"Swinging" the Compass

[skippydiesel]

 I have a very small (two systems) amount of experience with EFIS compass swinging - Both required all four cardinal points to be entered - WHY?
Surely the electronic gizmo can work out 45, 90 & 270 degrees from one set direction?
Okay, do one additional cardinal point, as a check - should be enough.
Anyone like to explain? (simple language please)

[facthunter]

Life is too short  to not just go by the Maker's directions. In this case it would be a cardinal sin.  Nev

[pmccarthy]

1 hour ago, facthunter said:

Life is too short  to not just go by the Maker's directions. In this case it would be a cardinal sin.  Nev

It would not be NEWS.

[skippydiesel]

So you to sinners don't have a simple answer for me?

 

RFguy may have more to say

[sfGnome]

Regardless of whether you’re dealing with a compass or a magnetometer (from your reference to ‘electronic gizmo’), the effect of equipment in the aircraft is not linear around the circle, so the errors need to be checked at at least the 4 cardinal points. The assumption then is that the errors will be reasonably linear between those points, though if they are quite disparate then there would be value in checking more points (or, better still, finding the cause of the disparity).

[skippydiesel]

5 hours ago, sfGnome said:

Regardless of whether you’re dealing with a compass or a magnetometer (from your reference to ‘electronic gizmo’), the effect of equipment in the aircraft is not linear around the circle, so the errors need to be checked at at least the 4 cardinal points. The assumption then is that the errors will be reasonably linear between those points, though if they are quite disparate then there would be value in checking more points (or, better still, finding the cause of the disparity).

Great response BUT;

 

While a magnetometer may have ways of accommodating the non liner characteristics of an aircraft moving through a flat 360 degrees plain , I don't see how a compass can be adjusted to make the same accommodations. The best you could hope for is noting the variations (assuming consistency) at each cardinal point, so that when turning you know to allow for the variation.

[spacesailor]

DEVIATION. ! . ( card ).

A' card '  with the error ' written on it . To be used at each " compass point " . .to correct the known error,  out .

spacesailor

[facthunter]

CDMVT. Compass, Deviation Magnetic, Variation, True. You adjust the Deviation internally in a particular aircraft with engines running etc at a place you can align the aircraft with the correct orientation, on the Ground,  Some better planes have a FLUX valve in the wing.  The compass has to be set up for northern or Southern hemisphere use , specifically in most simpler compasses. . When swung the "DEVIATION Card as SPACEY says is put in place after the compas has been adjusted as well as possible internally by moving small magnets, I think.  VARIATION is shown on many larger area Maps to assist the Navigator. Nev

[Thruster88]

2 hours ago, skippydiesel said:

Great response BUT;

 

While a magnetometer may have ways of accommodating the non liner characteristics of an aircraft moving through a flat 360 degrees plain , I don't see how a compass can be adjusted to make the same accommodations. The best you could hope for is noting the variations (assuming consistency) at each cardinal point, so that when turning you know to allow for the variation.

The type of compass found in certified aircraft, cessna piper beech etc do have several adjusting screws for north south east west. I have only been involved in this process once and don't remember much, normally error  will be within 1-2 degrees of the master compass.  

[spacesailor]

BUT

Every compass is different,  hence you Have to ' turn " the aircraft or boat , to all four cardinal points of said compass & write that error on a

" proper "  card. Not just a scrap of paper ( tea bag box is a strong cardboard ) , cut to fit in the appropriate ' holder beside your compass .

ALL very ' old school ' , just in-case your " gps stops working ,! & your brain retains the knowledge to use mental arithmetic 

spacesailor

[440032]

Anyone thought of consulting the CASA advisory publication on compasseseses?

Anyone?

Hello.... tap tap tap... is this thing on????

AWB 34-008.

[BrendAn]

9 hours ago, 440032 said:

Anyone thought of consulting the CASA advisory publication on compasseseses?

Anyone?

Hello.... tap tap tap... is this thing on????

AWB 34-008.

i learn't a new word . compasseseses. thanks

[Bruce Tuncks]

I have never worried about the compass, it's a leftover from olden days I reckon.

If you insist on worrying though, how 'bout worrying about the reversal of the magnetic field?  It is overdue by millennia, and will cause the extinction of life on this planet.

[facthunter]

Only worry about things you can have some control over.  Nev

[RFguy]

On 04/10/2023 at 11:07 AM, skippydiesel said:

So you to sinners don't have a simple answer for me?

 

RFguy may have more to say

mmmm. I was involved in the compass swinging for my GA plane a couple of weeks ago, 8 points,  couple of times, all within 2 deg, generally 1, there are  compensation screws for this to pull it in.

Was done on the grass region since pipes, reo etc are all trouble. 

however!

The magnetometer devices in the EFIS etc can be compensated / calibrated with a curve fit, or an interpolated look up table , pretty damn well. 

However they do need at least 8 points to be on the money if they are to work in any aircraft. And they'll still be sensitive to current flowing in wires in the instrument panel. 

Most certified EFIS systems required 12 points, which ensures they get plenty of datapoints.

The mags in most of the EFIS  both  internal and external magnetometers are 3D, ((three magnetometers XYZ) so they can work in any orientation.

 

Skippy, if your EFIS only requires 4 points for cal, then the performance is likely bollocks and wouldn't meet a Part91 certified aircraft requirement.

But probably, in these days of GPS, PLENTY good enough en-route nav IF  it is NOT your primary heading means. 

 

 

 

 

[skippydiesel]

While I can understand the rational of repeating a callibration, so as to acheive  more "data points" I am still not convinced that every cardinal point should need to be aligned, to have a modern compass (magnetometer) calibrated . The instructions for calibration of my Dynon, required the four cardinal pints, so that is what I have done -- makes little sense to me. The combination of magnetometer & GPS, should deliver a degree of instant accuracy (without sextants) , that navigators of just  a few years ago could only dream of.

[RFguy]

10 minutes ago, skippydiesel said:

While I can understand the rational of repeating a callibration, so as to acheive  more "data points" I am still not convinced that every cardinal point should need to be aligned, to have a modern compass (magnetometer) calibrated . The instructions for calibration of my Dynon, required the four cardinal pints, so that is what I have done -- makes little sense to me. The combination of magnetometer & GPS, should deliver a degree of instant accuracy (without sextants) , that navigators of just  a few years ago could only dream of.

It's not the calibration of the magnetometer that is required. Just like a (wet) compass doesnt need calibration-

It is COMPENSATION that is required - due to iron, electrical currents, ferrous metal etc. A wet comipass and solid state magnetometer  are affected precisely the same way.

[skippydiesel]

So when the gizmo knows where north (or any other point on the compass) is how come it cant figure out  - I'm a bit out here, need to adjust by a degree or two, cause if north is there, then west must be a bit further around (or closer)? ie do the compensating based on a known direction, after all that's what the pilot is expected to do when compensating for a known error in a "whisky" compass..

Edited October 8, 2023 by skippydiesel

[Blueadventures]

4 hours ago, skippydiesel said:

While I can understand the rational of repeating a callibration, so as to acheive  more "data points" I am still not convinced that every cardinal point should need to be aligned, to have a modern compass (magnetometer) calibrated . The instructions for calibration of my Dynon, required the four cardinal pints, so that is what I have done -- makes little sense to me. The combination of magnetometer & GPS, should deliver a degree of instant accuracy (without sextants) , that navigators of just  a few years ago could only dream of.

A compass is something you can rely on; so need to take the time and do the full required swing, then you know its deviation due to the aircraft.

[facthunter]

It's years since I've done one but it was on a plane that had a "FluxGate" which is out on the undersurface of one wing, which would be far preferred to within the cockpit.  The cockpit instruments got  data from the one source. It was still done with all engines running and I recall some of the first readings had quite large errors which were eventually corrected to 2 degrees max for the Correction card figures. . It's a good reliable reference used properly and doesn't require power. Used it for 10 hour overwater flights. There was nothing else bar star shots that only experienced navigators could use when you could see anything  IF you had one and they have been known to get it wrong Nev

[RFguy]

57 minutes ago, skippydiesel said:

So when the gizmo knows where north (or any other point on the compass) is how come it cant figure out  - I'm a bit out here, need to adjust by a degree or two, cause if north is there, then west must be a bit further around (or closer)? ie do the compensating based on a known direction, after all that's what the pilot is expected to do when compensating for a known error in a "whisky" compass..

ferrous objects or magnetic field generators (DC carrying wires() influence the local magnetic field, and give it a different shape.  In absence of interference, the earth's magnetic field  look appears homogonous and without deviation in the local region. 

Consider  the fields around a bar magnet  in a bath of iron filings in suspension.  In absence of the bar magnet, they will all line themselves up inside the tank  identically. the whole tank will be same.  If you introduce a tiny bar magnet into the solution, it will introduce the shaped fields and reorientate the particles in the classic bar magnetic shape.  The bar magnetic field will vectorially add to the static (earth) field.  Pick a 3d spot in the tank.

 

Now, if you rotate the tank 90 degrees, you'll find the point you chose in the tank , the vectors of the earth's field are now 90 deg to the previous influence, and the new resultant vector will be different.

 

If you had two vectors 1< 0  (earth's magnetic field) and 0.707<45 (bar nagnet) , the vector result is  1.58 < 18...... 

now if we rotate the tank 90 deg  CCW so that we have 1< 90 and still 0.707<45 (bar magnet) , the new vector result is 1.58< 71deg . quite different angles !

IE the earths field was at 0 deg to start with. we introduced the magnet to the tank . now it reads 18 degrees. OK, so if you 'simply' subtract 18 from the compass, you get 0 deg OK, 

but rotate the tank and now the compass reads 71 degrees. if you are still subtracting 18 degrees, now the compass reads (71-18 = 53deg)  instead of what it should read (=90  because you rotated the tank and compass 90) . so single point corrections do not work. 

 

Edited October 8, 2023 by RFguy

[skippydiesel]

Aaaaah! I would need a lot of time to work through your argument RF,  so I  unconditionally capitulate in the face of superior knowledge, or argument, whichever you prefer😕

Edited October 8, 2023 by skippydiesel

[old man emu]

If you can fly within one or two degrees of a magnetic heading in a VFR flight, you're a better man than most, Gunga Din!

 

The reason for swinging a compass to see how it compares to known directions is simply to see how far out of whack the particular compass is when installed in an aircraft that has its engine running and all electricals working. If it is off by a fair bit, then it can be adjusted to show something acceptably closer to the mark. Even if you do the swing and find the compass is reading 10 degrees off correct, there's nothing wrong with using the compass, as long as you have that little card that says " FOR ... degrees, STEER ....degrees"

 

How accurate to the second is the clock in your aircraft? Even a broken clock gives the correct time twice a day.

[facthunter]

The further you fly on a crook compass, the more lost you become. To be used it's required to be within set limits.   If you are a "near enough is good enough " person, flying is not for you, unless you are hooked on risk and fear.  Nev

[old man emu]

32 minutes ago, facthunter said:

The further you fly on a crook compass, the more lost you become.

That's what a Mk I eyeball is for. If you look where you are going when you first start out and keep updating your target, you will soon pick up if you are off track. 

 

Don't forget we are talking about flight below 10,000 ft in Day VMC in aircraft mostly cruising below 150 kts.