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2 Answers

Interpreting the WCA (-) and (+) values

Asked by: 943 views General Aviation

Dear CFIs

Do I understand correctly?

If the Variation and Deviation are written as 10°E, it is the same as +10° and you subtract the value.

If the Variation and Deviation are written as 10°W, it is the same as -10° and you add the value.

Correct?

And how about WCA figures? When written as e.g. -012 how should this be interpreted?

Many thanks!

Regards, Ben

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2 Answers

  1. Best Answer


    Bryan on Jan 17, 2023

    For magnetic variation, you want to look at page 16-7 in the Pilot’s Handbook of Aeronautical Knowledge:

    If the variation is shown as “9° E,” this means that MN is 9° east of TN. If a TC of 360° is to be flown, 9° must be subtracted from 360°, which results in a magnetic heading of 351°. To fly east, a magnetic course of 081° (090° – 9°) would be flown. To fly south, the magnetic course would be
    171° (180° – 9°). To fly west, it would be 261° (270° – 9°). To fly a TH of 060°, a magnetic course of 051° (060° – 9°) would be flown. Remember, if variation is west, add; if east, subtract. One method for remembering whether to add or subtract variation is the phrase “east is least (subtract) and west is best (add).”

    For magnetic deviation, the card will only give you the values for every 30 degrees or so. If your calculated heading is between those, you have to interpolate. Using figure 16-12 from the PHAK, if your calculated heading were 015°, the compass card says 0° for N and 28° for 30°. So you would fly 014°–the middle of the two marks. Deviation does not have exact math–it’s interpolation.

    Finally, for wind, think of how you calculated the wind correction angle–you had to know both the direction you wanted to fly and the direction of the wind. If you’re using the slide style E6B, if your dot is to the left of your course, you correct left which will ALWAYS be subtracting the appropriate correction from your heading. If your dot is to the right, you correct right which will ALWAYS be adding the correction to your heading.

    Based on your last two posts, it appears to me that you are trying to ram these three concepts together in your mind. Don’t. The reason for each of these three calculations is different and that affects how the corrections must be applied. If they could be simplified into a single action through some memory trick or mathematical formula, it would have been done years ago. Take it one step at a time every time and you’ll be fine. Mush them together and you’ll make mistakes.

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  2. Benni2000 on Jan 17, 2023

    Many thanks Bryan! Much appreciated.

    I am studying the Navigation Theory GS and then attempting the sample questions.

    Some of the terms and concepts in the exams are nowhere in the course material. Hence these 🙂

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