Long Range Heading Differences
 

Hello,

I noticed if you have a long leg, especially of a few hundred miles, the heading at the beginning of the leg and at the end of the leg differ by a few degrees. I'm thinking the difference is due to Magnetic Variation and the curvature of the Earth. Are there any other factors?

Thanks.

Have you heard of great circle navigation?:}

In the flat Earth society, the heading would indeed remain constant. However, when I was last up high enough the horizon definitely looked curved. If you can get the kit to navigate a rhumb line the heading would also remain constant, but most systems use great circle nav.

True heading will vary continuously along a route, assuming you are flying a Great Circle (unless your route is due north/south). It doesn't have to be a long route - if you overfly, say, Heathrow's east-west runways, your heading will differ at each end by a fraction of a degree.

To the OP, assuming you’ve studied to ATPL level look up convergence. It’s easily explained with a diagram but beyond my tech abilities! Hope that helps.

Great circle navigation in a nutshell:

Imagine you're flying from one point of the equator to the opposite side of the equator, over the North Pole. In the straightest path possible. First your true course will be 360, then, as you cross the pole, it will flip and be 180 for the second half of the trip.

Now imagine a similar flight where you pass kinda near the pole, but not directly over it. Your true course will start out something like 020, then as you pass near the pole quickly change to the south, (even though you're still flying a straight line, it is crossing each meridian at a different angle). Finishing the trip at 160.

A trip between pretty much any 2 points on the planet, is a less extreme example of the same thing, and the course angle relative to the meridians will always be changing.

Suggest you use Google Earth;use the little info box,top left ,press view,then press `grid`,and all the latitudes/longitudes should appear;next box is `tools. select `ruler`.
Now pick a point ,say anywhere in California,and draw a line around the World to London(England).It should go across Canada,Greenland to England in a curve/Great circle.Note it will also give you your initial heading,approx 035 *.Now draw another line from about 1/3 of the track ,(HudsonsBay area) parallel to the main track to the East coast of Greenland.IT should be about 056* appx. at the start of that line.Now draw the final track ,parallel to the main track to London.the initial heading will be about 097*.So ,you have travelled about 110* of Longitude,and the heading has changed about 60*....
If you travel E OR W around the Equator,your heading will not change(much)...

Go to gcmapper.com, enter LHR-LAX and look at the route on the map. The heading changes will be obvious.

Best way to understand is to get a globe and use a piece of string between two points and see what a great circle looks like.

Nothing at all to do with magnetic variation. There could be absolutely no magnetic field whatsoever and your true heading will still change enroute. Vessbot gives the best explanation as to why variation plays no part. Yes, your compass will change, but that's just referencing another north point. If you have TH selected vs MH, you would still experience the same effect of changing headings as you traverse the globe on an extended leg.

I think the NAV exam has been replaced by ‘how to load the FMS’, and the location of the DIR TO button! :}

That’s pretty much what you said, both Meriden convergence & FMC VAR tables are involved. Bill bulfer B737 NG FMC USER’s GUIDE talks briefly about in a section entitled DIR TO, it’s a brief review on the matter.

Jonkster.

We're all doomed I tell ye! Doomed1

This is nav 101.
You don't need to be a ATPL holder to know the answer - this sort of knowledge will be the routine province of PPL and above. The type of study and application to do with it tends to mean it's not forgotten, whether over 12 years or 20. (Or in my case, 40.)

Folks. I think all and sundry are well aware that I don't get excited too much about a bit of argy bargy. However, this thread has gone off the rails both significantly and rapidly.

The OP asked a question. Whether that question be good, bad, or indifferent is not a consideration. Please, either answer the question, as asked, or hold your peace and just shake your head, if necessary.

While we are quite happy with a bit of to and fro and humour, we really prefer to keep things on a pleasant and reasonably polite even keel.

The OP, perhaps reasonably, got a bit hot under the collar and was more than a bit out of order in his/her responses so a short period on the bench to think about things looks to be in order.

Above all, folks, let's play the ball and not the player.

Here endeth the lesson and homily.

So I missed the good stuff?

Not really.

A few minutes looking at the OP's posting history would have prevented all the unkind comments (happily now deleted).

Fair enough. I apologise for mine, (now deleted). But I question whether it is reasonable to have to spend time researching a poster's history - especially if they claim ATPL, then ask a basic ATPL navigation question.

My reaction came from genuine concern that an unqualified pilot might have been on the line.

Guys,

It's a difficult path one treads in moderation. My personal aim is to stay out of sight on the sidelines and let folks have their say. Every now and again, though, shirt collars start to get a tad sweaty and folks arc up a little when a calming coffee might have been the better strategy. Just part of the tapestry of life which, fortunately, doesn't cause us too much problem in this forum.

Keep in mind, also, that after problem posts are deleted, often there are others to tidy up where those others become irrelevant after the problems are removed. Not that there is any concern with the latter, just an editorial tidy up exercise.

I thought the moderation in this one top notch. Well done.

.. win some, lose some ... we can but try to do a reasonable job ... but thanks for the vote of confidence.