Magnetic Compass

[Old Koreelah]

Finally, after several half-hearted efforts to correct my compass over the years, it's fixed!

 

Yesterday I flew a big dodecagon over Goran Lake (which I couldn't find- no water, just crops) and tested my panel magnetic compass against the M compass on OzRunways.

North and south are spot on, all other compass points were within 3 degrees, except easterly headings which deviated by about 5.

After years of pretty much ignoring my magnetic compass because it was often out by 25 degrees, I can now rely on it.

 

What had I been doing wrong? Removing it and taking it to a specialist will cost lots of money and won't totally fix it.

The various online "how to swing your compass" tutorials are useful. This time I actually simulated in-flight conditions as closely as possible. In-flight attitude by propping the tail up in a trestle ladder, wheels chocked, engine running, aimed exactly at MN (which I had previously painted in the Tarmac) and followed the instructions.

[Mike Borgelt]

Why do you think the OZ Runways mag compass was correct?

[old man emu]

Is the GPS derived compass in a smart phone correct? I think so.

 

Also, there's a thing called a Compass Deviation Card that you can fill in and paste beside your compass. If you copy this image, you can paste it in Word and make up a whole heap of them.

[turboplanner]

This time I actually simulated in-flight conditions as closely as possible. In-flight attitude by propping the tail up in a trestle ladder, wheels chocked, engine running, aimed exactly at MN (which I had previously painted in the Tarmac) and followed the instructions.

There used to be a dedicated compass swinging bay at Moorabbin Airport. I never took much notice of it because it was only ever used by LAMES. Tried to find it using Google Earth but the are is now covered by retail factories and I couldn't see any other bays from the air. It had a white line datum.

 

If you're talking to a LAME some time, he might be able to explain the procedure he uses, but what you did sounds like what I used to see from a distance; an aicraft parked in the bay set to the datum line.

[Old Koreelah]

Is the GPS derived compass in a smart phone correct? I think so.

 

I'd hope so, OME, but I sometimes find that the iPhone compass gives a dodgy reading for a while.

Those iPhones and iPad compasses are pretty much the only thing we can rely on besides magnetic compasses; and they are suspect because the earth's magnetic field is getting weaker and wandering all over the place. In fact some sources say it's overdue for a reversal and in some parts of the planet there are crazy anomalies.

Another source says we may not have much to worry about:

https://www.smithsonianmag.com/science-nature/earths-magnetic-field-could-take-longer-flip-previously-thought-180972843/

 

But wait, there's more! Not even the planet's axis of rotation is fixed:

https://www.scientificamerican.com/article/humans-contribute-to-earth-rsquo-s-wobble-scientists-say/

 

Also, there's a thing called a Compass Deviation Card that you can fill in and paste beside your compass.

 

Yep, I've had one of those for a decade, but until recently the deviation numbers have been large enough to put me off using it:

 

There used to be a dedicated compass swinging bay at Moorabbin Airport...

 

That's interesting, Turbs. I've never seen one, but made up my own reference line, using every sort of compass I could get hold of. I painted MN and TN lines on the apron in front of our Aero Club building.

(The reaction was interesting; one crusty old pilot couldn't be convinced that north was anywhere near my painted pointer.)

Edited August 16, 2020 by Old Koreelah

[facthunter]

Watch out for those crusty old pilots. They are more worried abut going west than any other direction. Nev

[facthunter]

Magnetic compasses are often north or southern hemisphere due DIP of the lines of magnetic force. (except at the equator).. This also gives the annoying turning and acceleration errors as the moving card doesn't sit level. If that's compensated for it's the opposite in the other hemisphere. You usually swing on more than four points and have all electrics running. Can take hours..Nev

[old man emu]

Now there's a possible cause for a compass reading incorrectly. If you buy a compass fro one of the suppliers in the States, you have to make it clear to them that you want one set for southern dip.

 

https://www.skybrary.aero/index.php/Magnetic_Dip

[facthunter]

THAT info probably refers to remote sensing like a flux gate in a wing inputting the readout on the panel. Our simple ones are a bit different but the dip issue is there. Nev

[Dick Gower]

Finally, after several half-hearted efforts to correct my compass over the years, it's fixed!

 

Yesterday I flew a big dodecagon over Goran Lake (which I couldn't find- no water, just crops) and tested my panel magnetic compass against the M compass on OzRunways.

North and south are spot on, all other compass points were within 3 degrees, except easterly headings which deviated by about 5.

After years of pretty much ignoring my magnetic compass because it was often out by 25 degrees, I can now rely on it.

 

What had I been doing wrong? Removing it and taking it to a specialist will cost lots of money and won't totally fix it.

The various online "how to swing your compass" tutorials are useful. This time I actually simulated in-flight conditions as closely as possible. In-flight attitude by propping the tail up in a trestle ladder, wheels chocked, engine running, aimed exactly at MN (which I had previously painted in the Tarmac) and followed the instructions.

[turboplanner]

You're halfway there Dick; might have hit the Reply button too early by mistake.

[poteroo]

When calibrating an EFIS for magnetic readouts, you need to establish 'dip' for your location and it is done via some website or other, (can't remember exactly). Only after inserting that can you begin the actual swing - which I used to do over a painted compass 'rose' at our airport. (pic). Management have since managed to overspray it with hot bitumen, and cannot be cajoled into providing us with a site on which to paint another. I used a LAMEs prismatic compass to instal the original about 15 years ago. LAMEs usually use a calibrated prismatic compass and follow the aircraft around over each of the cardinal points, then the in betweens = deviation card for standard compass. An EFIS is calibrated directly into degrees M.

 

The best compass I've ever used is the 'vertical card' type, (pic), which is intuitive, steady, and large enough to read without too much error. Better than a DI because you don't suffer from precession error.

[Dick Gower]

I think the answer to your question may lie in your swinging the compass with the tail up in the flying attitude. That would change the relationship between the compass and the stray fields in the aeroplane and there are many of these.

As well as electrical instruments that contain permanent magnets (EGT gauges in particular) any wire carrying a current also generates a magnetic field (which is why the wires to a compass light are always twisted together). All of these fields must be present when the compass is swung.

A surpriusingly common source of stray magnetic fields is steel engine mounts, particularly when the engine grounding cable is dodgy and the mount becomes magnetized by starter motor current.

[Old Koreelah]

Does anyone know if Jab engines are 'degaussed' before dispatch?

I built my own engine mounts, but had a proper aircraft welder complete them; it didn't occur to me to ask if he'd demagnetised them and he's no longer on this world.

[facthunter]

Any structure containing steel will adopt some magnetism during building especially if extensively rivetted. A spring back binder will get you lost if you leave it on the glareshield. I wouldn't worry about your Jab engine . Any residual will dissipate with all the hammering and vibration any internal combustion device has to put up with . The easiest way to make a magnet useless is to have it bang around with no "keeper" on it. Nev

[Mike Borgelt]

The compass in an iPad is GPS based? Really? It is actually a 3D magnetometer chip. There are automatic calibration algorithms but if the aircraft structure/electrics etc induce stray fields the magnetometer will see them the same as the aircraft compass. The fields may also be different from on your lap to the top of the windscreen where the compass lives.

Swinging a compass properly is a pain and can take a while.

GPS will give you a TRACK while you are moving along that track but not a heading unless you are in a ground vehicle. Heading can be done by GPS but you need a fancier and rather special GPS setup known as a GPS compass which has two antennas.

[old man emu]

When calibrating an EFIS for magnetic readouts, you need to establish 'dip' for your location and it is done via some website or other,

 

Here it is:

http://www.geomag.bgs.ac.uk/data_service/models_compass/wmm_calc.html

[Old Koreelah]

...GPS will give you a TRACK while you are moving along that track but not a heading unless you are in a ground vehicle...

Good point, Mike. I'm sure OzRunways used to have a Heading option, but I couldn't find it for my compass test.

As you say, relying on Track means you have to allow for drift.

[Old Koreelah]

The compass in an iPad is GPS based? Really? It is actually a 3D magnetometer chip. There are automatic calibration algorithms but if the aircraft structure/electrics etc induce stray fields the magnetometer will see them the same as the aircraft compass. The fields may also be different from on your lap to the top of the windscreen where the compass lives...

Thanks for clearing up that misconception, Mike.

Mr. Google led me to this:

https://www.quora.com/How-does-the-compass-on-the-iPhone-work

[Riley]

Watch out for those crusty old pilots. They are more worried abut going west than any other direction. Nev

Hush yor mouf boy!

[Jabiru7252]

this is an interesting read. We're dicking around with this sort of stuff at work.

https://phys.org/news/2018-11-quantum-compass-satellites.html

[facthunter]

Inertial systems are independent of anything but they usually have drift over time. The early ones were hellishly expensive and touchy to operate. Nev

[Student Pilot]

Is the GPS derived compass in a smart phone correct? I think so.

 

Also, there's a thing called a Compass Deviation Card that you can fill in and paste beside your compass. If you copy this image, you can paste it in Word and make up a whole heap of them.

[ATTACH type=full" alt="1597533040990.png]55476[/ATTACH]

Always used to leave that one off if a DCA/DOT/CASA inspection was due. They used to fuss around and make a big thing of it saying it was essential for aircraft operation. One chap gave me a big lecture about how I should take more care. The aircraft was facing 230 the compass was indicating 330, he noticed the compass deviation card wasn't here but didn't look at the compass ? I admit I neglected to point it out to him. He snagged me and sent out a list of things he said needed attention. One of which he claimed was a cracked exhaust, I knew it wasn't. After taking it off inspecting and using dye penetrant there were no cracks, another $300 spent.

[Bruce Tuncks]

I like the quantum compass idea. For Australia's defence, we need a thousand or so of long range ground-hugging drones, each with nukes made from Roxby Downs uranium. The only weakness with this idea has been the navigation of those drones. Gosh anything reliant on GPS could be hijacked by a sophisticated enemy who swamped the GPS frequencies with bogus stuff.

Once we had those drones, we could save heaps on our military. And any enemy, no matter how powerful, would be risking that one or 2 of those drones just might get through and ruin their day , that is if they were stupid enough to attack us.

[pmccarthy]

Jabiru drones could carry a 300kg bomb each,