check it quite frequently, and realign it during flight.
If at any time you’re not sure of your heading during flight, check that your heading indicator is correctly aligned, as forgetting to align the Heading Indicator has been the cause of more than a few pilots getting well and truly lost!
Question: I’ve been Told that I can Use the Heading Indicator to Find the Runway when Approaching an Airfield. How do I do this?
Answer: It is very simple. You simply imagine the runway of the airfield you are approaching extending across the Heading Indicator. So long as your Heading Indicator is properly adjusted, when you look outside you will see that the runway is in the same orientation as you visualized it on the Heading Indicator.
Using this technique can keep you out of trouble by helping you line up for the right runway, as you can work out where it is long before you can see the runway numbers on the tarmac.
Question: Do Some Modern Heading Indicators Align Themselves Automatically?
Answer: Yes, they most certainly do. Some more expensive and modern Heading Indicators can continuously sense the Earth’s magnetic field, and a servo mechanism constantly corrects the Heading Indicator, by some very clever mechanism.
These ‘slaved gyros’ reduce pilot workload by eliminating the need for realignment of the Heading Indicator every ten to fifteen minutes.
I have never been lucky enough to fly a plane with one of these, but it sounds like a great idea, for while re-aligning the Heading Indicator is not complicated, it certainly adds to a pilot’s workload, and anything which reduces that is to be enthusiastically welcomed.
An aircraft’s Heading Indicator is a very common and useful instrument. It is not absolutely essential during flight, but it is certainly advantageous to have one. It is very simple to operate, and although the details of exactly how it works are quite complicated, using it is not.
So hopefully you now understand the use of the Heading Indicator better than was the case before you read this article. And happy flying!
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- Learn to fly blog, http://learntoflyblog.com/2016/11/14/flight-instruments-the-heading-indicator-and-magnetic-compass/
- Flight Literacy, https://www.flightliteracy.com/gyroscopic-instruments-part-one-attitude-and-heading-indicators/
- Flying Magazine, https://www.flyingmag.com/technique-tip-week-use-hi-find-runway/