Correction for magnetic deviation

[vanosten]

In the real Viggen, the difference between true heading and magnetic heading is corrected automatically. See chapter 3.4 on page 312 in Del 1 of the AJS manuals.

• There is an automatic correction done during take-off
• Or the pilot can override it manually by pressing a button (referensomkopplaren) on the stick.
  The current Viggen in FG does nothing like this and the HI correction in Instruments does not help neither (the only effect of that one seems to be to change the map direction in the TI in the JaDi version).

I propose the following:

• Add a keybinding for referensomkopplaren - will be needed for other stuff in the future, too.
• Use the difference between true and mag heading properties to set a course correction when the keybinding is used.
• Have a flag whether the correction has been done (it could be 0.0)
• Automatically set the course correction when reaching 200 km/h and the flag is false.

[vanosten]

In the JA37 flight manual volume 1, page 392 apparently some similar fixes are taken for true North.
In Bild 4 on page 309 of the AJS Del 1 manual the "Kurskorrektionsratt" is illustrated, however there is not description of how or when it would be used.

[colingeniet]

I think I misunderstood this issue completely. I thought course correction was used to display magnetic heading on the instruments, using the true heading from the INS (for the JA at least, I think true heading is used internally).

I now realise that you probably meant that the instruments are supposed to display true heading. This just seemed too strange to me at the time, sorry.

This seems to confirm it:
https://www.youtube.com/watch?v=fbSCnqFMNLc&t=50
JA 37 aligned on rwy 14 at Luleå, showing heading 137. That's the runway true heading.
A magnetic declination calculator gives a deviation of 5° at Luleå in 1990, and increasing.

Also, section 20.3.3 from the AJS manual seems to indicate that the heading gyro is indeed aligned with magnetic heading,
explaining the importance of this heading correction in order to display true heading, whereas the JA wouldn't need it because the INS is aligned with true north.

This is going to make flying with other aircrafts a lot more fun.