magnetic compass

The magnetic compass is the only instrument in the aircraft by which the pilot determines the direction of flight. Magnets in the compass cause it to align with the Magnetic North Pole. It is nearly always fitted on top of the visor that shades the instrument panel or is hung from the top of the windshield frame in order to keep it as far away from electrical gear as possible.

The compass card has the four cardinal headings shown as N, E, S, and W. Numbers appear every 30 degrees. Long vertical marks occur in 10 degree increments, with intervening short marks at 5 degree points. The compass card containing the magnets are mounted on a small pivot point in the centre of the card assembly. This allows the compass card to rotate and float freely. It is somewhat like suspending a paper cup, upside down, on a pencil point located at the centre of the cup bottom.

The enclosure is filled with white kerosene to provide a medium to dampen out some vibration and unwanted oscillations. A "lubber line" in etched on the glass face of the instrument to enable exact reading of the compass.

Magnets in the compass align themselves along a Magnetic North-South orientation. Whenever the aircraft is headed toward magnetic North, the compass will indicate N. If the aircraft turns from this direction, the magnets in the compass still align to this N-S direction. Similar to a gyro, the case of the compass and the lubber line is fixed to the aircraft. Thus when the airplane turns, the the case turns about the compass card. The lubber line will then show a reading other than North.

Compass Errors

Magnetic lines of force surround the Earth, flowing from the North to South Magnetic poles. The magnetic field strength is greatest near the magnetic poles and weakest at the Equator. Several compass errors can occur.

These are:

1. Magnetic Variation
2. Compass Deviation
3. Magnetic dip
4. Compass Card oscillation

Magnetic Variation

The Magnetic North Pole and the True North Pole are not at the same location on the surface of the earth. Magnetic Variation at any given location on the earth’s surface is the difference between the Compass North and True North. The map below shows the magnetic variation at various locations in the US.

The Agonic Line is the line of zero degree variation. It proceeds from upper Michigan through central South Carolina. Variation values to the East of the agonic line are called Westerly Variation; i.e. the magnetic north pole is West of True North. Likewise, the variation values west of the agonic line are known as Easterly Variation; i.e. the Magnetic North Pole is East of True North.

Magnetic North changes in small amounts each year. Aeronautical charts are updated periodically to correct for this yearly change.

When plotting a course on an aeronautical chart, the degrees of heading are measured against latitude and longitude lines. This is called a True Course (TC) because it is being measured relative to the True North Pole. Since the pilot relies on the magnetic compass for direction, the pilot will be steering the aircraft relative to the Magnetic North Pole. Therefore, the pilot must convert the True Course (TC), as plotted on the navigation chart, to a Magnetic Course (MC) by which to steer using the compass. . To convert from TC to MC, Westerly Variations must be ADDED to TC to get MC (see right hand example below). MC = TC + VAR. (MC = 45° + 10° = 55°). In other words, the pilot must steer 55° magnetic to fly over a true course of 45°. Likewise, Easterly Variation must be SUBTRACTED from TC to get MC (see left hand example below).

Compass Deviation

Magnetic deviation is the difference between the compass indications when installed in the aircraft compared to the indications when the compass is outside the aircraft. The cause of this difference is that the compass magnets can be influenced by magnetic fields within the aircraft due to electronic equipment and other factors. These magnetic disturbances may cause the compass readings to be slightly in error. Such errors are called Compass Deviation.. In other words, the compass reading when inside the aircraft "deviates" from a normal reading.

To determine compass deviation, the aircraft is parked on a compass rose painted on a level surface such as a ramp or taxiway. All of the electronic equipment is powered on as in normal operation. The nose of the aircraft is placed on the Magnetic North marking on the ground. Deviation in the compass reading (from North) is recorded. The aircraft is then rotated to 30 degrees to the right, and the deviation noted. The aircraft is turned in increments of thirty degrees through the 360 degrees, and deviation from the proper reading is noted. This procedure is called swinging the compass. These errors are posted on a Deviation Card placed at the lower portion of the compass. For example, it may state for a course of 180, steer 178°.

Usually the errors are only a few degrees, but should be taken into consideration by the pilot then tracking a given magnetic course.

Compass Dip Errors

Any time the compass card is not perfectly level, the magnets dip downward toward the earth. The result is that the compass does not correctly align with Magnetic North the same as when the card is level. This results in erroneous indication while in the non-level state. Dip occurs under 2 conditions.
1. During turns from the north and south. (i.e. Plane is in a bank.
2. During acceleration or deceleration while on an East or West heading.

Compass Turning Errors

When the aircraft initiates a right turn from the North, the dip of the compass causes the compass to initially indicate a turn IN THE OPPOSITE DIRECTION (i.e. the compass turns left). The amount of initial error is approximately equal to the Latitude position of the aircraft. If at a 30 degree latitude, and a right turn from North is initiated, the compass card will initially turn LEFT to 330 degrees. As the right turn to the EAST proceeds, the compass will start to catch up, so that when EAST (090) degrees is reached the compass will indicate correctly, even though the aircraft is still banked.

If the turn is LEFT from NORTH, the compass will turn right to 030 degrees, and will catch up by the time WEST (270) degrees is reached.

THEREFORE, WHEN TURNING FROM NORTH, THE COMPASS LAGS. If turning to the North, you will have to roll back to straight and level approximately 30 degrees prior to reaching North on the compass.

When turning from SOUTH, the opposite action occurs; the compass LEADS by the amount of the degrees Latitude. If at 30 degrees Latitude, the lead will be approximately 30 degrees. If you are turning to the South, you will have to roll back to straight and level approximately 30 degrees past South reading.

MEMORY AID: Turn to N, Under Shoot. Turn to South, Over Shoot.

Compass Acceleration Errors

When the aircraft is on an East or West heading, acceleration or deceleration of the aircraft causes the compass card to tilt forward or backward, respectively. This tilting causes the compass card magnets to swing downward toward the earth, which in turn causes the compass to rotate to an incorrect indication.

This error is maximum when on an East or West heading, and gradually diminishes to zero when a North or South heading is reached. Acceleration of the aircraft causes the compass to erroneously swing to the North. Deceleration causes erroneous rotation toward the South.

Again, the error is approximately equal to the Latitude degrees of the aircraft location. At 30 degrees Latitude, acceleration causes the swing to a northerly reading to be approximately 30 degrees. Once the acceleration ceases, and the aircraft assumes a constant forward velocity, the compass will return to it’s original East or West reading.

In like manner, deceleration of he aircraft causes an erroneous swing to a southerly reading of approximately 30 degrees at the same Latitude..

MEMORY AID: A N D S - Accelerate North, Decelerate South