Compass Swing

G’day Sean,

Not sure if you have an adjustable compass or not. Here’s a “how to” from EAA if you do.

Cheers,

Des.

https://www.eaa.org/eaa/aircraft-building/builderresources/while-youre-building/building-articles/instruments-and-avionics/swinging-the-compass

Thanks Des,

I know how to do (trained as part of PPL)  - just can't figure out why.

As I said, it's just a mechanical system linked to a magnet, how is it that it you get one cardinal point spot on,  all the rest somehow need adjusting??? What's the rational/explanation???

It's the WHY that I am after, not the how.

No worries Sean,

The way I think of it is….

Compass swing adjustments attempt to reduce Deviation, the “local” effects of the installation, I.e. structure, radios, wiring emf, engine ops., etc., etc.

If you adjust it for a northerly heading only, then fly that northerly heading and don’t make a turn it would be perfect as the effects of dip and those “local” influences would remain constant.

If you were to make a change of heading away from north however, the magnet of the compass would remain aligned with the Earth’s local magnetic field and the aircraft would rotate around it, placing all those influences in a different relevant position to the magnet, possibly/probably inducing errors on headings other than north. Small magnets within the compass are positioned by the adjustment screws to compensate for this during the swing process.

After the north-south and east-west errors are halved during the swing adjustment process we just have to live with any remaining errors that are noted on the “compass deviation card”.

Cheers again,

Des. 

Hi Sean, consider this.  You have a lot of metal and electronics in a plane.  Flying due north when adjusted is great, however turn your aircraft to  the east or the west the radio and other parts will swing more magnetic interference items in or out of line to north.  (deviation)  Adjusting this swing reduces that deviation and keeps flying other directions more accurate than just flying due north.  

Best as I understand it.

Cheers

Anorher factor,

Plus earth pole changes whicg will change this. This has been happening forever. In the older days the compass was all they had so they had to make adjustments so they didn't miss their target. And a compass cam get out of adjustment.