Compass variation and IRS systems.

[Bally Heck]

I have recently been told from a normally reliable source that the data which produces a magnetic heading from the true heading produced by the IRS system is "hard wired" into the system at the time of manufacture. If this is the case, then as time passes and variation changes, moderate to large errors will obviously be introduced into the aircrafts heading. Does anyone have information about the source of variation information in an IRS system?

[ICT_SLB]

There are two methods in use. Honeywell uses a world model (probably WG84-based) while Litton (now Northrop-Grumman) use (or used to use) a Swissair proprietary look-up table. Both produce the variation for a given Lat & Long.

Both manufacturers produce software updates that follow mag var changes but there will still be differences compared to whatever standard is used by the local NAV aid authority. As an example when we were doing HUD trials for CAT IIIa at our local field we had nearly 3 degrees difference between IRU and the Jepp chart ILS course.

Another way it manifests itself is that AHRS-equipped aircraft (that actually sense the local magnetic field) do not follow the same takeoff path as IRS-equipped ones - but the AHRS guys get the complaints from ATC.

[Blacksheep]

"Mag-Var" tables are included in the Flight Management Computer software. These tables are stored in memory and include predicted changes in the magnetic variations between Latitudes 82 north and 82 south. The tables are periodically revised to ensure they match the latest magnetic variation data available. These data are never completely up to date - its a never ending task like painting the Sydney Harbour Bridge - but its the best that can be done. The IRS itself doesn't bother with the magnetic information - it carries on navigating by reference to True North. The magnetic variation data converts 'true' to 'magnetic' headings for use in the external environment, which persists in using magnetic references for runway headings, Navaid alignment and the like. I suspect the continued use of the increasingly unstable magnetic reference is intended to keep the navaid calibration industry in employment. Or simple old fashioned "Tradition"

[ICT_SLB]

Blacksheep,
There may be military IRUs that do not bother with Mag Var but Honeywell Laserefs (all marks up to V in use on almost all Brand H-equipped aircraft like CL601,GX, Falcon 900 etc) and Northrup-Grumman LTN-101 Flagship IRUs (Airbus, CL604, CRJ) all have both Magnetic & True Heading outputs per ARINC 704.

The requirement is a bit more than tradition - VORs (remember using those?) and ILS still are centered on Magnetic Heading - hence my comment on our testing. It will be interesting if the FAA's essentially GPS-based future airspace comes to pass. There are already converter units that use GPS-derived parameters to generate True Heading from Magnetic and Lear Siegler proposed a cheap GPS-bounded inertial platform over a decade ago so how long will it be before a nearly pure GPS platform (or virtual platform) is built (but may be not certified)?

[BOAC]

The 'Classic' 737 -3/4/500 have a database in the IRS which adds 'variation' to 'true' to drive the displays. Whilst the NG (more modern) table is pretty much correct, the 'classic' tables do not seem to have been updated since around 1980 and produce an error of +3.5 degrees in the UK. The FMC variation (used for nav displays etc) is correct as it is updated on a fixed 14/28 day cycle.

This is noticeable when established on a VOR radial when the drift line will be 3.5 degrees to the right of the radial. It also explains why the 'runway heading' never points down the runway(!). That is how I noticed it before anyone accuses me of being able to fly within 3.5 degrees of a heading :-)). I had to spend a while at one stage flying VOR tracks and noticed the 'oddity'.

As to working out what it does with an ILS localiser - my head starts to hurt, but I shall continue using it as an excuse!!!:-))

[Bally Heck]

Thanks chaps. All answers appear to be correct depending on era.

[Blacksheep]

There's a subtle difference between 'using' and 'supplying' magnetic variation ICT_SLB. Inertial systems - whether FMS's utilising separate Lasergyro INU inputs as used on most large airliners, or self-contained General Aviation systems such as Honeywell's LaserRef products - do their navigating by reference to True North and produce magnetic variation outputs for external use only. The conversion in all variants is done using mag/var tables stored in memory. The updating frequency for the magnetic variations depends upon the technology of the system. As BOAC points out, old 'steam driven' Delco Carousel or Litton systems are hard wired and long out of date, whereas FMC sofware based tables may be updated as often as once a month. Neither method can ever be truly up to date as the data is only as good as the latest global survey.

BTW, those VOR and ILS systems that you mention are already scheduled to be de-commissioned under the long-term strategic plans for international ATC. In due course all ATC, including long-haul en-route navigation, will be under Area Navigation rules based on a combination of short-term inertial accuracy and long term GPS correction. When this is implemented, given the availability of cheap GPS systems for general aviation use, I still see no further use for inaccurate magnetic headings.

[ICT_SLB]

All EFIS display systems get their Heading information (Mag or True) from the IRS alone. FMS can not be used as a source as its software level is not high enough (IRUs are Level A, FMC Level C at best).

I've seen the latest Airspace plan for the US & it will still be close to 25 years before the current SRNs are discontinued. Even with the additional signals available when the new series of GPS satellites is finally up, it will still be a single source - arguably single failure - system. NavCanada did a study that cast doubt whether GPS can ever really be used with sufficient repeatable vertical accuracy for CAT I let alone CAT II or III (current GPS approaches have MDAs lower than normal but still well above CAT I (200 ft agl).

Either way we won't be out of a job soon with all the system updates this will entail.

[Shaka Zulu]

Can anyone tell me then why on the old 737 (classic) when you want to have the extended centreline of lets say a center fix then map display always shows this 3 degree displacement of the 'real' inbound track?

[Blacksheep]

The earth's magnetic field is in a state of turmoil at the moment. In some specific locations the magnetic variation has swung through more than 40° in just five years. Hardly a reliable means of navigation - and it is of course, a single source.

In 25 years the North Pole may quite possibly be somewhere in Antarctica. Or not, as the case may be. Its always wise to hope for the best and plan for the worst, well in advance. ;)

[BOAC]

Shaka - see my post above - it is actually 3.5 degrees. It is also why setting 'runway heading' is wrong by the same amount, and ALL headings we fly are 3.5 degrees less than asked for!:D