While the Magnetic Compass is a simple instrument, it's operation and limiting errors can be confusing for pilots to understand.
Here's what you need to know.
The acronym VDMONA helps pilots to remember the different errors that can affect a Magnetic Compass during flight.
V - Variation
D - Deviation
M - Magnetic Dip
O - Oscillation
N - Northerly Turning Errors
A - Acceleration and Deceleration Errors
Variation is the angular difference between True and Magnetic North. True North is the most North point on the globe (a step in any direction from this point would be a step Southward). Magnetic North is located deeper under the surface of the Earth (this is the location Magnetic Compasses point to). The problem is that True and Magnetic North are not in the same geographical location and VFR Sectional Charts are oriented in relation to True North while we (as pilots) fly in Magnetic Headings (in relation to Magnetic North).
In VFR flying this error is mitigated through the use of Magnetic Variation lines drawn on the VFR Sectional Charts.
In IFR flying this error is mitigated by orienting IFR En-Route Charts with Magnetic North instead of True North.
This error is caused by the electro-magnetic fields generated by the aircraft's electrical system and wiring. These fields can cause erroneous compass readings.
This error is mitigated in aircraft through the use of a Compass Deviation Card as seen below.
The closer an aircraft compass comes to the North Pole, the less reliable it becomes (it is said compasses are completely unreliable above 60 degrees latitude). This is because the compass wants to point "down" at the magnetic pull of the Earth.
Magnetic Dip causes the Northerly Turning Errors discussed below.
This is simply due to turbulence. Since the compass in sitting in a housing of fluid, turbulence can cause the compass to "bounce around" and make it difficult for the pilot to analyze.
NORTHERLY TURNING ERRORS
Magnetic Dip causes a compass to produce erroneous readings while making turns. The further North (or South) a compass goes, the more pronounced these errors will become.
Northerly Turning Errors are mitigated through the use of the acronym UNOS.
U - Undershoot
N - North
O - Overshoot
S - South
This means that if a pilot desires to turn to a Northerly Heading, he or she will need to rollout of the turn and Undershoot the Northerly heading by a pre-determined amount.
Conversely, if a pilot desires to turn to a Southerly Heading, he or she will need to roullout of the turn and Overshoot the Southerly Heading by a pre-determined amount.
The example below shows the amount of Undershoot and Overshoot necessary to make compass turns at 30 degrees North Latitude (this is most of the United States not including Alaska).
ACCELERATION AND DECELERATION ERRORS
Strong or rapid accelerations or decelerations can cause the Magnetic Compass to indicate a turn even if a turn is not initiated. This is due to the weights attached to the compass that help keep it upright.
Mitigation of Acceleration and Deceleration Errors is accomplished using the acronym ANDS.
A - Accelerate
N - North
D - Decelerate
S - South
This means, if a pilot Accelerates (in the Northern Hemisphere) the compass will show a slight/momentary turn toward the North before swinging back to the correct heading.
Conversely, if a pilot Decelerates (in the Northern Hemisphere) the compass will show a slight/momentary turn toward the South before swinging back to the correct heading.
Author - Nate Hodell
CFI/CFII/MEI/ATP - Creator of wifiCFI - Owner of Axiom Aviation Flight School.
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