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ND Track discrepancies

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ND Track discrepancies

Old 16th Mar 2021, 11:07
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ND Track discrepancies

Hello all,

Hope everyone has managed to keep well and healthy despite everything going on in the world at the moment. This is a bit of a random/geeky question that has come across my mind that is not really relevent therefore not written in the manuals so was wondering if someone may have had some insights on it.

I've noticed, particularly on some of the older aircraft (I fly the Boeing 737NG), that the Navigation Display tracks don't necessarily always align with the actual track, such as the final approach track of an ILS approach. They typically tend to be off by no more than 2 to 3 degrees, so I've always been wondering where this error may be coming from, but also how the aircraft is able to compensate for it? Another example I was able to find and have a picture of just to help illustrate things (https://prnt.sc/10n48qr), one evening we were flying on the UN860 airway over France and the lido flightplan and jeppesen chart shows that the magnetic track of that airway is 190 degrees. As you can see in the picture, the ND display and FMC legs page track was 192 degrees, but the VOR needles for both AGN and MLA were showing bang on 190 degrees.

Again, as I said before it really isn't an important question by any means, but as you may imagine the furlough boredom has struck me hard so if anyone happens to have an explanation for this I would be very interested to read about it

Thanks again everyone, and best wishes to you all!
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Old 16th Mar 2021, 11:25
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Not a boeing pilot here but could it be a discrepency due magnetic variation? Airbus publish an acceptable difference between FMS track and charted track ( FMS has a magnetic variation database ).
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Old 16th Mar 2021, 14:09
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My bet would be that the MagVar database in your ADIRUs isn't up to date. Revisions are normally only made every 10 years, but the upgrade isn't mandatory - moreover, it normally requires replacing of both ADIRUs for each aircraft, so it represents a significant cost to the operator.

Boeing has a good article on it, particularly on its effect when navigating using NDBs.
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Old 16th Mar 2021, 14:35
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Thank you for your reply and for sharing a really insightful article, it was a great read. I suspect that you are most probably right, and it all makes sense as I've already mentioned that I notice these discrepancies in the much older aircraft in our fleet which are over 10+ years old, versus the latest addition to the fleet everything works out spot on as per the charts and flightplans.

Also found this really interesting safety report from a B747 skipper after realising his IRS mag database was outdated and his tracks were off by 12 degrees, luckily I've never seen or experienced anything to this extent in my career!
http://www.37000feet.com/report/1203...tion-is-out-of
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Old 16th Mar 2021, 16:30
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Next time you line up on the runway (immaculately on the centre-line, of course), see if your heading exactly matches the runway QDM. If it doesn't, that is your MAG/VAR database error - in that location.
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Old 16th Mar 2021, 18:47
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In the days before we fitted GPS updating to our 757 fleet we had a similar issue with map shifting
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Old 17th Mar 2021, 05:16
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The map of the discrepancies in the Boeing document is very interesting.



The "greater than 10 " changes in the high latitudes are related to how the North Magnetic Pole started racing towards Siberia in the mid-90s.




Revisions are normally only made every 10 years, but the upgrade isn't mandatory - moreover, it normally requires replacing of both ADIRUs for each aircraft, so it represents a significant cost to the operator.
A question for the experts. What is it about how the MagVar database is stored in the ADIRU, that requires a replacement unit? Is the database "knitted" like the ROM in the Apollo Guidance Computer?

Last edited by India Four Two; 17th Mar 2021 at 06:40. Reason: missing "is"
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Old 17th Mar 2021, 08:42
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Should we just ditch magnetic in navigation redundant capable aircraft?

Last edited by CCA; 17th Mar 2021 at 09:59.
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Old 17th Mar 2021, 21:24
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The problem for that is that the lowest common denomenator aircraft are left adrift...
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Old 17th Mar 2021, 23:36
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Actually, it would be easier for GA aircraft with old avionics to accurately navigate to true north than it is for modern aircraft - read out the magnetic heading from the compass, add/substract the magnetic variation and set your DG accordingly, and voila - true north navigation.

On airliners, you rely on the database that could easily be 15 or 20 years old, and there is no way you can compensate for that from the flight deck.​​

Originally Posted by India Four Two View Post
A question for the experts. What is it about how the MagVar database is stored in the ADIRU, that requires a replacement unit? Is the database "knitted" like the ROM in the Apollo Guidance Computer?
AFAIK there is no way to update any part of the ADIRU software via onboard data loader, so the only way to update it, is to take it out and send it to manufacturer/avionics shop.
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Old 18th Mar 2021, 05:08
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To be pedantic, the compass still works, doesn't it?
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Old 18th Mar 2021, 08:23
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FlyingStone

Not sure I agree with that. The IRUs calculate true heading directly, without the need for a database. Its magnetic heading that needs the database.
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Old 18th Mar 2021, 08:32
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Ah, should have written my post more clearly, I guess it was getting late. You are absolutely right.

What I meant to say is, that for many airliners (except for those that operate in the polar regions), there is no way to actually force the displays to true north, unless you are actually in a polar region, where hopefully automatic switching to true north will occur.
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Old 18th Sep 2022, 00:11
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Originally Posted by 42go View Post
Next time you line up on the runway (immaculately on the centre-line, of course), see if your heading exactly matches the runway QDM. If it doesn't, that is your MAG/VAR database error - in that location.
Its QFU actually! QDM is magnetic bearing to a radio station.
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Old 18th Sep 2022, 10:48
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When I flew 737 Classics, you could display heading in true on the IRS on the overhead panel. Sat on the ground, I'd compare this to the displayed mag heading and charted variation to see where the aircraft was in terms of variation.
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Old 18th Sep 2022, 11:00
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As far as i know commercial ships have switched to true north decades ago. I always wondered why aviation doesn’t do the same as it would simply remove the need for those pesky databases.
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Old 18th Sep 2022, 12:26
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NAV Canada are planning to do precisely that.
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