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Guest macflegg

FMC and magvar

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Guest macflegg

All my nav data bases up to date but when an approach is selected the centre fixes are always off the cenre line.In a recent post it was suggested that magvar should be checked.If it is wrong can anyone tell me how this can be corrected?Malcolm Flegg

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Guest dt1951

Malcolm, this is a problem with FS and the ILS data. Not a problem with 777. I have seen several approacjhes that are not on centerline.

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Guest macflegg

DanThanks for the info.I have until fairly recently had the centre fixes in the correct position - I realise that it is not a 757/777 problem and more likely to be the FMC data.Norman replied to a previous thread about the possibility of the magnetic variation not being correct for the location but did not say how this could be corrected.This would have the effect of giving an incorrect localiser track and indeed that is what happens.Malcolm Flegg

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Guest NormanB

Magnetic variation is in a scenery file. I'm away at the moment so don't have access to FS to give you the details but pretty sure it is in basescenery and called magvar.bgl or magdec.bgl

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Guest macflegg

Thanks Norman.Do you know of anyway to correct the variation or do you think it is a problem with the nav data?Malcolm Flegg

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Guest NormanB

The navdata supplied with the planes is fine. Check the datestamp on your bgl file and post it here - hopefully somebody else can verify it as the same as theirs.

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Guest macflegg

NormanFile magdec.bgl is 04 03 2003 03:48.Malcolm Flegg

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The mag declination file is based on 2003 data and coded into all the scenery bgl files. When using a current approach plate, you will have an approach course based on the current mag dec model. The problem is that the mag north pole has been moving for those three years in between. The differences will be most apparent at locations near the magnetic north pole, e.g. Alaska, N Canada, N Europe etc.It's not as simple as changing the magdec.bgl file, as the ILS data coded into every bgl in the FS world is based on the old data as well, which would require you to change the magdec.bgl and every other scenery bgl too.I find it easiest, at least with ILS approaches, to leave the scenery and magdec.bgl file as-is, and use the current approach plate, but using the original ILS front course coded into the bgl at far northern locations...especially those that have changed runway designations as a result of the mag dev shifts.CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile


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Guest macflegg

Hi BobThankyou for your reply - the problem is really that on selecting an ILS aprroach on the FMC - the centre fix is not in line with the runway hence the autopilot flies an offset appraoch - not desireable in Cat3 conditions!The radnav also gives an incorrect localizer track.Annual variation since 2003 would only be about 1/2 a degee at most and I am seeing much more than that.I can't recall seeing this in the past - is anyone else getting this problem?I may be wrong but it seems to have happened since I down loaded the latest FMC data. Malcolm FleggEx ATPL B737,B757,B767

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Guest NormanB

Malcolm,I know you say you have the latest navdata but is this the set that comes with the download or the one available from www.navdata.at ?

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Guest macflegg

NormanI have the latest navdata from navdat.at but I think,despite having flown these aircraft, I must be a little rusty with the operation of the flight system display.It was in the PLAN mode looking through the route and it is there that the centre fix does not line up - I can't remember that happening in real life.Sorry for the wild goose chase!Malcolm Flegg

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Guest NormanB

Malcolm,No probs.

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Hi Malcolm, sorry to but in and forgive me if i've got the wrong end of the stick but I have a feeling I know what your talking about. All the latest Airacs are based on the latest navdata in the world, the poles are shifting, the magnatic variation's changing (was 4 degrees west for me last year, now its 3) hence the navdata you have is up to date for this year, new magnetic headings and all. The flight simulator world in FS2004 is based on 2003 data, hence some runway centreline headings, ILS approach courses etc will be different in your 'up to date' FMC (or up to dat Jeppesen charts!) than what you're actually seeing from you're heading indicator, some of the latest payware scenery corrects this as it is 'usually' designed with the latest data.A good fix that I use is one you can only use if you own FSX ( I own it, have it installed, but never use it and still use FS2004). From your FSX/BASE scenery folder copy the 'magdec.bgl' file and paste into FS2004/SCENERY/BASE/SCENERY (back up the original first just in case!) you'll then be using 2005 MAG data in FS2004, i've not come accross any problems with it yet and it matches my 2005 Jeppesen Airway Manuals. Give it a go! Kind RegardsMark (LBA ATC)

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There's a problem, as I pointed out before, with just replacing the magdec.bgl file as suggested above. Whether you use the updated file from FSX, or one of the updated magdec files already available elsewhere, the problem is the same: all the ILS approaches in FS have their course data coded into the scenery bgl files, which means you will have a mismatch with the approach course data (based on 2003 magdec file) coded into the sceneries.Localizer courses are laid out in FS as a point (the loc antenna) and a magnetic azimuth to that point. Runways, however, are defined as a line between two fixed endpoints, and therefore remain fixed in space regardless of the mag declination. In an ideal world, the designers of FS would have defined the localizer by placing an approach fix ~18nm out on the extended centerline of the runway, and the line between this point and the LOC antenna could be used practically forever without being queered by shifting magdec data.An an example, if the ILS 6L at airfield XXXX had a final approach course in FS9 of 064 deg magnetic based on 5.0 deg E variation (069 deg true heading), and the mag var has shifted 2 deg W to 3.0 deg E variation, then the final approach course azimuth needs to be shifted in the BGL to 066 deg to match up with the revised magdec data (which would result in the RWY deing renamed to 07L). If you don't change every scenery BGL (there are hundreds), then the approach courses will shift in position...in this particular case by 2 degrees. This doesn't sound like a lot, but it would be noticeable.My personal technique is to use the ILS front course coded into the BGL rather than that depicted on the approach plate. Fixes defined along the approach course (i.e. LOM beacons) are at a fixed point and do not move...if the fix appears off the approach centerline it's because the final approach course has moved as a result of the approach courses in the current charts being revised due to the shifting deviation...but without a matching change in the approach azimuth coded in the BGL.Clear as mud now?CheersBob ScottATP IMEL Gulfstream II-III-IV-V L-300Santiago de Chile


Bob Scott | AVSIM Forums Administrator | AVSIM Board of Directors
ATP Gulfstream II-III-IV-V

System: i9-10900K @ 5.2GHz on custom water loop, ASUS Maximus XII Hero, 32GB GSkill 3600MHz CAS15, EVGA RTX3090 XC3 Ultra
Samsung 55" JS8500 4K TV@30Hz,
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SoundBlaster XFi Titanium, TOSLINK to Yamaha RX-V467 HT Rcvr, Polk/Klipsch 6" bookshelf spkrs, Polk 12" subwoofer, 12.9" iPad Pro
PFC yoke/throttle quad/pedals with custom Hall sensors, Coolermaster HAF932 case, Stream Deck XL button box

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Guest macflegg

BobVery good explanation - thanks. If you read my previous post I now realize that the ils is ok (or near enough) - it was the PLAN view that confused me when stepping through the legs to CF then the runway.I have flown the real thing and can't recall seeing this appear the way it does on this FMC - try it - it looks odd especially at Goose Bay - however when the route is followed it is OK.Thanks againMalcolm Flegg

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