Our aeroplane, Tweety, has two compasses. One is a magnetic compass and one is a directional gyro.
To understand them better I did some study to refresh my memory on the subject of direction from the navs course. I’ve written it down because maybe if I can explain it, I can understand it
Direction – true, magnetic and compass
As a pilot I need to know where I am and where I’m going. And I need to be able to let other pilots know where I am. Direction and expression of direction is based on geographic and magnetic poles of the Earth.
True Direction – geographic poles
Early astronomers named direction as north, south, east and west according to how these points related geographically. This is what we find on our maps and charts.
Magnetic Direction – magnetic poles
The earth acts like a large magnet in space. It’s constantly moving in a circle slowly around and close to, the geographic poles.
‘Magnetic’ poles are measured using a free swinging magnet and described by the angle they have away from true North.
The difference in magnetic and geographic poles is measured in degrees east or west. This measurement is called a variance and is indicated on maps and charts by isoganals.
The variance is constantly changing. That’s why it’s important to use current charts with up to date isoganals.
Directions on our maps are true directions but directions in our aircraft are shown by a magnetic compass.
So we need to convert true directions to magnetic directions and then we can use our magnetic compass to steer ourselves.
Depending on the direction East or West, the conversion is:
Direction (T) minus Variation (E) = Direction (M)
Direction (T) plus Variation (W) = Direction (M)
A good way to remember this is to think:
‘East is Least and West is Best’
- Draw a line between two points on a map
- Measure the distance in NM
- Add or subtract the variance shown by the isoganals to convert true direction to magnetic direction
The magnetic compass in our airplane is affected by electrical, magnetic and metallic interference. This can cause the compass to line up in a direction different from magnetic north.
This is called a deviation.
Engineers adjust the magnetic compass (the compass is ‘swung’) to minimise the deviation.
The deviation is measured again and a deviation correction card is kept in the cockpit so the pilot can check it.
In practise, deviation can be ignored if it’s small.
Variance, however, can never be ignored. In Australia, variation can be from 5° west to 15° east. Ignoring variation can result in a significant navigational error.
Two Magnetic Compasses?
A pilot uses a compass to establish a heading.
A magnetic compass will be affected by the slope of the Earth’s magnetic field (called the ‘dip’). This will cause it to read incorrectly when the aircraft is in a bank or during acceleration. It will work well when flying straight and level.
A directional gyro (or heading indicator) is a gyroscopic flight instrument not affected by the ‘dip’ when the aircraft is banking or during acceleration. It should be set every 15 minutes using the heading shown on the magnetic compass.
South Coast Recreational Flying Club Navigation Course notes
Ground Training Manual by Jan and Val Dyson- Holland
Fly high; Fly safe,