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!