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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.

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Cruising Altitude

When flying anywhere you need to climb, cruise, and then descend. But you must comply with the rules and fly at certain predetermined altitudes.

The Rules: Neodd and Sweven

When you are flying above 3000 feet AGL you must fly at an even-numbered thousand feet if you are traveling west. That is what sweven means. If you are traveling directly South, or on any Westerly heading then fly on the even-numbered thousands.

Conversely, if you are traveling East or directly North, fly at an odd thousand (neodd).

For IFR flights to the East you will fly at 5000 or 7000 or 9000 feet, etc….

For VFR flights you must be 500 feet above these altitudes. So a VFR flight to the West would cruise at 4500 or 6500 or 8500 feet, etc….

These rules are for cruising altitude, meaning that if you are flying up above 3000 to practice an emergency descent then you can just climb to whatever altitude you want.

Why is this rule in place?

This rule is a bit of a compromise between safety and simplicity.

When two planes approach head-on their closure speed, the speed at which they are approaching each other, is very high. Even a relatively slow 152 will approach another 152 at around 180 knots TAS if they are head-on. For faster planes like an arrow traveling at 150 knots the closure speed is 300 knots!

It is safer to fly at these altitudes because VFR planes flying East will always be 1000 feet vertically separated from VFR planes flying West. Furthermore, they will be separated by 500 feet from all IFR traffic.

This is great, but there is a problem! What if planes approach nearly head-on but both traveling East? One of them could be traveling 010 degrees (which is East of North) and the other could be flying at 170 (which is East of South).

This is where the compromise comes in. The rule could split the compass into 4 segments but then it would be more complicated and difficult to remember.

Always stay vigilant looking for traffic that might be climbing/descending, not following the rules, or might be at a near head-on angle.

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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

Deviation

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.