Teaching Compass Turning Error
 

I have, in the past, taught that this error is caused by the compass needle losing its best alignment with the Earth's magnetic field line of flux (gauss line)while the aircraft is banked, and that the needle will seek out another "best alignment" but need help in best explaining why the compass LEADS on some headings (North where I come from) and lags on others (South, ditto.). What is different about the two? why don't they both lead or lag? How best to explain it?

Thanks
DH

The compass is set on a pivot with the magnets suspended below the pivot point, so that the centre of gravity is below the pivot. This is to allieviate the problem of the magnets dipping towards the nearest pole, giving the compass "Horizontality".

The compass errors in turning through north and south and accelerating on east and west are due to the inertia in the suspended magnets causing them to be "left behind" leading to a reaction and a turning moment. Errors are also due to the z field effect (the effect of the earth's vertical field component) and to a slight extent liquid swirl (a bit like the workings of the inner ear)

Like most things, this is best explained with pictures on a whiteboard, or using whiteboard markers representing the magnets in conjunction with a model aircraft.

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It is becuase the magnets do dip towards the pole (follwing the magnetic field) that the centre of gravity is not precisely below the pivot point. This is why the compass swings during acceleration.

I applaud the fact you teach this. Are we talking commerical students or PPL?

WWW

Getting a bit too technical maybe, guys ?
I'm with Checkboard on this one - just tell 'em its due to dip.Then.....
uNdershoot onto Northerly headings and overShoot onto Southerly headings.
Accelerations onto E and W, apparent turn towards North etc.

FLY NAVY.

Magnetic compasses suffer from acceleration and turning errors, Bloggs. Using the compass, roll out of a 30 deg AoB turn on North. Now adjust to 340 deg. You can't, can you?? So don't try - keep the DI synch'd and use that instead. If it dies, turn using a stopwatch, Rate 1 and the Turn and Slip (or useless bl**dy turn co-ordinator if you're unlucky!!). That demo takes about 5 minutes maximum - after which, forget the theory, apply the practice!!!

I teach that acceleration error is mainly caused as a direct consequence of hanging the needle on a pendulous pivot, (to minimise the angle of dip, of course) thus separating the needle's CofG from the pivot, creating potential for a turning moment to be created. I don't have a problem with that, my query is more to do with the uncommanded deflection of the needle when the aircraft is banked. I don't believe this has anything to do with a turning moment as for acceleration error, but is created when the needle banks with the aircraft and is therefore taken out of its best alignment with the gauss line. You can demonstrate this by taking a broomstick, facing your class/student with one end of it on the floor pointing to them, the other on your shoulder/chest/paunch/nether regions. This simulates the gauss line, dipping down toward the nearer pole. Now take a model and point its nose at them, in front of the broomstick and with your fourth hand (it can be done!) use a marker pen above the model as a compass needle with it pointing at north. I am in the southern hemisphere (NZ to be exact) so I will tell the students I am therefore facing south. With the wings level, it is easy to show the needle, pointing behind the aircraft, maintains alignment with the broomstick/gauss line. Now, bank the model, (maintaining heading, yes I know this is impossible without crossing the controls, but ..) AND THE NEEDLE WITH IT, and you can see the needle will be taken out of alignment albeit only slightly. See if you can move the needle to improve the alignment. If you have done it correctly, you will see that there is a swing, in this case, against the movement of the aircraft, and this is what causes the needle to lag. The students get a simple visual demo, without verbal diarrhoea or histrionics on the whiteboard.

This error is absent on east and west, since the needle lies athwartships in both these cases, maintaining best alignment with the gauss line/broomstick and therefore cannot improve its alignment any more. The error is also absent on the ground, since the aircraft is not banked. (Try it!)

My question is that, I can't readily explain why the movement should be the opposite on north as it is for south. Every time I try to demo it the swing seems to be against the movement of the aircraft in both cases.

I was wondering if anyone was familiar with using this method to teach this. I abandoned the pivot-point CofG method when I discovered this error does not occur on the ground.

In NZ this is required to be taught at all levels. The PPL requirement is identical to the CPL, ATPL, IR and instructor rating requirements. The only difference is in the flight tests, where the professionals are required to hit the correct headings first time (or have a good reason why not and explain exactly what happened) whereas there is a bit of give and take at PPL level.


Cheers
DH

[This message has been edited by deadhead (edited 27 January 2000).]

[This message has been edited by deadhead (edited 27 January 2000).]

[This message has been edited by deadhead (edited 27 January 2000).]

I teach:

Compass is full of alcohol. If you were full of alcohol and swung around in circles you wouldn't make much sense either!

Good enough for Phantom crews (F4)

Don't forget its the angle of dangle that causes the lead lag on N/S [C.U.N.O.S.] There's no dangle on east west so compass best.

[This message has been edited by StrateandLevel (edited 27 January 2000).]

[This message has been edited by StrateandLevel (edited 27 January 2000).]

How about OSUN - Overshoot South, Undershoot North and ANDS - Accelarate North, Decelarate South?

VD MONA
Variation
Deviation
Magnetic Dip
Oscillation
North/South turning Errors
Acceleration/Deceleration Errors
no offence meant to any Mona's ,keep smiling

Deadhead

You seem to be having trouble with your compass as well as falling off your broom stick. By the way if you taxi at 90 knots you'll get the same effect on the ground I promise.

The compass needle is pivoted north of the c of g to counteract dip thus when subject to a turning acceleration this causes a deflection of the needle. On South the pivot point is effectively on the opposite side of the C of G i.e. its still North of it and the deflection is in the opposite direction. Fore and aft acceleration on North and South just cause the needle to rise or dip, on East or West the angular acceleration does not cause a turning moment but a speed change will.

I think you should practice a lecture on the effects of coreolis on the compass for your next renewal.

britavia .......

Fine in the North, but here in the southern hemisphere the ONUS is on you to know to Overturn North, Underturn South) and SAND (South Accelerate, North Deccelarate.

Beagle has it right. Two minutes max in the air as demo, then a couple of timed turns.

Compasses (apparently) have some sort of mass trimming device - a screw, in short - that is used to balance it against the vertical component of the Earth's magnetic field in it's normal area of operations. Older long ranging aircraft had a manual adjustment that had to be altered in flight (on hearsay) Might be worth raising in the tech log forum?

A senior instructor gave me this explanation after hearing my (very dodgy) explanation for Overturn / Underturn turning onto north / south:

The compass needle is counterweighted on the north-seeking end (here in NZ at least). When you bank, the weight will tend to point down towards the ground due to gravity. (It will of course be almost held in alignment by the flux lines, but not quite!)

This bank - induced rotation causes the effect of Overturn North, Underturn South. Try it with a pen, lump of plasticine.

This also explains why acceleration errors do occur while taxying, but turning errors don't - no bank angle.

Works for me, but I confess I haven't tried it on a student yet...

O8

I'm with Okta8 on this one. The counter balance in towards the North seeking pole and thus the CoG is slightly removed from the Pivot point. Whilst heading East or West the CofG is 90* to the direction of travel. When you accelerate/decelerate the CofG has inertia and of course want to continue in its previous direction which gives us our apparent turns to the North/South. This offset CofG is also responsible for the under/overturning effect during turns...INERTIA!!

I demo a full 360 turn at rate1 and stop every 90 degrees to show how the compass is slow or fast and that the errors between DI and wet compass are nil when arriving on E and West and maximum when arriving on North and South. Whilst heading East/West I raise and lower the attitude to demo acceleration errors.

Then its on to the practise.

ONUS Overturn onto North and Underturn on to South.

Select reference heading
Are we turning onto North or South?
Do we underturn or overturn?
By how much?
What heading will we rollout on to give us our reference heading?
Lookout
Rate 1 turn
Rollout
Let compass settle down.
How did you go?
Fine tune heading using 3*/sec i.e 10* off = Rate 1 turn for 3 seconds.

Whilst I understand the concept, what practical application does all this have?? I think it's a total waste of a student's time and money to do more than demonstrate WHY one can't use the compass alone in turns - and then HOW to carry out Rate 1 'timed turns'!!

BEagle

The practical application is that with this knowledge you CAN use the compass whilst turning. It's not difficult to learn (all PPLs in NZ are tested on its application) and it is 100% reliable. Timed turns will also work, but why resort to watch and compass when you only need the compass?

Thank you everyone for what has been a most enlightening conversation. Yes I have learned and relearned a lot - and am happy to report that after careful consideration my broomstick works just fine now.

To strateandlevel: I will certainly re-read my notes on the Coriolis effect if you will spell it correctly :) I am also willing to try anything new, except demonstrating via 90 kt taxi turns that turning error can be generated on the ground.

The straight up fact is that in the southern hemisphere, the needle will turn further than it should on south because of the CG vs pivot point and all that jazz - we know that - but it wants to parallel the gauss line as well. It won't ever get its best alignment due to limitations in the plane of rotation and the battle with gravity, as explained above, and yes, liquid swirl etc etc. I have gone back to my broomstick and after a bit of practice indeed saw that the needle movement, when on north, was indeed in the opposite direction. Thanks for that one strateandlevel http://www.pprune.org/ubb/NonCGI/redface.gif

I am happy now, and must apologise for those that think this subject should not be taught like this because it is wasting students' money ;) or is too complicated http://www.pprune.org/ubb/NonCGI/tongue.gif

It's a fun exercise to teach, easy to practise on the ground calculating headings to turn to, so you can do it first time in the air, and above all it's in our syllabus (of course I could start a campaign to get it taken out... but that's definitely another story!)



[This message has been edited by deadhead (edited 03 February 2000).]

Is the turning error increasing linearly or lograthmically(or something!) as you turn from West to North (northern Hemisphere)? Only once had a DI fail on me and sort of continued on VOR to desination and then tuned in ILS in dodgy weather.No problem but timed turns(or at least for the final correction) would be my preference. Some compasses are not as damped as they should be and the value of turning error is varying madly in turbulence which all adds to the problem.
I prefer Compass 'lags'and becomes 'sluggish' as you turn North and 'races ahead'and becomes 'lively' as you turn South ( too lively at times- swings back and forth so the timed turn is probably better on Southerly headings.
I have I got anything wrong? - quite prepared to be corrected.