Compasses & Navigation, I
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.
So we 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 could even fail or be unserviceable at the time of your flight.
Magnetic and True North
The rotating Earth contains a lot of iron and as such it can be seen as a very large magnet. This magnetic field is not constant all over the Earth as iron concentrations are not evenly distributed. Even the magnetic North pole is not located were the true North pole is. This is called variation or declination and due to the movement of the magnetic pole this varies too.
Making things complicated is navigating with a magnetic compass when the variation is not known for your location. To add to this situation: the magnetic lines (which run from north to south) are parallel near the equator but tend to dive to the ground near the poles, called inclination or dip. Which causes a compass dip error in the higher latitudes, i.e. near the North and South pole.
Adding up to this is the fact that the magnetic North pole is not even fixed in one location. According to the National Geophysical Data Center it is moving northwest with a speed of 40 km per year.
A compass aligns itself with the magnetic lines around the Earth. As explained above, this doesn't mean it is pointing to true north (N90.00). To find true north this compass indication must be adjusted for local magnetic declination and/or variation. The National Geophysical Data Center has a variation calculator online where you can enter your location (lat, long) and obtain the current magnetic declination/variation.
Correcting for true bearing
To get the true bearing from a compass (magnetic) bearing you must add the variation (for your location) to the compass bearing. Just remember that when calculating from magnetic to true: west variations are negative and east variation are positive (its exactly opposite when calculating for your navigation plan). The next links provide you with examples for an east variation
East Variation and a west variation
West Variation (all pictures courtesy from NGDC).
When the aircraft lines up on the runway, verify the compass indication is against the runway numbers. This is just to make sure that you are on the correct runway and that the compass hasn't failed.
Runway numbers (RWY 24) are chosen based on the magnetic heading within 10° for that runway. So that a runway with a magnetic heading of 242° will be designated as 24. Wind reports from the tower controller are also based on the magnetic direction the wind is blowing from. This makes is very easy to assess the crosswind component by the pilot.
In areas where compass indications are unreliable, true headings are used. This is indicated by a T following the runway number, thus RWY 24T.