[ EXPLAINED ] Grid Questions

Having problems solving the ATPL Questions on grid and true tracks? Making a simple sketch on a piece of paper will make your life much easier! Below, I will explain the procedures step by step.

Let's first take an example Question to start with:

"Route A - B is drawn on a Southern Polar Stereographic Chart whose grid is aligned with the Greenwich meridian. The True Track of the straight line at A is 120°. When passing the meridian of 100°E, the true track is 090°. The grid track of this route on the chart is?"

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Step 1

Step 1 is to construct the basics and check that you have everything correct before you start to draw the meridians on it.

Take a pair of dividers and draw a circle (not too small). Mark the centre dot and name it NP or SP depending what hemisphere the Question states (in our example Question we are in the Southern Hemisphere).

Remember that right always stays EAST and left always stays WEST, these do not change when we view from NP or SP.

The solid straight line pointing up (000°) is the prime meridian. Since we are in the Southern Hemisphere, the anti-meridian (180°), represented by a dotted line is pointing towards us.

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Step 2

Once this is drawn, we are ready to add the meridians. Remember that the more accurate you measure the angles and draw them on your sketch, the easier it will be to just measure the answer at the end.

Note: The pictures provided here and the angles on them are not accurate, they are just added to help you visualise the procedure!

Place your protractor and measure 100° to the EAST (from 000°). Be careful not the measure them starting from 180° anti-meridian!



In our Question we only need to add 1 meridian (some Questions require you to plot more but this is just the same technique).

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Step 3

Now let's add the rest of the information given. The Question says that when we are crossing the 100°E meridian, our true track is 90°.

Remember that true tracks are measured from TRUE (NORTH) POLE in clockwise direction!

We add the straight line (our track) through the meridian so that it intersects at an angle with true north of 90°. Our sketch has SP in the centre so we cannot measure from there, NP will be at the other end of the meridian so start measuring from that point in clockwise direction.

In the sketch below you can see by the red numbers how you should be looking at it. The north pole is on the opposite side of the meridian than the SP and we measure 90° clockwise. Our route (orange line) is now 90° True Track at meridian 100°E

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Step 4

Now we are at the final step. The last thing we need to do is add the grid over our location and measure the angle from that line.

The Question states that the grid is aligned with Greenwich Meridian (in sketch shown as the blue arrow pointing up). In the ATPL Questions they will usually only give you grids that are aligned with Greenwich Meridian or its anti-meridian which makes it easy because those straight lines are simple to slide to our location.

Slide the grid over our location (to the right) as shown by the 2nd blue arrow in the sketch. The top of the arrow is Grid North, from where we measure the grid track.



Place your protractor with N aligned with GN and now measure clockwise. The grid track of the orange arrow (our track) is 190°(G).

Tip: Placing an arrow for the direction you are flying on the track (orange line) will help you to minimise the mistake of measuring the opposite side (10°). From GN follow the protractor clockwise until you hit the arrow head, that is where you read the angle!

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Summary;

1) Draw your basic sketch with NP or SP in centre, 000° and 180° at the right place accordingly and remember right is EAST and left is WEST

2) Add the meridians (be sure to measure from the correct datum 000°)

3) Add the track (remember that true track is measured from TRUE NORTH NP)

4) Slide the grid over your location and measure the track from Grid North