Compasses & Navigation, II
To navigate using a compass without getting lost it is necessary to understand how they work. On this page we talk about the Earth's magnetic field, where true and magnetic north are and the effect of this variation on the compass onboard the aircraft. Navigating the aircraft by using only the magnetic compass, clock and sectional is not regularly done by every pilot these days, it is almost becoming a lost art with all these GPS driven EFIS navigation systems available.
We are going to expand on that subject a little as I believe that basic flying by compass, map and clock should be the primary navigation method and be complemented by radio- and satellite navigation as these systems could even fail or be unserviceable at the time of your flight.
We all know that nothing is perfect and so is the magnetic compass, it suffers from a number of peculiar effects which influence the way it operates and the pilot needs to be aware of these effects.
There is a mechanical construction in the compass which is the main cause of turning and accelerating errors: The card is mounted in such a way that its center of gravity is lower than the pivot point. And any movement from the aircraft will cause a force to act on that center of gravity thereby moving the compass card.
Variation and Deviation
Variation (or declination) is caused by the fact that the magnetic north pole is not located at the same spot as the true North pole. Nothing we can do about that.. Some other definitions: lines of equal variation are called isogonic and were the variation is zero it is called an agonic line.
Deviation is a error caused by influences of the vehicle where the compass is installed. This can be caused by ferrous materials, wiring with running currents from, for example, radio installations. Deviation is written on a little chart located on or near the compass, it could show: to fly heading 090 stear 093.
Angle of Dip Error
Magnetic lines of flux around are parallel at the magnetic equator but they dive down to the ground as they approach the magnetic poles. This is called dip. The magnet in the compass will align itself with the magnetic flux lines and as such will dip too. But the construction of the magnet is such that they are below the pivot point and residual dip is reduced to about 3°. Lines of equal dip are called isoclinals.
When accelerating or decelerating on an easterly or westerly heading the compass will make a turn. Through inertia in the compass and because the acceleration/decelerating vector is more or less perpendicular to the earth magnetic lines this will result in a turning of the compass. On a northerly or southerly heading the compass will usually not turn but dip. Remember ANDS: Acceleration gives a turn to the North and a Deceleration to the South in the northern hemisphere.
In the southern hemisphere it is exactly the opposite: an Acceleration gives a turn to the South and a Deceleration a turn to the North.
When the aircraft is in a banked turn, the card also banks because of centrifugal force. And in this attitude, the vertical component of the Earth's magnetic field causes the compass to dip to the low side of the turn.
This compass turning error is most apparent when turning through headings close to north and south. When the aircraft makes a turn from a heading of north, the compass briefly indicates a turn in the opposite direction. When the aircraft makes a turn from a heading of south, the compass indicates a turn in the correct direction but at a considerably faster rate than is actually occurring.
Thus, when making a 360° right turn beginning at north, the card initially turns in the wrong direction; then, as the aircraft passes through East, the compass "catches up" with the actual heading. Passing through South, the compass leads the turn considerably. As the aircraft heading passes through West, the compass should approximate the correct heading. Then, as the aircraft heading approaches North again, the compass lags.
To compensate for this error and turn on headings you should overshoot turns on northerly and undershoot on southerly headings to maximum of 30° on a heading of 360° or 180°. Rolling out on a heading of 090° or 270° requires no compensation for this turning error.
Turbulence is not a real error. It makes the compass unstable and it has therefore more or less unreliable indication. Gyro compasses give a much stabler reading in turbulence, problem with these gyro's is that their indication changes due to drift in the instrument and the rotation of the Earth, 15°/hour (as they are fixed to one point in space). Earth's rotation can be compensated.