PPL exam compass errors

[baleares]

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

[Mach Jump]

Yes. The compass errors in the northern hemisphere are reversed in the southern hemisphere.


MJ:ok:

[tecman]

Yes..we use 'SAND' to help us down here :)

[500ft]

I use ONUS in New Zealand

Oversteer North Understeer South

[baleares]

Can someone confirm that the clockwise/anticlockwise direction of movement is also opposite in opposing hemispheres?

many thanks for the help

[keith williams]

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.

[Mach Jump]

Can someone confirm that the clockwise/anticlockwise direction of movement is also opposite in opposing hemispheres?

Yes, it is. :)


MJ:ok:

Ps. Nice one Keith. Very well put.:D

[Cusco]

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;)

[UV]

Academic claptrap. Isnt it time we got rid off all this nonsence?

[Mach Jump]

Isnt it time we got rid off all this nonsence?

Not quite sure what you mean UV. Are you suggesting that we just ignore the compass errors, or pretend they don't exist?


MJ:ok:

[Johnm]

I have a compass in my aeroplane and I have an ADF I can't remember the last tine I looked at either of them

[Mach Jump]

...I can't remember the last tine I looked at either of them

Just curious Johnm, how do you determine which way you are heading?:confused:


MJ:ok:

[Gertrude the Wombat]

Just curious Johnm, how do you determine which way you are heading?:confused:
Maybe he has a slaved HSI or a G1000?

Either way, I still look at the compass. At least the once before take-off.

[Mach Jump]

Maybe he has a slaved HSI or a G1000?

Ah, you mean he uses his other compass? ;)

Either way, I still look at the compass. At least the once before take-off.

Me too. :)


MJ:ok:

[Cessnafly]

Become a proper pilot......ditch the compass.......get yourself a SkyDemon
account and just follow the magenta line. :oh:

[Big Pistons Forever]

Academic claptrap. isnt it time we got rid off all this nonsence?

I am with you on this one. Know the compass has errors, know the mnemonic to remember them and you are done.:ok:

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 ?

[Cusco]

Me sir, me sir,

Left mag earth lead disconnected.

Prop live even if keys out.

Moving the prop will chop yer head off.

[hegemon88]

Left mag earth lead disconnected.

Right, I think? Not as in "correct", but as in "starboard". Not that it affects the conclusion about live prop and decapitation risk in the slightest.

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? :E

[riverrock83]

Right, I think? Not as in "correct", but as in "starboard".
...
P.S.2. Would one be OK to fly with one live magneto? :E
:ooh: 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?

[Big Pistons Forever]

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 :O

[UV]

Quote:
Isnt it time we got rid off all this nonsence?

Not quite sure what you mean UV. Are you suggesting that we just ignore the compass errors, or pretend they don't exist?


MJ

No, what Im saying is that people dont need to be examined on this bullsh1t.
All a student needs to know is that the aircraft must be in stable straight and level flight to avoid compass errors. End of.
KISS...remember!

[fireflybob]

So when the gyro stabilised HSI fails how are you going to turn onto a heading using the compass?

It's nonsense to say a pilot shouldn't have a working knowledge of compass errors.

[lasseb]

So when the gyro stabilised HSI fails how are you going to turn onto a heading using the compass?

Std IFR procedure is to fly a rate-1 turn and time it.
So note your heading (when straight and level); figure out how many degrees you want to turn. Then start stopwatch and start turning at rate-1.
This works a LOT better than trying to figure out a heading from a magnetic compass, and is the only procedure I teach IFR students.
That said, its always good to have the knowledge as to why you should not rely on the magnetic compass during turns/acc..

[dubbleyew eight]

all this because there is a tiny blob of solder on one of the magnets to make it sit level. sheesh.

[fireflybob]

Std IFR procedure is to fly a rate-1 turn and time it.
So note your heading (when straight and level); figure out how many degrees you want to turn. Then start stopwatch and start turning at rate-1.
This works a LOT better than trying to figure out a heading from a magnetic compass, and is the only procedure I teach IFR students.
That said, its always good to have the knowledge as to why you should not rely on the magnetic compass during turns/acc..

Yes that's the way I've taught it for several decades but that's not the point - you still need to start off and finish with a heading reference. Also Turn Coordinates/Needles are not always calibrated correctly.

Pilot's need to have an appreciation and basic understanding of compass errors - is it really that difficult?

[Mach Jump]

Pilot's need to have an appreciation and basic understanding of compass errors - is it really that difficult?

It's about VFR flying with just a compass. I'm with you, Bob.;)

...there is a tiny blob of solder on one of the magnets to make it sit level.

Errrrm, No there isn't.:=

(I wonder if you wrote this with your tongue in your cheek and I'm taking it a bit too literally?)

The errors are caused by the horizontal displacement of the magnets due to the pendular suspension system used to reduce the 'dip angle' at higher latitudes.


MJ:ok:

[Big Pistons Forever]

Flying instruction theory, particularly ab initio instruction, should concentrate on items which have a practical significance.

The test on whether a particular item of theory should be taught ought be " is there a potential circumstance where the student can apply the bit of knowledge I am about to teach "

So going back to the OP's question, knowing a mag compass has errors and learning how to compensate for those errors has obvious practical significance.

Teaching the design of the compass and how/ why the design produces those errors has zero practical use so IMO it is waste of time. That training/ study time could be used to teach things that are actually useful.

I think there is far too much emphasis on theoretical subject which have absolutely no use in the real world of flying an airplane. The UK is IMO particularly bad in this respect

Here is a heretical thought. Given the, on average, once a month carb ice induced engine failure and resultant forced approach maybe ground school less time should be spent on the theory of compass errors and more time on
recognizing the symptoms of carb icing and knowing those conditions when it is most probable

Hat, Coat, Door ....

[what next]

...knowing a mag compass has errors and learning how to compensate for those errors has obvious practical significance...

The "knowing a mag compass has errors" part of this statement is really all that is required. In ten years, this thing will have disappeared from aeroplanes. If one really wants to look at a magnetic compass, he can use his smartphone...

[fireflybob]

Here is a heretical thought. Given the, on average, once a month carb ice induced engine failure and resultant forced approach maybe ground school less time should be spent on the theory of compass errors and more time on
recognizing the symptoms of carb icing and knowing those conditions when it is most probable


BPF,

For me nothing heretical at all about it - I've been teaching such for decades.

There is a huge difference between doing things right and doing the right thing.

[Mach Jump]

Here is a heretical thought. Given the, on average, once a month carb ice induced engine failure and resultant forced approach maybe ground school less time should be spent on the theory of compass errors and more time on recognizing the symptoms of carb icing and knowing those conditions when it is most probable

No problem with this at all. I just don't think they are mutually exclusive.


MJ:ok:

[fireflybob]

Well we could have an even more philosophical debate as to what the instructor's job is about.

The root of the word education is from the Latin "educo" meaning to "draw out" or another one is "educere" meaning to "draw out from within" or to "lead forth".

The best instructors I have had always teased out my innate curiosity to actively go out and find out more about the subject - they didn't just tell me what they thought I needed to know.

[Big Pistons Forever]

The best instructors I have had always teased out my innate curiosity to actively go out and find out more about the subject - they didn't just tell me what they thought I needed to know.

If they were doing ab initio insruction then they were doing it wrong. There is quite a lot of information that the student must know. Now in an Air Force or College program you have the luxury of having full time students doing nothing else but learning to fly and you can afford to gold plate the training with a lot of non essential theory.

However this is the private aircraft forum and I can say on the basis of a lot of personal experience teaching ab initio and low hour pilots, getting the time to make sure the student has mastered all the must know stuff and a reasonable amount of the nice to know stuff leaves very time for unstructured satisfactions of curiosity. In fact digressions into theory often come at the expense of study on stuff that really, really matters.

No problem with this at all. I just don't think they are mutually exclusive.


MJ:ok:


All knowledge in flight training is not of equal value. The challenge is teaching the right amount of information at the right time. Since flying is an innately practical exercise there is not all that much theory required to safely and efficiently fly the aircraft.

Knowing and understanding the basics of how a carburetor functions is IMO vital to fully understand how to avoid carburator icing and how to correctly use the mixture control, both of which are obviously must know areas.

Understanding the theory of why a compass has errors has no practical use in flight. So it would not be mutually exclusive only if teaching this was not at the expense of something that actually matters, which is going to be pretty much everything else. Given the time constraint realities of flight training in the private flying arena, this is IMO a tall order.....

[fireflybob]

BPF, with respect you have missed my point.

Of course at the ab initio level we need to teach the essentials but it's also a question of how you do that.

[Johnm]

The problem is, for me and many people, how much can we remember? I fly IFR most of the time with AI HSI turn coordinator and GPS. If HSI packs up the GPS tells me heading and track if HSI and GPS both pack up at the same time, then turn coordinator and stop watch are the answer and compass confirms heading when straight and level. I can't therefore remember the compass errors and would have to go and look them up if I needed to know.

[Piper.Classique]

My VFR aircraft has a compass, turn indicator, ASI and altimeter. I am entirely capable of doing timed turns but being VFR can't be **sed to do rate one turns. So I turn onto landmarks found under the wings.....
THe club aircraft have DI and radio nav. Mine doesn't.
Even so, turning on to 90 degree landmarks works well, and keeps heads out of cockpits. It works thus...
Turn 360 right? Find a landmark, bush, tree, field, farm, whatever under rigt wing. Turn until it is under left wing. That is half the turn. Repeat. That is 360.
Easy. Let aircraft settle down wings level. Check heading and adjust if needed. Vary as needed for lesser changes of heading.

I teach turning and acceleration errors, in gliders, microlights, and SEP. It takes about five minutes on the ground and ditto in the air. Compass not very useful in turns or other accelerated flight, read the book if you want to know why. Show the effects in flight. Job done.

I spend a lot longer on engine failure modes, how to diagnose and cope, full and the more insiduous partial. (yes, and some gliders have an engine.....)

Now my IMPORTANT instruments take up a bit more room. a proper glider vario, CHT and oil pressure and temperature, RPM. Would like carb temp, a CHT per cylinder, and EGT but can't find room in the panel or the money to buy them

[fireflybob]

If HSI packs up the GPS tells me heading

Johnm, how does GPS tell you the heading?

[dubbleyew eight]

mach jump, err yes it is.
the magnets are actually balanced to level by a blob of solder.
one of my mates does a very nice job of changing a northern field compass to a southern field compass just by melting off the old blob and tacking on a new one.
refilling and sitting a night in the freezer etc. etc.

[Mach Jump]

...err yes it is.

Err... No it isn't.

Field compasses may be balanced in that way, but aircraft compasses are not.


MJ:ok:

[Johnm]

Quote:
If HSI packs up the GPS tells me heading
Johnm, how does GPS tell you the heading?

Because it shows the angle I have to fly the aircraft to stay on the magenta line

[fireflybob]

Because it shows the angle I have to fly the aircraft to stay on the magenta line

Ok so it shows heading indirectly - thanks

But if ATC asked you to fly a heading (assuming drift) that wouldn't be quite as straightforward although you could offer to fly a track?

[gemma10]

Thread starter probably wondering why he asked the question now. He got a satisfactory answer after two replies, then the thread drifts all over the place even to carb ice. It would be great if we all understood every technicality of every aircraft subpart but we dont as we are all still learning. Brilliant piece by Keith Williams, I wonder how many ATPL`s could recite that. :D

[Big Pistons Forever]

. Brilliant piece by Keith Williams, I wonder how many ATPL`s could recite that. :D

Not very many because it is useless information they memorized to pass their ATPL exams and then flushed from their brain to make room for information that actually can be used in flight.

[dubbleyew eight]

I suggest that mach jump goes and gets himself an Airpath brand Aircraft compass and pulls it apart.
some people only ever learn by having the evidence in front of them.

[Mach Jump]

I suggest that mach jump goes and gets himself an Airpath brand Aircraft compass and pulls it apart.
some people only ever learn by having the evidence in front of them.

I guess I'm going to have to do that now. I'll let you know what I find.


MJ:ok:

[Mach Jump]

I couldn't find an Airpath compass to dismantle, so I Wrote to Airpath and asked the question. Here is their reply.

I don't know if this will settle your argument, but basically you are both correct. The card assembly is balanced for the vertical component of the earth's flux lines based upon the surveyed strength by NOAA for a given latitude and hemisphere. A small weight is applied to level the display level, non-accelerated flight. The pendular design of the card assembly helps minimize (but not eliminate) the dip errors when turning and accel/decal environments. Hope this helps.

Gil Stone
President
Airpath Instrument Company

Sorry, dubbleyew eight, you are right.


MJ:ok:

[Baikonour]

Thanks to MJ for that and well done to Airpath for responding :ok:

Interestingly, it implies that compasses sold at different latitudes therefore should have different counterweights. Which makes sense but is not something I think would have been practically possible or commercially desirable. A compass made for Florida will be a nuisance in Scandinavia and vice versa!

B.

[Mach Jump]

Interestingly, it implies that compasses sold at different latitudes therefore should have different counterweights. Which makes sense but is not something I think would have been practically possible or commercially desirable. A compass made for Florida will be a nuisance in Scandinavia and vice versa!

Yes. I believed that the whole point of the pendular suspension of the compass card was to eliminate the need for balance weights, so that the compass could be used anywhere in the world.

It looks like anyone thinking of flying from London to Capetown will have to consider the need to change the compass half way.


MJ:ok:

[Mach Jump]

Yes. I believed that the whole point of the pendular suspension of the compass card was to eliminate the need for balance weights, so that the compass could be used anywhere in the world.

It looks like anyone thinking of flying from London to Capetown will have to consider the need to change the compass half way.

A little more info from Airpath, after further enquiry, including the above London - Capetown example.

Only if airplane was moving base from north to south would there be any noticeable difference and any compass will function in any location. Static balance would be slightly off (1-3 degrees) but design incorporates 18 degrees of tilt/turn.

GS

Since the magnetic dip is 60 odd degress+ in the uk, and 60 odd degrees- in South Africa, I'm more confused than ever now! :confused:



MJ:ok:

[fireflybob]

Since the magnetic dip is 60 odd degress+ in the uk, and 60 odd degrees- in South Africa, I'm more confused than ever now!

Me too!

Not so sure this is correct.

The pendular design of the card assembly helps minimize (but not eliminate) the dip errors when turning and accel/decal environments.

But surely similar magnitude errors in both hemispheres?

Confused?

[FullWings]

I fitted one of these to my last glider, no errors at all!

http://www.zweefvliegopleiding.nl/LAPL(S)/algemene%20kennis%20van%20het%20zweefvliegtuig/instrumenten/afbeeldingen/bohli-kompas.JPG

You can even use it as an attitude reference should all the rest of your instruments fail.

[fireflybob]

I fitted one of these to my last glider, no errors at all!

Apart from the obvious answer, FW, what is it?