Compass calibration: simulate electrical current influence

[hbeni]

The compass calibration card was made with „avionics on“. It is influenced by both, electrical power to the avionics and also the metal of the plane itself. It would be cool, if switching on/off of the avionics would be having a simulated effect.

• [x] determine if compass correction card includes only the avionics-on error

• [x] Implement error difference avionics on/off

• [x] Correct the cockpit card - 90° shows 96 correction, but its actually 4° by the numbers we have configured

• • - Ref:

• https://www.cfinotebook.net/notebook/avionics-and-instruments/magnetic-compass

• https://www.flight-study.com/2021/04/compass-systems-flight-instruments.html?m=1

• https://www.faa.gov/documentlibrary/media/advisory_circular/ac_43-215.pdf

• https://www.youtube.com/watch?v=WC-iOlckdf4 (Compass errors explained)

C172 also affected, sister PR: https://github.com/c172p-team/c172p/pull/1462

[TheFGFSEagle]

If no one else can, I guess I could get a calibration on a real aircraft, at the nearest airfield EDRO - but 1. I most probably won't go there again before april / may (it's too cold right now ! 🧊 ), and 2. it probably won't be from a C182 since the one that is stationed there is flown only rarely ... just tell me if that would be of any help.

[hbeni]

We already have a calibration, just not the „elec off/only metal influence“. No worries.

a simple implementation would be to make the error depending on a simple calculation: apply 50% always and the remainint depending on activated electrical Equipment. But i want to read up more on this subject.

A question i still have: with avionics off, is the compass card already compensated for the aircrafts metal, or do we have a compass that shows an undefined/roughly correct value? (You might ask this at the airfield, eagle!)

ie: does the compass compensator compensate fully when avionics=off, resulting in a „perfect“ compass reading? (In this case the error from the card is completely depending on the electrical/acionics on)

[hbeni]

Reading this: https://www.cfinotebook.net/notebook/avionics-and-instruments/magnetic-compass Explains it somewhat. So the correction card shows the remaining error after adjusting the compensator (ie the variable error depending on aircraft orientation, which affects both, the bare metal influence and avionics elecrical fields)

ie: does the compass compensator compensate fully when avionics=off

Answer: no

[hbeni]

Got another piece of information here, an example of the influence of radios: https://www.flight-study.com/2021/04/compass-systems-flight-instruments.html?m=1

in this case the radios counter-compensate, (radio off=more error)

[hbeni]

FAA advisory describes the procedure how the compensator is adjusted (p. 5f). Tldr; half of the error will remain, resulting in an averaged errornous reading (what is the „remaining error“ noted on the correction card).

https://www.faa.gov/documentlibrary/media/advisory_circular/ac_43-215.pdf

sometimes there is an „with avionics off“ table too, so best to model it for our purposes might be to

• add a table with „avionics off“ deviation
• apply the two tables weighted by avionics switched on In the future we might easily add an „avionics off“ table to the cockpit, if we desired to do so