"Grid" in higher latitudes....what is it?

[RPRIEVER 52]

Greetings all,

 

In the 777 FCOM 2, there's a reference to a grid heading on higher latitudes (70 and up). The picture on the ND shows "Grid 237". What exactly does this mean?

 

Many thanks.

[Stephen88 6]

Correct me if I'm wrong, but i believe this stems from the difference between grid north and true north on maps. Basically, as you get closer to the poles, the vertical lines on maps no longer point exactly to true north. They area a few degrees off. This is simply a result of having a flat map of a spherical earth. Thus grid 237 means that you are at 237 degrees compared to the vertical lines on the map, instead of at 237 degrees compared to a line pointed to true north.

 

The reason for using this is that it makes your life a lot easier when you are plotting your course on a map, as you do not need to correct for the difference in the angles.

 

Hope that helps and is vaguely understandable.

[Richard McDonald Woods 168]

Ron,

 

GRID 237 indicates that the airplane is navigating with reference to true/grid north, and is trying to fly a course of 237 degrees true.

 

My only knowledge is from the B744.....

 

The heading reference switch permits selection of a magnetic north (MAG) or a true north (TRU) reference for the navigation systems. The switch can be positioned to either TRUE or NORM. If NORM is selected, the system automatically determines the reference as follows:

 

The reference is TRU when the airplane is north of 73 degrees north latitude or south of 60 degrees south latitude, otherwise the reference is MAG.

 

When the reference is TRU, there are no AFDS roll modes other than LNAV available. When the reference changes from MAG to TRU with the HDG SEL mode engaged, the AFDS automatically changes to HDG HOLD.

 

I hope that this helps.

[Boris_The_Spider 0]

Picture, if you will, a polar stereographic projection of the polar region (the projection you'd get if you stuck a light bulb at the centre of a hollow, semi-transparent globe, and projected onto a flat sheet of paper that touched the earth at the pole, and was perpendicular to the surface at that point). So the meridians are straight lines, fanning out in a circular way from the middle of the page (the pole), and the parallels of latitude are concentric circles.

 

Now great circles are, to a close approximation, straight lines on a polar stereographic, so you draw your intended great-circle track on the chart, and unless you happen to be heading due north or due south, you're going to find your true direction is going to change a lot as you continue along the line. That's convergence, and it's a big problem near the poles where it's very large, becoming smaller and smaller as you move towards the equator.

 

So align your DI to North, fly a little way (you're near the pole, so you'll soon cover a good few degrees of longitude) and all of a sudden, your DI no longer points North - it continues pointing to the space direction that you aligned it to, but Earth North has shifted beneath you.

 

That's not even all though, since the magnetic variation would also be changing a lot near the poles, so all in all you'd be lost pretty rapidly.

 

Nowadays, you'd use GPS or INS. Convergence wouldn't be a problem, and nor would magnetic variation.

 

In days of yore, for polar flying, a "gyro north" was arbitrarily chosen, and a grid of straight lines was overlaid over the chart. Say gyro north was chosen to be equal to true north at 20E. You could then fly a grid heading which would not change along your straight line drawn on the chart.

 

Along the 20E meridian in our example, grid track and true track would be the same - everywhere else, the two would differ.

 

You'd use various voodoo to convert back and forth from grid to true, so as to relate your grid position to your true position, since the grid position/track/heading is, as I said, arbitrary and means nothing to anyone else such as ATC. It's just a practical way to actually accomplish the navigation in the air, since it eliminates using a reference (True North) which is changing rapidly. Instead, a reference that doesn't change has been arbitrarily chosen.

 

Now if you can find me a commercial pilot (or at least one who doesn't actively fly in polar regions) who can remember what grivation (or indeed an isogriv) is without looking it up, I'll eat my hat.

[RPRIEVER 52]

Thanks for your responses. However, until I can get my head around some of those concepts, I'll stick to flying near the equator. :wacko:

[Datalink 26]

How does FSX handle in these high latitudes for polar flights?

[Flamin_Squirrel 71]

Thanks for your responses. However, until I can get my head around some of those concepts, I'll stick to flying near the equator. :wacko:

 

I'm not surprised; there are lots of overcomplicated examples above, when it's actually fairly simple. Take a look at the image below:

 

[Richard McDonald Woods 168]

Nicely done, Jordan!