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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
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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
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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,
　
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