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.
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.
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.
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.
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.