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MAGNETIC COMPASS


A Magnetic Compass may be old school and very simple but is still a required instrument per Title 14 CFR part 91 for both visual flight rules (VFR) and instrument flight rules (IFR) of flight.

A magnet is a piece of metal containing iron that holds lines of magnetic flux, regardless of the size every compass has two poles, the North and South Pole. When two similar poles are placed together, they repel from each other.


Compass Errors

Variation:

Variation is the difference between magnetic north and true north. Magnetic north is what a magnet points to, unlike true north which is a geographic location. Magnetic north has the ability to shift over time. The distance between magnetic and true north are about 1,300 miles apart.


VFR Sectional charts are oriented in relation to true north, even though we as pilots fly magnetic headings. On VFR charts there are lines drawn on the charts called isogonic lines that connect magnetic variations, whereas IFR En-Route charts are oriented with magnetic north.

Since we are flying in the Southern California area, the variation is 14° east. To be able to fly a true course of 180°, the pilot would have to subtract the variation and fly a magnetic course of 166°.

Another example is if you are flying over Washington, D.C., the variation is 10° west. To be able to fly a true course of 180° (south), the pilot must add the variation to be able to fly a magnetic course of 190°.


Deviation:

Deviation errors are caused by electro-magnetic fields within the aircraft, from either flowing electrical currents, magnetized parts, as well as Earth magnetic fields. The compass is placed in a location of the aircraft away from any electrical currents that may affect it.

This errors in the aircraft magnetic compass are corrected by using a Compass Deviation Cards.


Dip Errors:

As you move closer to the magnetics poles the angle created by the vertical pull of the Earth’s magnetic field increases in relation to the Earth’s surface is known as the dip angle. The dip angle increases downward as you move closer to the Magnetic North Pole.

The closer you are to the magnetic pole, it becomes too small to align a compass thus, becomes unusable to use for navigation.


Turning, Acceleration & Deceleration Errors:

When an aircraft turns in flight, the force causes the float assembly to lead and swing resulting in a false northerly turn. To correct this, a northerly turn should be stopped prior to the desired heading. A good rule of thumb is the pilot, he or she should stop a turn 15° prior to a new desired heading.

For turns in a southerly direction, the forces cause the float to lag, resulting in a false turn to the south. This rule of thumb for southerly turns is, that the pilot he or she should turn passed the new designed heading by 15°.

We use a memory aid that may help you remember Northerly Turning Errors, this is “UNOS”.

  • U - Undershoot

  • N - North

  • O - Overshoot

  • S – South


When an aircraft accelerates or decelerates, the compass will indicate a slight turn even though the aircraft is on a straight heading. If the aircraft is accelerating on a westerly or easterly heading, the compass will indicate a slight turn to the north. If the aircraft is on a westerly or easterly heading and decelerates, the compass will indicate a turn to the south.

We use a memory aid that may help you to remember acceleration errors, this is “ANDS”.

  • A - Accelerate

  • N - North

  • D - Decelerate

  • S - South

Simplified, acceleration compass turns to the north, and decelerates compass turns to the south.

Oscillation:

Is simply the bouncing around of the compass caused by turbulence, since the compass is incased if fluid, it can make it difficult to read.



Remember:

Northerly Turning Errors, this is “UNOS”.

  • U - Undershoot

  • N - North

  • O - Overshoot

  • S – South

Acceleration errors, this is “ANDS”.

  • A - Accelerate

  • N - North

  • D - Decelerate

  • S - South


References:


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