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ATPL Theory - Module 1 > General Navigation - Lines on the Earth > Flashcards

General Navigation - Lines on the Earth Flashcards

1
Q

What do all children know about the shortest distance between two points?

A

It is a straight line.

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2
Q

How might children define a straight line?

A

As a line that does not bend or deviate or perhaps a line that has a constant direction.

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3
Q

What do we find about the definition of a straight line when looking at lines on the Earth?

A

The definition is no good; there are lines of constant direction, but they are not the shortest distance between two points.

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4
Q

What must we look back at to find out why lines of constant direction are not the shortest distance between two points?

A

The way direction itself is described.

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5
Q

How is direction in navigation measured?

A

As an angle from a datum.

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6
Q

What causes the change of direction on a straight line on the Earth?

A

The converging meridians.

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7
Q

What is the effect called when the change of direction on a straight line is a result of the converging meridians?

A

Convergency.

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8
Q

Where are true directions most obvious according to the diagram?

A

Near the pole itself.

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9
Q

What are directions referenced to the geographic north pole called?

A

True directions.

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10
Q

How are directions measured?

A

In degrees in a clockwise direction from the north datum.

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11
Q

How are true directions shown?

A

With the letter T after the degrees sign, as in 070°T.

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12
Q

What is the proper name for lines that, if extended, would form circumferences on the Earth?

A

Great circles.

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13
Q

What do great circle tracks mark?

A

The shortest distance between two points.

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14
Q

What do great circles usually have because of convergency?

A

A changing track direction along their length.

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15
Q

What tracks are considered great circle tracks?

A

Tracks that run north/south along a meridian and the equator.

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16
Q

What are small circles?

A

Any circle that is not a great circle, such as parallels of latitude.

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17
Q

What is true about the plane of a small circle?

A

It does not pass through the center of the sphere, and its radius is less than the radius of the sphere.

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18
Q

What is a great circle?

A

The intersection of the surface of a sphere and a plane passing through the center of the sphere.

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19
Q

What does a great circle represent on a sphere?

A

The largest circle that can be drawn on the surface of the sphere, and the shortest distance along the surface between any two points.

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20
Q

What is the shortest distance between two points along the surface of the spherical Earth called?

A

A geodesic.

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21
Q

How many great circles can connect any two points on Earth?

A

Only one, unless the points are antipodal (180° apart on the Earth).

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22
Q

What does every great circle do to every other great circle?

A

Bisects it.

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23
Q

What is the point of greatest latitude on a great circle called?

A

The vertex.

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24
Q

What happens to the direction of a great circle at its vertices?

A

The direction changes progressively until the intersection with the equator is reached.

25
Q

What is an advantage of taking the shortest track between two points using great circle tracks?

A

They are used for navigation whenever possible.

26
Q

What is a disadvantage of great circle tracks?

A

The constantly changing track direction.

27
Q

What systems are needed to fly great circles?

A

Inertial navigation systems, FMS, or GPS.

28
Q

What is convergency?

A

The change of track direction between two points.

29
Q

On what is convergency dependent?

A

Latitude and how far we travel.

30
Q

What is the formula to calculate convergency?

A

Convergency = change of longitude x sine mean latitude.

31
Q

How does the convergency formula work best?

A

For calculations between two positions relatively close to each other (within 30° difference of longitude).

32
Q

What happens to convergency at the equator?

A

It is zero because the meridians are parallel.

33
Q

How can convergency be tested for the poles?

A

Convergency equals the change of longitude.

34
Q

What happens if you change your longitude by 180°?

A

You change your track direction from north to south.

35
Q

How do you find the mean latitude?

A

Add the two latitudes together then divide by two.

36
Q

How do you calculate convergency?

A

Convergency = change of longitude x sine mean latitude.

37
Q

How do great circle tracks change direction over long distances?

A

More significantly than over short distances.

38
Q

How do you find the convergency between London (51°30’N 000° 00’W) and Bristol (51°30’N 002°30’W)?

A

Use the formula: convergency = change of longitude x sine mean latitude.

39
Q

How does convergency affect short distances?

A

It is minimal.

40
Q

What must you do to calculate the final track direction?

A

Calculate the difference between the initial great circle track direction and the convergency.

41
Q

Are you expected to calculate convergency between a point in the northern hemisphere and a point in the southern hemisphere?

A

No.

42
Q

What does navigation along great circles require?

A

Advanced navigation equipment.

43
Q

What can aircraft that do not have advanced navigation equipment only fly?

A

A constant direction between two points.

44
Q

What is true about lines drawn with a constant direction compared to great circles?

A

They are longer tracks than the great circle.

45
Q

When can the rhumb line be used for plotting the course of an aircraft?

A

Over the Earth’s surface at low latitudes or over short distances.

46
Q

When is the great circle route significantly shorter than the rhumb line?

A

Over longer distances and/or at higher latitudes.

47
Q

How does the distance compare for an East-West passage at 20 degrees north for great circle and rhumb line?

A

Great circle distance is 4997 nautical miles (8042 km), rhumb line distance is 5074 nautical miles (8166 km), about 1.5% further.

48
Q

What is a more extreme case for rhumb line vs great circle distance?

A

Air route between New York and Hong Kong. Rhumb line path is 9700 nautical miles (18,000 km), great circle route over North Pole is 7000 nautical miles (13,000 km), 5 1/2 hours less flying time.

49
Q

Where does the rhumb line always lie compared to the great circle?

A

On the equatorial side of the great circle.

50
Q

What is the definition of a rhumb line?

A

A line of constant direction.

51
Q

What is the conversion angle between the great circle and rhumb line tracks at either end?

A

Half the convergency.

52
Q

What is the formula for the conversion angle?

A

Conversion angle = 1/2 convergency.

53
Q

What are the basic rules for drawing sketch diagrams for great circles?

A

Draw the meridians sloping in to the top in the northern hemisphere and sloping out in the southern hemisphere; look at the given track, if it is generally easterly draw it left to right, if generally westerly draw it right to left; put in the angle you are told about and check it looks roughly correct; make sure the great circle is poleward of the rhumb line.

54
Q

What are great circles?

A

The shortest distance between two points with a constantly changing track direction except for meridians and the equator.

55
Q

What are rhumb lines?

A

Lines with a constant track direction.

56
Q

What is convergency?

A

Change of longitude x sine mean latitude.

57
Q

What is the conversion angle?

A

1/2 convergency.

58
Q

How do rhumb lines compare to great circles?

A

Rhumb lines lie on the equatorial side of great circles and never follow the same path over the Earth as great circles except for meridians and the equator.

ATPL Theory - Module 1 (17 decks)

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