Compasses showing true heading instead of magnetic

[Nealius]

When lining up on one of the Jokkmokk dispersal runways and inputing magnetic heading from F10 map (147 degrees), I noticed both compasses were showing about 152 or 153. Referencing the info bar 152 is the true heading.

[Rudel_chw]

The Viggen has a magnetic declination knob, set it to 0 and the compasses will display magnetic.

[Nealius]

53 minutes ago, Rudel_chw said:

The Viggen has a magnetic declination knob, set it to 0 and the compasses will display magnetic.

All manuals and tutorials say to set magnetic declination knob to local magnetic declination on startup.

[Rudel_chw]

53 minutes ago, Nealius said:

All manuals and tutorials say to set magnetic declination knob to local magnetic declination on startup.

I don’t understand what you want: compasses corrected to show true headings or not?

[Nealius]

Previously, not setting magnetic declination FUBAR'ed the nav system. So ideally I'd like to have magnetic compasses show magnetic heading and a nav system that works.

[renhanxue]

9 hours ago, Nealius said:

Previously, not setting magnetic declination FUBAR'ed the nav system. So ideally I'd like to have magnetic compasses show magnetic heading and a nav system that works.

The Viggen doesn't have any actual magnetic compasses. It has two course gyros, main and backup.

The backup course indicator (bottom row, immediately to the right of the CI) uses the backup gyro. That gyro is always referenced to a magnetic sensor that points to magnetic north, so it always displays magnetic heading and should not be affected by the KURSKORR knob (declination). I'm not sure if that last part about the declination knob has ever been correct in DCS Viggen but it doesn't really matter, nobody ever looks at that thing anyway since nav system faults aren't really emulated in a meaningful way.

The FLI 37 (ADI) and the course ring around the CI both use the main gyro. If there is weight on the nose wheel, the main gyro is referenced to a magnetic sensor, so on the ground they should normally show magnetic heading if (and only if) the magnetic declination knob is set to 0. If you turn that knob on the ground then the indicators should move. When you spawn in it may be initialized to the correct value for the area already, and if it is set to the appropriate value then the course indicators should show true heading.

During the takeoff roll, the CK37 compares the stored true heading of the runway with the magnetic heading (corrected for declination, as input with the knob) and if they match within +/- 15° then the course is initialized to the runway's true heading. After that point the magnetic sensor is disconnected from the course gyro and only true heading is used everywhere (except in the backup course indicator). If the magnetic heading and the runway heading do not match, the CK37 first checks the opposing runway heading, then the heading of any other runways on the same base (assuming the base was input with a REF code) and if any match is found, the true heading of that runway is used as the initial course. If no match is found then no course initialization is done and you get a NAV SYST warning. That last bit about matching opposing heading and other runways I don't think is implemented in DCS Viggen. In the real aircraft the magnetic reference is reconnected again if there's a CK37 fault or a primary data fault but I don't think that's emulated in DCS either.

In other words, the nav system really doesn't care about magnetic heading very much; it's only used on the ground and to sanity check the initial heading. The declination in this case is only about 5° which is within tolerance, so as long as the runway's true heading is programmed into the CK37 it doesn't matter if the KURSKORR knob is set to 0 or to the actual declination for the area.

The backup course indicator is probably wrong, though.

[Rudel_chw]

2 hours ago, renhanxue said:

The Viggen doesn't have any actual magnetic compasses. It has two course gyros, main and backup.

 

Thanks a lot for the insight, I didnt knew that  

[Nealius]

@renhanxue so if I want to fly using magnetic headings, I just need to set the KURSKORR knob to 0 and everything should be fine? Last year when I was flying the Viggen I was having some issues with the nav system error light illuminating and some nav accuracy issues even after setting BANS/GRANA course, because I hadn't been setting magnetic declination properly on the KURSKORR knob. 

[Machalot]

On 5/19/2024 at 2:56 PM, renhanxue said:

That last bit about matching opposing heading and other runways I don't think is implemented in DCS Viggen.

DCS currently models a single runway heading for each airfield, and it does check the reciprocal heading.  It doesn't check any other runways though.

[hilmerby]

The heading on the Cockpit Instruments of an Aircraft are allways MAGNETIC. In the AJS37, there is a module (called ”Kursgivare” in Swedish)
 installed in the Fin that reads the earths magnetic field and tranfer that information to the CK. 
So what You should read when lined up on the Runway is the magnetic heading of that Runway. 
All ATC Radar Displays are oriented in magnetic north, all Runways are designated according magnetic heading,
 every Aircraft have a Compass that
indicates magnetic heading. 

IRL we never turned the KURSKORR, we just let it be in 0.

Stellan

Edited September 4, 2024 by hilmerby

[renhanxue]

5 hours ago, hilmerby said:

The heading on the Cockpit Instruments of an Aircraft are allways MAGNETIC. In the AJS37, there is a module (called ”Kursgivare” in Swedish)
 installed in the Fin that reads the earths magnetic field and tranfer that information to the CK. 
So what You should read when lined up on the Runway is the magnetic heading of that Runway. 
All ATC Radar Displays are oriented in magnetic north, all Runways are designated according magnetic heading,
 every Aircraft have a Compass that
indicates magnetic heading. 

IRL we never turned the KURSKORR, we just let it be in 0.

Stellan

 

With all due respect, that is completely contrary to how the SFI describes the nav system. The magnetic "kursgivare" is described as not having sufficient precision, and is also explicitly said to be disconnected from the main course indicator at takeoff. The system completely disregards magnetic north from that point on (except for the backup course indicator). As I described above though, setting KURSKORR to 0 does work (in Sweden) because the declination is not big enough to confuse the CK37.

I always understood this as the system being referenced to true headings from takeoff because that would be convenient for navigation. I guess though if the stored heading of the takeoff runway isn't actually the true heading but rather a magnetic heading, then you get a sort of "pseudo-magnetic" frame of reference instead, where all headings in the system are referenced from the magnetic north at the takeoff base. Actual declination at the aircraft's current position in the air would not be reflected by the instruments. That seems very confusing, but I guess it could work that way?

 

Edited September 5, 2024 by renhanxue

[Nealius]

Coming from primarily US aircraft where compasses are always magnetic by default and there's no need to faff with magnetic declination, I'm still confused. 

1. KURSKORR to magnetic declination sets all compasses in the aircraft to true?
2. KURSKORR to 0 sets all compasses in the aircraft to magnetic?
3. KURSKORR to 0 does not cause a nav error as it used to in previous updates?

[renhanxue]

9 hours ago, Nealius said:

Coming from primarily US aircraft where compasses are always magnetic by default and there's no need to faff with magnetic declination, I'm still confused. 

1. KURSKORR to magnetic declination sets all compasses in the aircraft to true?
2. KURSKORR to 0 sets all compasses in the aircraft to magnetic?
3. KURSKORR to 0 does not cause a nav error as it used to in previous updates?

KURSKORR only affects the main course indicator, not the backup one (standby compass). The standby compass always shows magnetic heading no matter what.

On the ground, KURSKORR 0 means both course instruments show magnetic heading. While in the air, KURSKORR does nothing, unless you get a CK37 failure.

During the takeoff roll, if the heading shown on the main course indicator is within +/- 15° of the stored heading of any runway at the takeoff base (as entered into your flight plan in CK37), the computer says "okay, the runway heading is the aircraft's heading now". You may see a "jerk" in the HUD and on the course indicator when this happens. From that point on, the computer and hence the main course indicator use a gyrocompass with no magnetic influence whatsoever. All headings are effectively relative, referenced from the stored runway heading. If the runway stored heading was a magnetic heading then all headings are magnetic (or, well, would be magnetic if the aircraft was located at the takeoff base - local declination effects do not affect you). If it was a true heading all your headings are true.

If the computer does not find a stored runway heading that matches within +/- 15° the heading during the takeoff roll, the gyrocompass is left uninitialized. Instead you get a NAV SYST warning and the main course indicator keeps working like it does on the ground (that is, it shows magnetic heading plus the correction from KURSKORR). The KURSKORR knob exists to fix this situation; if you for some reason have magnetic disturbances or deviation issues in the aircraft so you can't get the course indicator to show correct within +/- 15° on the ground, then you can use KURSKORR to get close enough.

In order to check what the stored runway heading is in the CK37, go BANA/GRÄNS -> UT -> LS; the first four digits shown are the heading to within 1/10th of a degree (e.g. 123.4°) and the last two are the TILS channel. To check all stored runway headings at the base you need to first need to select it as a destination (in mode AKT POS), then go into BANA/GRÄNS UT as above and repeatedly press the appropriate waypoint button to step through the runways (including their opposing headings). If you entered the waypoint using lat/lon only the opposing heading of the one runway you've entered will be available; for full information you need to use the REF codes.

All of the above is a description of the real aircraft according to its flight manual. I don't know to what extent it is correct in DCS.

 

In Sweden, declination is never really as great as 15° anywhere, and the real aircraft had all runways on all bases stored in the computer, so in IRL procedures setting KURSKORR to 0 makes sense since it "just works". The CK37 should always initialize the heading correctly, and whatever definition of north was used for the stored runway heading becomes the aircraft's north. I had previously assumed that this north would be true north, but judging by the testimonies from actual Viggen pilots it seems that was wrong.

Edited September 5, 2024 by renhanxue

[MYSE1234]

Pretty sure KURSKORR is not actually hooked up to anything, other than the backup compass () so it's not really doing much other than being a visual thing.
Setting it to 0 instead of random should be easy enough though.

[Snappy]

2 hours ago, MYSE1234 said:

Pretty sure KURSKORR is not actually hooked up to anything, other than the backup compass () so it's not really doing much other than being a visual thing.
Setting it to 0 instead of random should be easy enough though.

Now I’m confused, you and @renhanxue are seemingly contradicting each other.

With him saying KURSKORR only affecting the main course indicator and not the standby compass and you saying it’s exactly the other way around.

[MYSE1234]

8 minutes ago, Snappy said:

Now I’m confused, you and @renhanxue are seemingly contradicting each other.

With him saying KURSKORR only affecting the main course indicator and not the standby compass and you saying it’s exactly the other way around.

I'm talking DCS, not what it's actually supposed to be.

[Snappy]

1 hour ago, MYSE1234 said:

I'm talking DCS, not what it's actually supposed to be.

Yes ok, but isn’t the DCS module supposed to simulate the real aircraft as accurately as possible/ reasonable?

I get though that it’s not necessarily a high priority item. 

Edited September 5, 2024 by Snappy

[Rudel_chw]

13 minutes ago, Snappy said:

isn’t the DCS module supposed to simulate the real aircraft as accurately as possible/ reasonable?

 

Yes, and it does do that ... however compromises have to be made when you have an aircraft designed to operate in one theater, but on a game like DCS it is expected to be used on any map. For example, how would you implement the navigation to work on Nevada, with Longitudes over 99° and yet a computer that offers only six digits (ddmmss) ? 

 

[Snappy]

5 hours ago, Rudel_chw said:

 

Yes, and it does do that ... however compromises have to be made when you have an aircraft designed to operate in one theater, but on a game like DCS it is expected to be used on any map. For example, how would you implement the navigation to work on Nevada, with Longitudes over 99° and yet a computer that offers only six digits (ddmmss) ? 

 

Thats a valid&good point in general , however I don’t see how the KURSKORR functionality falls under this category.

Edited September 5, 2024 by Snappy

[Rudel_chw]

50 minutes ago, Snappy said:

however I don’t see how the KURSKORR functionality falls under this category.

 

I just used an example, but if you need all the aircraft systems simulated with full accuracy, your only option will be to ask the developers to correct this aspect of the simulation.