ATPL General Navigation - Class 5: Direction.

Summary

In this educational video, Grant explores the complexities of navigation using a compass, which isn't as straightforward as it seems. He explains the Earth's magnetic field, generated by the molten iron core, and its misalignment with the geographic poles. The video delves into concepts like magnetic variation, deviation, and the impact of wind on navigation. Grant teaches viewers how to convert between magnetic and true north and highlights the importance of compass calibration, especially in aviation, to account for metal and electrical components that can affect compass readings. The summary also touches on the difference between heading and track, and the necessity of wind correction angles to maintain course.

Takeaways

• 🧭 A compass points towards the magnetic north pole, not the true geographic North Pole.
• 🌐 The Earth's magnetic field is generated by its molten iron core, which is offset and changes over time.
• 🔍 The magnetic field's offset causes a difference between magnetic and true north, known as variation.
• 🌀 At the magnetic equator, the compass would not point downwards but would spin flat due to the lack of vertical force.
• 📍 Close to the magnetic poles, the magnetic dip can be so high that the compass becomes unusable.
• 🔄 The horizontal component of the magnetic force is called directive force, which spins the compass, while the vertical component is known as magnetic dip.
• ⚖️ Variation can be either east or west, and it affects the accuracy of a compass reading.
• 🔄 To convert between magnetic and true north, use the equation: Magnetic + Variation = True.
• 🛫 Aircraft generate their own magnetic fields, causing an error in compass readings known as deviation.
• 🔄 Deviation is corrected using the equation: Compass + Deviation = Magnetic, with similar rules for east and west as variation.
• 🚀 For accurate navigation, pilots must account for both variation and deviation, as well as wind effects on heading and track.

Q & A

Why doesn't the Earth's magnetic field align perfectly with its geographic poles?

The Earth's magnetic field doesn't align perfectly with its geographic poles because it is formed mainly by the molten iron core at the center of the Earth, which is liquid and has a life of its own, causing the magnetic field to be slightly offset and change year to year, day to day.

What happens to a compass if you stand directly on the magnetic north pole?

If you stand directly on top of the magnetic north pole, the compass would be pulled all the way down and try to stand up vertically because the magnetic force is directly beneath it.

What is the term for the imaginary line at the halfway point of Earth's magnetic field?

The imaginary line at the halfway point of Earth's magnetic field is called the magnetic equator.

What are the two components of the magnetic force experienced by a compass?

The two components of the magnetic force experienced by a compass are the horizontal component, known as the directive force, which spins the compass around, and the vertical component, known as magnetic dip.

Why can't a compass be used near the magnetic north pole?

A compass can't be used near the magnetic north pole because the horizontal component of the magnetic force becomes so weak and the magnetic dip is so high that the compass becomes unusable and can't spin effectively.

What is the difference between magnetic north and true north called?

The difference between magnetic north and true north is called variation.

What is the significance of the term 'variation' in navigation using a compass?

Variation signifies the difference between magnetic north and true north. It is crucial for accurate navigation as it accounts for the inaccuracy when using a compass due to the misalignment of the true and magnetic north poles.

How can you convert between magnetic and true north?

You can convert between magnetic and true north using the equation: Magnetic + Variation = True. Remember that east is positive and west is negative.

What is deviation in the context of aircraft navigation?

Deviation refers to the error in the compass readings caused by the aircraft's own magnetic fields due to its metal structure and electric components. It is corrected through a process called compass swinging.

How is the error check for variation and deviation performed?

The error check for variation and deviation can be performed using the phrase 'if the variation is east, the magnetic will be least; if the variation is west, the magnetic will be best' and similarly for deviation.

What is the difference between heading and track in aircraft navigation?

Heading refers to the direction in which the aircraft's nose is pointed, while track is the actual path the aircraft is following. The difference between the two is due to factors like wind, which can cause the aircraft to drift off course unless corrected.

How can you remember the order of operations for converting between compass, magnetic, and true north?

You can remember the order of operations using the acronym 'CDMVT', standing for Compass, Deviation, Magnetic, Variation, and True, which can be associated with 'Cadbury's Dairy Milk', a popular chocolate bar in the UK.

Outlines

00:00

🧭 Understanding Compass and Earth's Magnetic Field

The script begins by explaining the basic concept of a compass, which points towards the North Pole. However, it clarifies that the Earth's magnetic field, generated by the molten iron core, does not align perfectly with the geographic poles, causing an offset. The video introduces the idea of magnetic poles and how a compass interacts with the Earth's magnetic field, including the concepts of magnetic dip and directive force. It also discusses the inaccuracy caused by this misalignment, known as variation, and how it can be corrected using a simple equation: Magnetic + Variation = True. Additionally, the script touches on the issue of compass deviation in aircraft due to their metallic structures and moving parts, which generate their own magnetic fields.

05:01

🔍 Calculating Compass Bearings with Variation and Deviation

This paragraph delves into the specifics of calculating compass bearings, taking into account both variation and deviation. It explains the difference between compass north and magnetic north, and how to convert between them using the equation: Compass + Deviation = Magnetic. The script provides a method for determining the deviation of an aircraft compass through a process called compass swinging, which involves aligning the aircraft with known magnetic directions and noting the difference. The importance of correcting for these variables is emphasized, as even small discrepancies can lead to significant errors over long distances. The paragraph concludes with a practical example that combines variation and deviation to find the compass bearing, using the mnemonic 'CDM VT' to remember the order of calculations.

10:02

🌬 Impact of Wind on Aircraft Heading and Tracking

The final paragraph discusses the impact of wind on an aircraft's heading and tracking. It explains how wind can push an aircraft off course unless a wind correction angle is applied. The difference between heading (where the aircraft's nose is pointed) and track (where the aircraft is actually traveling) is highlighted, with the wind drift angle being the difference between the two when uncorrected. The script illustrates how to maintain a desired track by steering into the wind, effectively using a wind correction angle to counteract the wind drift angle. The summary reinforces the importance of these concepts for accurate navigation, especially in high-latitude regions where the Earth's magnetic field's dip can severely affect compass readings.

Keywords

💡Compass

A compass is a navigational instrument that shows direction relative to the geographic cardinal directions. In the video, it is explained that while a compass points towards the magnetic north pole, it does not perfectly align with the true north due to the Earth's magnetic field being offset. This concept is central to understanding the complexities of navigation as discussed in the video.

💡Magnetic Field

The Earth's magnetic field is a complex system generated primarily by the molten iron core at the center of the planet. The video describes how this field does not align perfectly with the geographic poles, leading to the existence of magnetic declination. This is a fundamental concept for understanding why compasses do not always point directly to true north.

💡Magnetic North Pole

The magnetic north pole is the point on the Earth's surface where the planet's magnetic field points directly downwards. The video explains that compasses align with this magnetic north pole, not the true geographic north pole, creating a discrepancy known as variation.

💡Magnetic Equator

The magnetic equator is an imaginary line that represents the midpoint of the Earth's magnetic field. At this line, the force exerted by the magnetic field is horizontal, causing a compass needle to point directly north-south without any vertical magnetic dip. It is mentioned in the video as a point where the compass would not experience any pull to dip.

💡Magnetic Dip

Magnetic dip, also known as inclination or magnetic variation, is the angle a compass needle makes with the horizontal when at rest. The video illustrates that as one approaches the magnetic north pole, the dip increases, potentially rendering the compass unusable due to the weak horizontal component of the magnetic force.

💡Variation

Variation is the angle between magnetic north and true north. In the video, it is described as east or west depending on the location on Earth, and it is crucial for accurate navigation. The script uses the term to explain how to convert between magnetic and true bearings.

💡Directive Force

The directive force is the horizontal component of the Earth's magnetic field that acts on a compass needle, causing it to align with the field. The video explains that this force is what allows the compass to spin and point towards the magnetic north pole.

💡Deviation

Deviation refers to the error in a compass's indication caused by the magnetic influences of the compass's surrounding environment, such as an aircraft's metal structure and electrical systems. The video describes how deviation must be accounted for to ensure accurate navigation, using the process of compass swinging to determine and correct for it.

💡Compass Swinging

Compass swinging is a calibration process used to determine the deviation of a compass, especially important in aviation. The video script explains that by aligning an aircraft with known magnetic directions and comparing the compass reading to the actual direction, one can note the deviation and create a correction table.

💡Heading and Track

Heading refers to the direction in which an aircraft's nose is pointed, while track is the actual path the aircraft follows. The video discusses how wind can cause a discrepancy between heading and track, necessitating a wind correction angle to maintain the desired track.

💡Wind Correction Angle

The wind correction angle is the adjustment made to an aircraft's heading to compensate for wind drift and maintain the intended track. The video script uses this concept to explain the difference between where an aircraft is pointed (heading) and where it actually travels (track), emphasizing the importance of wind correction for accurate navigation.

Highlights

A compass points towards the magnetic north pole, not the true north pole, due to the Earth's magnetic field.

The Earth's molten iron core creates a magnetic field that is slightly offset from the true north and south poles.

The magnetic field's alignment changes year to year and day to day, affecting compass accuracy.

Compass points vertically at the magnetic north pole and spins flat at the magnetic equator.

There are both vertical and horizontal components to the magnetic force experienced by a compass.

The horizontal component, known as directive force, spins the compass, while the vertical component is called magnetic dip.

At the magnetic north pole, the compass becomes unusable due to weak directive force and high magnetic dip.

The difference between magnetic and true bearings is known as variation, which can be east or west.

A simple equation, magnetic plus variation equals true, can be used to convert between magnetic and true north.

Deviation is the error in compass readings caused by the aircraft's own magnetic fields.

Compass swinging is a process to determine the deviation of an aircraft compass.

Combining variation and deviation, the equation compass plus deviation equals magnetic, then magnetic plus variation equals true, helps in accurate navigation.

Wind can affect an aircraft's course, requiring a wind correction angle to maintain the desired track.

The difference between heading (where the aircraft nose is pointing) and track (where the aircraft is traveling) is due to wind.

Cabbies Dairy Milk (CDMT) is a memorable acronym for compass, deviation, magnetic, and true north.

Error checks using the phrases 'magnetic least' and 'compass best' help verify compass and deviation calculations.

Transcripts

everyone knows that a compass points us

towards the north pole it's a bit of

knowledge you pick up when you're

growing up and it's very easy to

understand but it's not quite as simple

as it might seem

[Music]

hi i'm grant and welcome to the fifth

class in the gnab series today we're

going to be looking at direction and how

it isn't quite as simple as just north

east south and west

the earth has a magnetic field formed

mainly by the molten iron core in the

center of the earth

this core has a life of its own and it's

liquid so annoyingly it doesn't create a

magnetic field that lines up perfectly

with the true north and south poles it

is slightly offset

and actually changes year to year day to

day

an easy way to think of it is if there

was a

giant stereotypical magnet straight

through the earth at an angle like this

and the magnetic field comes out of the

south field and around and back in to

the north

magnetic pole as so it is important to

note that the compass will always point

towards the magnetic north pole so if we

were standing directly on top of the

magnetic north pole our compass would be

pulled all the way down and it would try

to stand up vertically

conversely when we are at the magnetic

equator this kind of imaginary line at

the halfway point of this magnet

then there would be no force pulling us

down

and the

compass would spin perfectly flat and

point towards the magnetic north pole

essentially what i'm trying to say is

there's a vertical

and horizontal component to the magnetic

force experienced

by the compass

the horizontal component is the

component that

spins the compass round and it is known

as the directive force so you can

imagine if you're looking at a compass

from a side on angle it would be the one

that kind of spins it this way

this is what we're seeing down here this

is the sideways angle and it's just

gonna um

spin the compass

and then

there's also the vertical component

known as magnetic dip and as you get

further and further towards the

magnetic north pole you get more and

more dip and the compass becomes less

and less able to spin

and at a certain point when we are very

close to the magnetic north pole you can

find that the horizontal component of

this magnetic force is so weak and the

magnetic dip is so high

that the compass actually becomes

unusable

so because the true north pole and

magnetic north pole are not aligned it

means that there is a certain amount of

inaccuracy when using a compass the

compass aligns itself with magnetic

north using the directive force for that

position that we are standing in and the

true north is always at that point very

at the top of the earth

so if we measure a magnetic bearing

there would be a difference between it

and the true bearing the difference

between the two is known as variation

and depending on where you are in the

earth variation is either east or west

in this example is east which we

consider to be positive variation

because the magnetic north is to the

east of true north

so while our true bearing is about 90

degrees

our magnetic bearing would only be from

the magnetic north and it would be this

angle in here

which is about 80 degrees

and you put a little m to signified

magnetic or a t to signify true we can

use a simple equation to convert between

magnetic and true north

so the easiest way

is doing magnetic

plus variation

equals true

and

remembering that uh east

is positive

and west

is negative

and then you can do a simple error check

using a little um

phrase

you basically say if the variation is

east

the magnetic will be least

if the variation is west

the magnetic will be best

as in the higher number or the lower

number when we're using our compass to

navigate using magnetic directions which

is common for most regions of the earth

apart from very close to those magnetic

poles where the dip

is too high to be able to use compasses

then there is often an error in the

compass

this error comes around because

planes are metal basically and they've

got lots of electric and moving parts so

they generate their own magnetic fields

when they're powered up so we have to

account for this error

or the aircraft would point us in

slightly the wrong direction

this difference is known as deviation

and it works very similarly to variation

so basically we have a compass north

which we give this little c arrow to

and that is pointing to a different

um north than our magnetic north and

just like variation we can use a simple

equation to

convert between one and the other so

what we say is compass

plus deviation

equals magnetic with the same rules

where east is positive and west is

negative

so for this example here we can see that

the west

variation here

means it's negative so we could do our

compass which we don't know plus the

deviation would be minus 10

equals the magnetic which is 90 then we

would take it over and it would be a

heading of 100 which as you can see from

our diagram makes sense

and again you can use that simple tool

to error check if the deviation is

leased

the compass will be leased and if the

deviation is west the compass will be

best as in the higher number

to figure out the deviation of an

aircraft compass a process known as

compass swinging is carry out where an

aircraft is lined up with known magnetic

directions and the reading on the

compass is compared the difference is

then noted down and you create a table

which looks

like this but probably a bit neater and

more tidy so you would say four zero

zero zero for a heading of north you

need to steer zero zero one

for a heading of

east zero nine zero you would need to

steer zero eight nine

this is typical the differences are only

ever

a few degrees off but a few degrees off

over long distances makes a huge

difference

so we can combine the concepts of

variation and deviation together and the

best way to do this is through this

little example that i've shown

so if the true bearing of an aircraft is

305 degrees true

deviation is 3 degrees west and

variation is 7 degrees east what is the

compass bearing

for once and probably the only time

in g nav i'm going to tell you to don't

bother drawing a picture

these equations are actually easier

if here or sorry these examples are

actually easier if you just use the

equation

so we can combine the two

equations that we've just seen together

so we know that compass

plus deviation equals the magnetic

and then the magnetic plus variation

equals true

an easy way to remember this is that c

d

m

v

t

and the way that i was taught to

remember this is

there's a type of chocolate bar here in

the uk called a cadbury's dairy milk

or chocolate dairy milk i suppose you

could call it

so cadbury's dairy milk very tasty

cdm vt

that way you can remember the order and

then you just plug in the numbers that

we've got here remembering the east is

positive and west is negative

so we're looking for the compass so

we're gonna leave that as a c

deviation is three degrees west so

that's going to be plus

minus three suppose i could have just

written minus three

equals the magnetic which we don't know

and plus the variation which is seven

degrees east east is positive

equals true which is 305.

and then we would find out our magnetic

so our magnetic

in this case equals we take 7 away from

305 and get 2

9 8

plus 7 equals 305 that makes sense

and then we have to

take away 3 from our compass to get to

this so our compass

would be

3

0 1. and then you can just error check

that so 301 minus 3 equals 298 yep

298 plus 7 equals 305 that's correct so

our compass bearing is 301 degrees

and then you give it a little c for

compass instead of magnetic or true

and then you could error check it again

with that magnetic least and

compass least sort of check so if the

variation is

west which it

isn't sorry if the variation is east

that magnetic is least compared to the

true yeah

if the variation sorry if the deviation

is west

the compass is best so compass is best

compared to the magnetic

and you get a final answer which we've

already discussed

so another level of complication can be

added through wind

so let's say we have the aircraft this

aircraft here point is fully true north

and we're traveling straight up a

meridian

if we have wind from the right

then we will be pushed off course

as we travel along unless we correct for

it

in this example we would have to point

the aircraft to the right and as we fly

along the wind would push us and we'd

keep

flying along the correct track

this is the difference between heading

and track where the aircraft is pointed

that is our heading and where the

aircraft is traveling that is our track

and the difference between them

if we didn't correct for it would be

called a wind drift angle and when we do

correct for it would be called a wind

correction angle

so in this example our heading and our

track are the same but that would push

us off course

so i'm going to draw what it would look

like corrected

so as you can see our heading would have

to be into the wind

by a wind correction angle

and the wind drift angle would only

exist if we didn't correct for this and

the wind drift angle will be completely

opposite to the wind correction angle

you only really have one or the other

so for this example while our track

might be true north our heading is maybe

0 1 0 degrees true

because

our wind drift

is 10 degrees to the

west

so that would mean that our wind

correction angle is 10 degrees to the

east

and that would mean that we maintain a

track

of

zero zero zero degrees true or true

north

so in summary then the earth has a

magnetic field which is offset and

constantly changing because of the

molten core

easiest way to think of it is this if

you've got this bar magnet through the

top and the earth's magnetic field

projects out and around from south to

north as so

this magnetic field is also offset from

true north and through south

a compass lines up with these local

lines of the magnetic field so it means

that we get

something known as magnetic dip when we

get close to the poles

and at high latitudes the dip can be so

severe

that it means that the direction

directive force

isn't strong enough to actually pull the

compass around and give us any useful

information

this is sort of a side on view

so at high latitudes you'd be looking

outside on and it'd be dipping down so

much the directive force won't be too

strong

and then at low latitudes aka like the

equator

then you have

a lot of directive force and very little

dip so they're actually more effective

because of this offset of the

magnetic poles it means we get something

known as variation because the true

norse and magnetic north are in

different points and depending on where

you are on the earth

that means that you get a difference

between the two and it could be

as extreme as 180 degrees if you were

standing in between the two

magnetic north could technically be

behind you whilst true north is right in

front of you

and then you also have an error in the

compasses that we use caused by the

aircraft themselves

and the easiest way to think of

all of these things combined together is

with cabbie's dairy milk very tasty

chocolate dairy milk very tasty some

other acronym to remember

and its compass plus the deviation

equals the magnetic

and then you plus the variation and that

equals the true

remembering that east is positive and

west is negative and then you can do

those error checks where

if the variation is east the magnetic

would be least if the variation is west

the magnetic would be best

or if the deviation is east the compass

would be least if the deviation is west

the compass would be best then we can

add a further complication to it by

talking about heading and track

heading think of it as where the

aircraft nose is pointing track is where

we're actually traveling and the reason

we there's a difference between the two

is because of the wind

if we didn't correct for it we would

drift off course by a drift angle

and if we

invert that drift angle and steer into

the wind that's known as our wind

correction angle and that allows us to

stay on the desired track