Magnetic variation, ILS problems, AIRAC data, and so on...
May 25, 2016 12:03:22 GMT
Post by Herve Sors on May 25, 2016 12:03:22 GMT
I read more and more frequently on specialized sim forums and/or in mails I receive, simmers who do not understand what the magdec.bgl does or are in trouble with ILSs that are not correctly aligned on runways (sometimes leading to landing problems), ILS magnetic courses that are not those published on charts or ILS that are not working properly (missing GS, DME, etc).
Usually follow some comments/answers regarding possible AIRAC data inaccuracies, incorrect magnetic variation data, errors in ILS characteristics, etc..
In many cases, users make use of updatable FMS data (as provided by Navigraph or NavDataPro) on equipped aircrafts and frequently ask questions regarding disagreements between FMS display and aircraft instruments and/or charts
To correctly analyze these problems, the following understanding may help:
1) The current magnetic variation (that is the earth magnetic field) is a FIXED parameter that only depends on the position of aircraft and date. On our sim, it is provided by the magdec.bgl file. So there is absolutely no reason not to use the more up to date data (exactly as if it was a real situation) for indicating on compass/heading indicator instruments the current magnetic heading,
2) In FS/P3D, instrumental ILS localizer tracking does not depend on any FMS data neither it is affected by current magnetic variation. It only depends on ILS characteristics as defined in the active scenery BGL file containing the ILS data that are (a) the position of the localizer antenna (b) the ILS TRUE course (by reference to runway true heading) (c) when available, the GS and DME positions. Instrumental means what is provided by the active scenery (that is the one containing ILS data with the highest priority, in case several exist) on VOR/ILS receiver,
3) When flying an ILS course (with a centered LOC indicator), displayed heading in no wind conditions will be the ILS LOC true course - Magnetic variation as defined in the sim MAGDEC.BGL file (W- E+)
Consequently, final path ending up out of runway may only be due to (i) an "offset" ILS (you'll easily identify on charts) (ii) or an error in properly coding the ILS localizer position or its true course in the BGL file. It is never the result of any magnetic variation wrong value or AIRAC inaccuracy.
4) ILS records in BGL files also include a "magnetic variation" value; ILS magnetic declination (as published in AIP and approach charts) is a calibration value that is set by airport authorities and is updated from time to time; it could differ from current (that is today) magnetic variation at ILS position. Published charts use this value to depict magnetic ILS course (and AIRAC data usually provides the same value). In FS/P3D, it is only used to display magnetic ILS course in the GPS and MAP views of the simulator and has nothing to do with (1) (2) and (3).
4) FMS make use of AIRAC data (as installed) to properly display ILS course taking into account ILS magnetic course as defined by Jeppesen or LIDO (depending on provider). It could differ from what you read on your FS instruments, just because simulator instrumental data do not depend on AIRAC data but on what is provided by the simulator in its BGL files.
5) FMS also make use of own AIRAC data to display en route magnetic courses/distances for defined legs; whether magnetic variation is calculated by FMC from its own data or retrieved from sim data via the magdec.bgl file may depend on FMS internal program
6) Airport records in BGL files also include a "magnetic variation" value that, as for ILSs, may differ from the current magnetic variation. Airport charts and/or AIP data publish these values that are updated on a variable time basis (some airports still use 1985 values..). If there are some ILSs on the airport, airport magnetic variation value may differ (although usually they are identical). Airport magnetic variation is the basis for calculating and displaying runway magnetic headings on charts. It appears from all performed tests that simulator doesn't use this value at all.
For some more details and some examples, you may read my "Understanding magnetic variation use in the simulator" document.
Hope it will help you sorting out discrepancies you could observe. Feel free to discuss it and ask any question (make a new thread for that)
Usually follow some comments/answers regarding possible AIRAC data inaccuracies, incorrect magnetic variation data, errors in ILS characteristics, etc..
In many cases, users make use of updatable FMS data (as provided by Navigraph or NavDataPro) on equipped aircrafts and frequently ask questions regarding disagreements between FMS display and aircraft instruments and/or charts
To correctly analyze these problems, the following understanding may help:
1) The current magnetic variation (that is the earth magnetic field) is a FIXED parameter that only depends on the position of aircraft and date. On our sim, it is provided by the magdec.bgl file. So there is absolutely no reason not to use the more up to date data (exactly as if it was a real situation) for indicating on compass/heading indicator instruments the current magnetic heading,
2) In FS/P3D, instrumental ILS localizer tracking does not depend on any FMS data neither it is affected by current magnetic variation. It only depends on ILS characteristics as defined in the active scenery BGL file containing the ILS data that are (a) the position of the localizer antenna (b) the ILS TRUE course (by reference to runway true heading) (c) when available, the GS and DME positions. Instrumental means what is provided by the active scenery (that is the one containing ILS data with the highest priority, in case several exist) on VOR/ILS receiver,
3) When flying an ILS course (with a centered LOC indicator), displayed heading in no wind conditions will be the ILS LOC true course - Magnetic variation as defined in the sim MAGDEC.BGL file (W- E+)
Consequently, final path ending up out of runway may only be due to (i) an "offset" ILS (you'll easily identify on charts) (ii) or an error in properly coding the ILS localizer position or its true course in the BGL file. It is never the result of any magnetic variation wrong value or AIRAC inaccuracy.
4) ILS records in BGL files also include a "magnetic variation" value; ILS magnetic declination (as published in AIP and approach charts) is a calibration value that is set by airport authorities and is updated from time to time; it could differ from current (that is today) magnetic variation at ILS position. Published charts use this value to depict magnetic ILS course (and AIRAC data usually provides the same value). In FS/P3D, it is only used to display magnetic ILS course in the GPS and MAP views of the simulator and has nothing to do with (1) (2) and (3).
4) FMS make use of AIRAC data (as installed) to properly display ILS course taking into account ILS magnetic course as defined by Jeppesen or LIDO (depending on provider). It could differ from what you read on your FS instruments, just because simulator instrumental data do not depend on AIRAC data but on what is provided by the simulator in its BGL files.
5) FMS also make use of own AIRAC data to display en route magnetic courses/distances for defined legs; whether magnetic variation is calculated by FMC from its own data or retrieved from sim data via the magdec.bgl file may depend on FMS internal program
6) Airport records in BGL files also include a "magnetic variation" value that, as for ILSs, may differ from the current magnetic variation. Airport charts and/or AIP data publish these values that are updated on a variable time basis (some airports still use 1985 values..). If there are some ILSs on the airport, airport magnetic variation value may differ (although usually they are identical). Airport magnetic variation is the basis for calculating and displaying runway magnetic headings on charts. It appears from all performed tests that simulator doesn't use this value at all.
For some more details and some examples, you may read my "Understanding magnetic variation use in the simulator" document.
Hope it will help you sorting out discrepancies you could observe. Feel free to discuss it and ask any question (make a new thread for that)