PPL exam compass errors
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PPL exam compass errors
Would love some input from any kind souls.
I understand that, in the northern hemisphere:
ACCELERATION = apparent turn to north. Heading west this is anticlockwise
DECELERATION = apparent turn to south. Heading west this is clockwise.
Am I correct that in the souther hemisphere this would be completely reversed ?
i.e.:
ACCELERATION = apparent turn to south. Heading east this is anticlockwise
DECELERATION = apparent turn to north. Heading east this is clockwise
my book implies it but doesn't specifically confirm.
many thanks
I understand that, in the northern hemisphere:
ACCELERATION = apparent turn to north. Heading west this is anticlockwise
DECELERATION = apparent turn to south. Heading west this is clockwise.
Am I correct that in the souther hemisphere this would be completely reversed ?
i.e.:
ACCELERATION = apparent turn to south. Heading east this is anticlockwise
DECELERATION = apparent turn to north. Heading east this is clockwise
my book implies it but doesn't specifically confirm.
many thanks
Instead of using memory tricks it is far better (and easier in the long term) if you try to get a proper understanding of why the errors occur.
The lines of force produced by the earths magnetic field flow vertically upwards, out of the ground at the magnetic south pole and vertically into the ground at the magnetic north pole. The degree to which they are inclined vertically at all other points on the earth is determined by the magnetic latitude. At the magnetic equator they are horizontal or parallel with the surface. As magnetic latitudes increase towards the magnetic poles the degree of inclination also increases.
Freely suspended magnets will align themselves with any lines of magnetic force around them. This means that the inclination of the lines of force in the earth’s magnetic field causes the magnets in compasses to dip below the horizontal. But only the horizontal component of the lines of force give north-south direction to the compass, so this dipping reduces the accuracy of the compass.
In order to minimise this problem, compasses are typically suspended such that their C of G is lower than their pivot. In this way the weight of the magnet is made to oppose the dipping caused by the lines of magnetic force. This is termed pendulous suspension. Although this reduces compass dip, it does not entirely eliminate it. This means that whenever an aircraft not on the magnetic equator, is on a heading other than magnetic north or south, the magnet is slightly dipped towards the nearest pole. This dipping causes the C of G of the compass magnet to be displaced slightly away from the nearest pole. This means that the C of G of the magnet no longer hangs directly below its suspension point.
Whenever an aircraft accelerates, the acceleration forces are applied to the compass magnet through its suspension point. But the inertia of the magnet acts at its C of G. If the suspension point is not directly above the C of G, the combination of acceleration force and inertia will exert a turning force on the compass magnet. So whenever an aircraft accelerates or decelerates on a heading other than magnetic north-south, the lateral displacement of the C of G and the inertia of its compass magnet, causes the magnet to rotate.
The magnitude and direction of this rotation is determined by the aircraft heading, the hemisphere and the acceleration or deceleration rate. The key facts to use to predict the direction of the magnets rotation are:
1. When the aircraft is not at the magnetic equator the magnet suspension
point always lies between the nearest pole and the magnet C of G.
2. When accelerating, the displaced C of G will tend to lag behind the
suspension point, so magnet rotation will be clockwise if the nearest
pole is on your left and anticlockwise if the nearest pole is on your right.
3. When decelerating, the displaced C of G will tend to move ahead of the
suspension point, so magnet rotation will be anticlockwise if the nearest
pole is on your left and clockwise if the nearest pole is on your right.
4. The compass magnet is fixed to the compass card, so clockwise
rotation causes the heading number to decrease and anticlockwise rotation
causes the heading number to increase. This information can be used to
predict whether the apparent turn to towards or away from the nearest
pole.
The above effects can be demonstrated using the wind side of your whiz wheel. For an easterly acceleration in the northern hemisphere for example set align 090 with the True heading pointer. Hold the whiz wheel in front of you with the true heading pointer away from your body. Now imagine that the north pole is on your left, and the C of G of the compass is on your right. If you accelerate the lagging inertia of the C of G causes the compass rose to rotate clockwise. So rotate the centre disc of the whiz wheel clockwise and see that the number aligned with the true heading pointer has decreased. This indicates an apparent turn to the north.
Although this message is very long, the concept itself is quite simple. If you establish a clear understanding of it your mind, you will always be able to predict the effects.
The lines of force produced by the earths magnetic field flow vertically upwards, out of the ground at the magnetic south pole and vertically into the ground at the magnetic north pole. The degree to which they are inclined vertically at all other points on the earth is determined by the magnetic latitude. At the magnetic equator they are horizontal or parallel with the surface. As magnetic latitudes increase towards the magnetic poles the degree of inclination also increases.
Freely suspended magnets will align themselves with any lines of magnetic force around them. This means that the inclination of the lines of force in the earth’s magnetic field causes the magnets in compasses to dip below the horizontal. But only the horizontal component of the lines of force give north-south direction to the compass, so this dipping reduces the accuracy of the compass.
In order to minimise this problem, compasses are typically suspended such that their C of G is lower than their pivot. In this way the weight of the magnet is made to oppose the dipping caused by the lines of magnetic force. This is termed pendulous suspension. Although this reduces compass dip, it does not entirely eliminate it. This means that whenever an aircraft not on the magnetic equator, is on a heading other than magnetic north or south, the magnet is slightly dipped towards the nearest pole. This dipping causes the C of G of the compass magnet to be displaced slightly away from the nearest pole. This means that the C of G of the magnet no longer hangs directly below its suspension point.
Whenever an aircraft accelerates, the acceleration forces are applied to the compass magnet through its suspension point. But the inertia of the magnet acts at its C of G. If the suspension point is not directly above the C of G, the combination of acceleration force and inertia will exert a turning force on the compass magnet. So whenever an aircraft accelerates or decelerates on a heading other than magnetic north-south, the lateral displacement of the C of G and the inertia of its compass magnet, causes the magnet to rotate.
The magnitude and direction of this rotation is determined by the aircraft heading, the hemisphere and the acceleration or deceleration rate. The key facts to use to predict the direction of the magnets rotation are:
1. When the aircraft is not at the magnetic equator the magnet suspension
point always lies between the nearest pole and the magnet C of G.
2. When accelerating, the displaced C of G will tend to lag behind the
suspension point, so magnet rotation will be clockwise if the nearest
pole is on your left and anticlockwise if the nearest pole is on your right.
3. When decelerating, the displaced C of G will tend to move ahead of the
suspension point, so magnet rotation will be anticlockwise if the nearest
pole is on your left and clockwise if the nearest pole is on your right.
4. The compass magnet is fixed to the compass card, so clockwise
rotation causes the heading number to decrease and anticlockwise rotation
causes the heading number to increase. This information can be used to
predict whether the apparent turn to towards or away from the nearest
pole.
The above effects can be demonstrated using the wind side of your whiz wheel. For an easterly acceleration in the northern hemisphere for example set align 090 with the True heading pointer. Hold the whiz wheel in front of you with the true heading pointer away from your body. Now imagine that the north pole is on your left, and the C of G of the compass is on your right. If you accelerate the lagging inertia of the C of G causes the compass rose to rotate clockwise. So rotate the centre disc of the whiz wheel clockwise and see that the number aligned with the true heading pointer has decreased. This indicates an apparent turn to the north.
Although this message is very long, the concept itself is quite simple. If you establish a clear understanding of it your mind, you will always be able to predict the effects.
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Just use ANDS and SONU in the northern hemisphere.
(meaning of letters as above posts)
No harm in just memorising it then getting on with the next bit of knowledge cramming required to pass the PPL exams, never to be used again)
Cusco
(meaning of letters as above posts)
No harm in just memorising it then getting on with the next bit of knowledge cramming required to pass the PPL exams, never to be used again)
Cusco
Isnt it time we got rid off all this nonsence?
MJ
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Just curious Johnm, how do you determine which way you are heading?
Either way, I still look at the compass. At least the once before take-off.
I am with you on this one. Know the compass has errors, know the mnemonic to remember them and you are done.
If you want stuff to study get into the POH. You should be very familiar with the systems descriptions and the amplified procedures in the emergency section'
Read the operators manual provided by the engine manufacturer and understand how the major accessories on our engine work (eg mags, starter, alternator, fuel pump, vacuum pump) with an emphasis on knowing what symptoms they will show when they are not on their way out.
Here is a skill testing question in a subject way more useful than knowing the theory of why the magnetic compass has errors. You are doing your run up and there is no appreciable drop when you select the left mag and a 50 RPM drop when you select the right mag. Is this OK, and if not why not ?
If you want stuff to study get into the POH. You should be very familiar with the systems descriptions and the amplified procedures in the emergency section'
Read the operators manual provided by the engine manufacturer and understand how the major accessories on our engine work (eg mags, starter, alternator, fuel pump, vacuum pump) with an emphasis on knowing what symptoms they will show when they are not on their way out.
Here is a skill testing question in a subject way more useful than knowing the theory of why the magnetic compass has errors. You are doing your run up and there is no appreciable drop when you select the left mag and a 50 RPM drop when you select the right mag. Is this OK, and if not why not ?
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@cusco
Left mag earth lead disconnected.
Or am I having a brain fart again?
/h88
P.S. Of course I am being pedantic as you beat me to giving a good answer.
P.S.2. Would one be OK to fly with one live magneto?
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Confused - there are 2 mags for redundancy. If either are live then the prop could chop your head off. One of them might be an impulse magneto - but even if the other is live, your head could still get the chop.And some aircraft (mainly older) only have one mag. Are they safe?
when you switch to Left Mag you are grounding the Right Mag, that way the right mag is out of the picture and the engine is running on only the left mag. Why should a pilot care about this detail ? Well one good reason is when you are talking to the engineer he/she will know which mag to have a look at first.
The problem with the scenario I presented, is that there is no way to test the mag selected because selecting one mag still means both are providing juice to the spark plugs.
Aside from the obvious danger of having a live prop regardless of the mag switch position, you can not complete a proper mag check and so therefore the aircraft is not airworthy
Sorry for the thread drift
The problem with the scenario I presented, is that there is no way to test the mag selected because selecting one mag still means both are providing juice to the spark plugs.
Aside from the obvious danger of having a live prop regardless of the mag switch position, you can not complete a proper mag check and so therefore the aircraft is not airworthy
Sorry for the thread drift
