MAGNETIC COMPASS

Magnetic Compass

How it works

The magnetic compass is a simple instrument that uses magnetization to give cardinal direction information to pilots.

The magnet in the compass points to “Magnetic North” and helps pilots to know their directional headings.

The magnetic compass is considered a very reliable instrument because it does not require an external power source to operate.

Pilot’s must also know that Magnetic North and True North are not the same geographical location as seen in the picture below:

Compass Errors

While magnetic compasses are very reliable, they experience many different factors that can lead to errors.

It is the pilot’s duty to understand these errors for safe aircraft operation.

The errors include: (VDMONA)

V: Variation

D: Deviation

M: Magnetic Dip

O: Oscillation

N: Northerly Turning Errors

A: Acceleration Errors 

Variation

Variation is caused by the angular difference between “True North” and “Magnetic North.”

True North: The North Pole. The furthest North geographical point on the Earth.

Magnetic North: Where the magnetic pull is concentrated.

These two points are located in different geographical areas.

Often time maps and VFR Sectionals are drawn in relation to True North.

This means pilots must make the necessary math calculation to convert True North Headings to Flyable Magnetic Headings.

However, many IFR charts are drawn in relation to Magnetic North.

This lessens a pilot’s workload by eliminating the need to convert True North Headings to Flyable Magnetic Headings.

Aviation systems in relation to Magnetic North Include:

IFR En-Route Charts

Instrument Arrival and Approach Charts

Instrument Departure Charts

Runway Headings

The GPS System

How to mitigate Variation:

These errors are mitigated in IFR flying by drawing charts and creating other aviation related systems in relation to Magnetic North instead of True North.

Deviation

Compass Deviation Errors occur due to the electro-magnetic fields of the airplane.

These fields can cause small errors in accurate compass readings.

They are mitigated through the use of a “Compass Deviation Card.”

Magnetic Dip

Magnetic Dip is caused due to the “dipping” of the Magnetic Compass.

The closer the compass gets to Magnetic North, the more it will “dip” as an attempt to point directly at the magnetic source.

This means, above certain latitudes, a Magnetic Compass is unusable due to the amount of dip error.

Magnetic Dip causes Northerly Turning Errors which will be discussed after Oscillation Errors.

Oscillation

Oscillation is a simple compass error.

It is caused by encountering turbulence in flight.

Since the compass floats in a housing of fluid, turbulence can “bounce” the compass around and make it difficult to use the magnetic compass for heading information.

Northerly Turning Errors

When a compass dips, it will cause precessions in turns that pilots need to be aware of.

This is due to the magnet in the compass having weight.

Let’s start with the basics.

We must also remember that headings on a compass are printed in reverse order.

Making a right turn while on a North Heading would mean the aircraft is turning to the East.

However, on the magnetic compass, East is drawn to the left of North.

This means a right turn from a North Heading would look like this on a Magnetic Compass…

Now, let’s examine how a compass will precess when turning to a Northerly Heading from a West Heading.

This will require a right turn as shown.

Because the North Seeking Magnet is on the “low” side of the compass…

The compass will get “hung” on a West Heading momentarily.

Now, let’s examine how a compass will precess when turning to a Southerly Heading from a East Heading.

This will require a right turn as shown.

Because the North Seeking Magnet is on the “high” side of the compass…

The compass will “accelerate” the turn to the East.

In order to mitigate these turning errors, a pilot must remember the acronym “UNOS.”

U: Undershoot

N: North

O: Overshoot

S: South

This means, anytime the pilot is making a compass turn to a Northerly Heading, he/she must Undershoot the turn.

Conversely, anytime the pilot is making a compass turn to a Southerly Heading, he/she must Overshoot the turn.

How much should a pilot Undershoot and Overshoot North and South Headings?

This depends on the aircraft’s latitude. 

The example shown below is for an aircraft flying in the United States.

Flights in different latitudes will have differing amounts of Overshoot and Undershoot.

These types of turns are called “Compass Turns.”

Example 1:

Let’s assume we are flying a heading of 270 and wish to make a right turn to a heading of 360.

Our pilot would roll wings level on a Rollout Heading of 330.

The compass would then settle on the Desired Heading of 360.

Example 2:

Let’s assume we are flying a heading of 090 and wish to make a left turn to a heading of 360.

Our pilot would roll wings level on a Rollout Heading of 030.

The compass would then settle on the Desired Heading of 360.

Example 3:

Let’s assume we are flying a heading of 090 and wish to make a right turn to a heading of 180.

Our pilot would roll wings level on a Rollout Heading of 210.

The compass would then settle on the Desired Heading of 180.

Example 4:

Let’s assume we are flying a heading of 270 and wish to make a left turn to a heading of 180.

Our pilot would roll wings level on a Rollout Heading of 150.

The compass would then settle on the Desired Heading of 180.

In addition to the “Compass Turns” (covered on the last slide), a pilot may also complete “Timed Turns.”

As a rule of thumb, Timed Turns are utilized when the pilot can see the desired heading on the compass.

If the pilot cannot see the desired heading on the compass, then he/she should perform a Compass Turn.

How to perform Timed Turns:

Timed turns are made at half standard rate on the turn coordinator.

At this rate, every “tick mark” on the compass is achieved every 3 seconds.

Timed Turn Example:

Let’s assume our pilot is flying a heading of Due North (360).

The pilot wishes to make a right turn to a heading of 030.

Since the pilot can see heading 030 on the compass, he/she will initiate a Timed Turn.

The pilot counts the number of “tick marks” between 360 and 030 on the compass and multiplies that number by 3.

This correlates to the number of seconds the pilot must turn to achieve the desired heading.

Review:

When performing “Compass Turns”

Done when the pilot CANNOT see the new/desired heading.

Completed at Standard Rate.

Remember UNOS

U: Undershoot

N: North

O: Overshoot

S: South

When performing “Timed Turns” 

Done when the pilot CAN see the new/desired heading.

Completed at Half-Standard Rate

Count the “tick marks” and multiply by 3.

This gives the number of seconds for the turn.

Acceleration Errors

Compass Acceleration Errors are most pronounced when an aircraft is on an East or West Heading.

Because the compass is encased in a housing, it will move as the aircraft accelerates or decelerates.

Acceleration Example:

Just as a persons head is pulled backward during an acceleration, a compass will tilt backward as well.

When the compass tilts like so, the weight of the magnet will pull down on the compass and indicate a turn toward the North momentarily.

Again, this is most pronounced when on an East or West Heading.

Compass Acceleration Errors are most pronounced when an aircraft is on an East or West Heading.

Because the compass is encased in a housing, it will move as the aircraft accelerates or decelerates.

Deceleration Example:

Just as a persons head is pulled forward during an deceleration, a compass will tilt forward as well.

When the compass tilts like so, the weight of the magnet will pull down on the compass and indicate a turn toward the South momentarily.

Again, this is most pronounced when on an East or West Heading.

The acronym to remember for Compass Acceleration Errors is ANDS

A: Accelerate

N: North

D: Decelerate

S:  South

This means than when an aircraft is accelerated, the compass will show a slight/momentary turn toward the North.

Conversely, when an aircraft is decelerated, the compass will show a slight/momentary turn toward the South.

Review:

Acceleration Errors are most pronounced when on an East or West Heading.

The error will most likely be slight and momentary.

Remember the acronym ANDS:

A: Accelerate

N: North

D: Decelerate

S: South

FAA Sources Used for This Lesson

Instrument Flying Handbook

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