VHF Omnidirectional Range

A VHF Omnidirectional Range is commonly called a VOR. In some ways, it is like a more-advanced NDB.

To understand what the VOR does take a look at the chart below. Notice the large blue VOR ring, indicating that there is a VOR station at the center. We will dig into what that all means to you as a pilot but first, take a look at the thin blue lines radiating out in several directions from the edge of the VOR ring. Continue reading “VHF Omnidirectional Range”

Non-Directional Beacons and Automatic Direction Finders

The Automatic Direction Finder is an instrument built shortly after the discovery of fire.

It is out of usage now and Non-Directional Beacon (NDB) stations are simply turned off if they break down.

However, despite its slow disappearance, you may very well find one in a plane you fly, and it doesn’t hurt to know how to use it.

The ADF at its core is very simple. It’s just an arrow that points to the NDB station.

Some ADF’s have a moveable compass card in them that can be set when using the ADF. However, many do not contain a gyro and the card will not turn when the plane turns. Rather, you set it while straight and level and use it to navigate. Once you turn you will need to set it again.

There are also ADFs with a slaved compass that rotates to show your current heading. These are more convenient, but either way, it is still a simple instrument.

How to use the ADF

To use it locate an NDB station on your chart with the symbol below.

See the Rainbow NDB station in the center of the image below. The magenta information box to the left contains the name of the station, its frequency (363), its ID (RNB) and the morse code to identify it.

To fly to an NDB station simply tune its frequency and turn so the arrow is pointed up. Don’t forget to listen to the station ID just like you would for a VOR.

It isn’t too much more complicated than that. If you want to approach the station on a specific heading then it helps to visualize your current situation before acting.

For example, say you want to approach Rainbow from directly South of the station. The needle is currently pointing to a heading of 330. This means that you are South East of the station. I like to look at the needle and then point out the window in the direction of the station. It helps me to get more situational awareness and determine which way to go. If you want to approach heading 360 then you will need to go to the left and watch the needle until it is pointing directly North.

Try this out next time you have access to an ADF, because it may be your last chance!

Bonus Feature

The ADF has one more important but little-known feature. It happens to fall on the same range of frequencies as AM radio so if there are any good stations in your area you can listen to them. Just tune to the station and press the ADF button on your audio panel. Instead of hearing morse code, you will hear AM radio. As you listen, the needle will point to the radio stations antenna as well!

Instrument Flight Rules

Instrument Flight Rules, commonly called IFR, are a set of rules that apply to planes flying by instrument reference. This is in opposition to VFR (Visual Flight Rules), flown by visual references. Basically, if you can see where you are going, then VFR is an option, but IFR is always an option.

This set of rules requires a pilot to have an instrument flight plan and follow a set of procedures that govern communication and navigation.

IFR flying requires constant communication with ATC and a mixture of visual traffic separation when you are in VMC and reliance on ATC for separation when you are in IMC.

How do I know if I’m flying IFR?

If you have to ask, you’re not flying IFR!

But seriously, to fly IFR you will need to file an IFR flight plan, get a clearance from air traffic control, and comply with that clearance. You can fly IFR in VMC, but you can only log IFR, or “flight by reference to instruments” time when you are flying in actual IMC.

Flight Instruments: Directional Gyro

The directional gyro is a fairly simple instrument. It indicates the direction that the aircraft is heading. However, it does not sense the direction that the aircraft is heading.

How does the DG know which way the aircraft is heading?

The heading must be set in advance and the DG will keep track of whatever was set.

To keep track of the heading it uses a gyroscope. This method has some serious improvements over magnetic compasses, but a few drawbacks.

Source: FAA Pilot’s Handbook of Aeronautical Knowledge

The DG does not have the acceleration and turning issues that a compass has, so it is much easier to turn to a given heading without having to think about the lead and lag of the compass.

The presentation is just plain nicer and easier to read, but it also allows for more advanced add-ons like a heading bug all the way up to a full HSI.

For these reasons, you will be very hard-pressed to find a plane without a DG, even though a magnetic compass is sufficient to determine your heading.

When do I need to set my DG?

The DG needs to be set before takeoff and sometimes in flight.

Look at the knob in the image above. If you push that knob in and turn it, the whole card will rotate left or right as you turn. The tiny airplane image stays fixed in place as the headings turn under it. This knob is how the DG is set. Usually, this is done before taxi and then checked again after the runup.

In order to set it, we need to know the plane’s current heading. Luckily, on the ground a magnetic compass works pretty well so we simply read the magnetic compass and then turn the knob until that heading is set at the top of the instrument.

The instrument can be wrong for a number of reasons. Simply put, once the gyro spools down (when the plane is off), there is nothing to hold the compass card rigid in space. It tends to naturally be off by a few degrees every time you start up.

Imagine if you shut down with the DG pointing to 050 (like in the image above). Then you hook up the tow bar and move the plane into a tie-down spot facing the opposite direction. You have just turned the plane all the way around. What will happen to the DG? You will get in the plane and start-up facing 230, but the DG will still show 050.

Checking that the DG matches the compass is critical to removing these errors and being able to rely on the DG.

But there’s more. You will also need to set the DG in flight. Over time, due to internal friction, the gyro will precess out of place a little and the heading will be off by a few degrees. Personally, I set this using a mixture of procedures and situational awareness.

If I am on a cross-country, travelling somewhere, it is good to do this periodically as part of a cruise checklist (which you should be doing every so often during cruise).

As for situational awareness, there are times when the heading doesn’t seem to make perfect sense, or some reference point on the ground is 10+ degrees from where you thought it would be. This is a great time to check the DG and see that it matches the compass.

To set the DG in flight make sure that the wings are level and the plane is not accelerating (or slowing down). Then simply set it like you would on the ground. That’s all there is to it.

Instrument Meteorological Conditions

Instrument meteorological conditions (IMC) exist when an aircraft is in weather conditions that are not within the VFR visibility and cloud clearance requirements.

This means that you are in IMC if:
• You are inside of a cloud
• The visibility is too low
• You are too close to a cloud

When you are in IMC you need to be flying an instrument flight plan. Flying VFR into IMC is very dangerous and there are a multitude of accidents that occur this way. Don’t be a statistic! Stay away from the clouds.

In fact, I recommend having personal minimums for visibility and cloud clearance that are even more restrictive than the rules.

Understanding Gyroscopic Precession

Airplanes use gyroscopes in many of their instruments. There are also some aerodynamic effects from the rotating movement of the propeller that require an understanding of gyroscopes. This post will focus only on precession and not on the main attributes of gyroscopes.

Gyroscopic precession affects rotating objects like a propeller or the classic example, a bicycle wheel. As an object rotates it will translate forces applied to it by 90 degrees.

If you push sideways on the top of a moving bicycle wheel, the bicycle will not fall over. Instead, it will turn because the front of the wheel will move in response to the force.

Now let’s think about the rotor of a helicopter, which is also subject to gyroscopic precession. Imagine that the blades are spinning around flat and just one of them is tilted for an instant to gather more lift. During this instant the tilted blade will have an upward force applied to it. It also already has an enormous amount of momentum and will continue spinning at a high speed.

The lift will cause it to begin accelerating up, but the momentum will very quickly move it farther around the rotor disk. Combining these two actions results in the blade rising until it reaches its peak 90 degrees from the point where it was pushed up.

Source: FAA Helicopter Flying Handbook

For example, if the blade (rotating counter-clockwise if you look at it from above) tilted momentarily while passing over the back of the helicopter then the whole rotor disk would tilt left because the right side would go up.

Confused yet? Gyroscopic precession is weird but it makes sense. If you can understand this 90 degree translation of force it will help you to understand:

  • Why airplanes have left turning tendencies
  • Why helicopter controls are mounted 90 degrees from the direction intended
  • How some airplane instruments work
  • Why your bike or motorcycle doesn’t tip over

Here are some additional resources I recommend:


Flight Instruments: Magnetic Compass

Airplane instruments and systems are usually as simple as possible. This is because simple systems will break less often. The magnetic compass is one of the simplest instruments there is.

How the magnetic compass works

A compass is made up of a housing with a lens on the front and a vertical line (called a lubber line) inside the glass representing the current heading. Inside of the housing, there is a liquid with the compass “float” suspended on a pivot. The float itself has the sensing magnet inside and markings for every heading. As the aircraft turns the float spins and indicates the planes heading along the lubber line.

There is also a second adjustable magnet in the bottom of the unit to correct for errors.

Finally, most magnetic compasses will have a light mounted above the lens so it can be viewed at night.

Source: FAA

Magnetic Compass Errors

The construction of the compass causes a few problems when reading it during turns and changes in speed. There are 3 basic types of errors.

Oscillation Errors

This is the simplest type of error. In turbulence, the indicator may bounce around because it is floating. If the compass is moving around continuously don’t expect to get a precise heading from it.

To determine your heading during turbulence, look for the midpoint or average of the oscillations.

Dip Errors – Turning

When banking the compass will turn to follow the vertical component of the earth’s lines of magnetic flux. In other words, the compass is drawn down towards the earth.

So if you are heading North and you start a turn the compass will try to point down towards the low wing. As the magnet is drawn down it will turn the indicated magnetic heading indicating a turn in the wrong direction.

Conversely, if you begin a turn while heading South the compass will indicate a turn in the right direction but it will turn more than your actual heading.

During turns, the compass lags when you are heading North and leads when you are heading South.

Remember OSUN or UNOS.

  • Undershoot North
  • Overshoot South

Dip Errors – Accelerating

When the aircraft accelerates the inertia of the heavy magnet causes the compass to rotate. It pulls towards the Northerly heading. Conversely, when slowing down the magnet pulls the card towards a southerly heading as it is moved forwards. This effect is most prominent when heading East or West and doesn’t have any effect when heading North or South.

Remember ANDS.

  • Accelerate North
  • Decelerate South


An aircraft is full of magnetic parts and flowing electrical currents that can interrupt the magnetic compasses ability to sense magnetic North. The adjustable compensator is set by a technician to account for these errors.

However, the compensator can’t fix this completely so a compass card is included with deviations corrections. When flying a magnetic heading read the heading you want under “For” and then turn to the indicated heading under “Steer”. Don’t forget about this when setting your directional gyro to match your compass.