The Magnetic Compass
The Knowldedge Tests have lots of questions on the compass, especially turning errors. The Handbook of Aeronautical Knowledge covers the questions well so I’ve quoted it here with bold for things to remember. Items in brackets […] is added.
Variation
Although the magnetic field of the Earth lies roughly north and south, the Earth’s magnetic poles do not coincide with its geographic poles, which are used in the construction of aeronautical charts. Consequently, at most places on the Earth’s surface, the direction- sensitive steel needles that seek the Earth’s magnetic field will not point to true north, but to magnetic north. Furthermore, local magnetic fields from mineral deposits and other conditions may distort the Earth’s magnetic field, and cause additional error in the position of the compass’ north-seeking magnetized needles with reference to true north.
The angular difference between magnetic north, the reference for the magnetic compass, and true north is variation. Lines that connect points of equal variation are called isogonic lines. The line connecting points where the magnetic variation is zero is an agonic line. To convert from true courses or headings to magnetic, subtract easterly variation and add westerly variation. Reverse the process to convert from magnetic to true.
Compass Deviation
Besides the magnetic fields generated by the Earth, other magnetic fields are produced by metal and electrical accessories within the airplane. These magnetic fields distort the Earth’s magnetic force, and cause the compass to swing away from the correct heading. This error is called deviation. Manufacturers install compensating magnets within the compass housing to reduce the effects of deviation. The magnets are usually adjusted while the engine is running and all electrical equipment is operating. However, it is not possible to completely eliminate deviation error; therefore, a compass correction card is mounted near the compass. This card corrects for deviation that occurs from one heading to the next as the lines of force interact at different angles. [Figure 6-22]
[Note: A non-magnetic screwdriver, usually brass, must be used to make the corrections to the compass card. This is often referred to as “swinging the compass”. Normally, corrections are made at a compass rose painted on the surface of an airport. Airports with compass roses can be found in the A/FD. To swing your compass, the A&P will put the airplane on a north heading, then adjust the compensator so the compass points north, then they will turn to a south heading and whatever the error is they will split the difference. Then do the same with east and west. After that they position the airplane to the cardinal headings and record the compass heading, constructing a new deviation card. The process can be repeated with the avionics off if desired. Link]
Magnetic Dip
Magnetic dip is the result of the vertical component of the Earth’s magnetic field. This dip is virtually non-existent at the magnetic equator, since the lines of force are parallel to the Earth’s surface and the vertical component is minimal. When a compass is moved toward the poles, the vertical component increases, and magnetic dip becomes more apparent at higher latitudes. Magnetic dip is responsible for compass errors during acceleration, deceleration, and turns.
Using the Magnetic Compass
Acceleration/Deceleration Errors
Acceleration and deceleration errors are fluctuations in the compass during changes in speed. In the Northern Hemisphere, the compass swings towards the north during acceleration, and towards the south during deceleration. When the speed stabilizes, the compass returns to an accurate indication. This error is most pronounced when flying on a heading of east or west, and decreases gradually when flying closer to a north or south heading. The error does not occur when flying directly north or south. The memory aid, ANDS (Accelerate North, Decelerate South) may help in recalling this error. In the Southern Hemisphere, this error occurs in the opposite direction.
Turning Errors
Turning errors are most apparent when turning to or from a heading of north or south. This error increases as the poles are neared and magnetic dip becomes more apparent. There is no turning error when flying near the magnetic equator.
In the Northern Hemisphere, when making a turn from a northerly heading, the compass gives an initial indication of a turn in the opposite direction. It then begins to show the turn in the proper direction, but lags behind the actual heading. The amount of lag decreases as the turn continues, then disappears as the airplane reaches a heading of east or west. When turning from a heading of east or west to a heading of north, there is no error as the turn begins. However, as the heading approaches north, the compass increasingly lags behind the airplane’s actual heading. When making a turn from a southerly heading, the compass gives an indication of a turn in the correct direction, but leads the actual heading. This error also disappears as the airplane approaches an east or west heading. Turning from east or west to a heading of south causes the compass to move correctly at the start of a turn, but then it increas- ingly leads the actual heading as the airplane nears a southerly direction.
The amount of lead or lag is approximately equal to the latitude of the airplane. For example, if turning from a heading of south to a heading of west while flying at 40°north latitude, the compass rapidly turns to a heading of 220°(180°+ 40°). At the midpoint of the turn, the lead decreases to approximately half(20°), and upon reaching a heading of west, it is zero.
[FROM a North or South heading turning TO a heading of East or West: Opposite first, then Lag North, Lead South due to Northerly turning error.]
The magnetic compass, which is the only direction-seeking instrument in the airplane, should be read only when the airplane is flying straight and level at a constant speed. This will help reduce errors to a minimum.
Instrument Check—Prior to flight, make sure that the compass is full of fluid. Then during turns, the compass should swing freely and indicate known headings.
Vertical Compass Card
A newer design, the vertical card compass significantly reduces the inherent error of the older compass designs. It consists of an azimuth on a rotating vertical card, and resembles a heading indicator with a fixed miniature airplane to accurately present the heading of the airplane. The presentation is easy to read, and the pilot can see the complete 360° dial in relation to the airplane heading. This design uses eddy current damping to minimize lead and lag during turns.
Handbook of Aeronautical Knowledge p 6-15, 6-16
14 CFR § 23.1547 Magnetic direction indicator.
(a) A placard meeting the requirements of this section must be installed on or near the magnetic direction indicator.
(b) The placard must show the calibration of the instrument in level flight with the engines operating.
(c) The placard must state whether the calibration was made with radio receivers on or off.
(d) Each calibration reading must be in terms of magnetic headings in not more than 30 degree increments.
(e) If a magnetic nonstabilized direction indicator can have a deviation of more than 10 degrees caused by the operation of electrical equipment, the placard must state which electrical loads, or combination of loads, would cause a deviation of more than 10 degrees when turned on.
14 CFR § 91.205 Powered civil aircraft with standard category U.S. airworthiness certificates: Instrument and equipment requirements.
(b)Visual-flight rules (day). For VFR flight during the day, the following instruments and equipment are required:
(3) Magnetic direction indicator.