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6 Answers

Should the compass deviation card be used?

Asked by: 4098 views General Aviation

As you know, We set at least every 15 minutes, Heading Indicator's Number from the number of the magnetic compass. In this situation, should I consider the deviation card below the compass card or not?? And plus... in the answer, reference please?(by FAA 8083 or so) Thank you!

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6 Answers



  1. Craig on Jan 09, 2020

    The PHAK 8083-25B 2016 p. 8-22 reads as follows:

    It is important to check the indications frequently (approximately every 15 minutes) and reset the heading indicator to align it with the magnetic compass when required. Adjust the heading indicator to the magnetic compass heading when the aircraft is straight and level at a constant speed to avoid compass errors.

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  2. dgssss on Jan 10, 2020

    Well No,,, I meant that

    “”DEVIATION CARD””

    Anyway thank you for your answer.

    DEVIATION CARD looks
    Something like this

    360 90 180 270
    359 92 179 270

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  3. Craig on Jan 10, 2020

    As I previously wrote, both the PHAK 8083-25B and the Instrument Flying Handbook (FAA-H-8083-15B) indicate that the directional gyro/heading indicator should be aligned with the magnetic compass (MC), not the compass deviation/correction card (CDC).

    After that, you figure out what heading you actually want to fly by referencing the compass deviation/correction card.

    Example: Assuming you’ve done your calculations for true course, variation, winds, etc….. align the DG with the MC. Then if you want to fly a magnetic heading 080 and the CDC indicates to do so you would fly a MC heading of 085, you also use the DG to fly 085. You used the MC to align the DG so the DG should have the same deviation error as the MC…until it starts precessing.

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  4. Warren Webb Jr on Jan 10, 2020

    I set the DG to the compass including the adjustment for compass deviation. I.e. if I see the compass card at 90, and the correction card indicates For 090 Steer 095, I know the nose of the airplane is really at 085, so 085 is what I set on the DG. The nose of the airplane and the DG are then in sync with the correct magnetic heading. When I turn to a new heading, I’m done. The nose and the DG are on the correct heading. I don’t have to look at the correction card for any deviation and then make another turn to fly the new heading, and I’ve seen deviations as much as 10 degrees. The only cross-check needed should be the regular check for precession. The whole idea of the DG is to have a compass reference that is free from all of the typical errors of the compass (lead/lag, acceleration, turbulence, deviation, etc), that minimizes the workload, and is suitable for instrument flight.

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  5. dgssss on Jan 10, 2020

    Craig – Yeah. in the cross country I remember calculating the TC / MH / WCA / DEV / VAR -> to get the CH. So when HI = Compass then, I could probably use the calculated numbers by the xc plan. However the problem is..

    as Warren mentioned,

    When changing the heading…

    For example, if the Dev is +5 deg in North, and for other headings, there were no error, and then the HI indicates very inaccurate heading from the original heading. In this case(without matching HI from magnetic compass after turns), The error will be bigger and bigger…

    So, I think I can figure out that, every time I have to change the heading, before and after changing heading set the HI form magnetic compass in straight and level flight.

    I think this way is the best that I can choose with prior way of xc plan and minimizing compass errors and precession.

    If I’m wrong, let me know.

    I didn’t choose the answer cuz I’m not certain yet.

    Thanks.

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  6. Warren Webb Jr on Jan 11, 2020

    If the HI is set correctly, there will be no deviation error on other headings.

    Setting the HI on the ground or in the air is done in these steps. Note the compass card, go to the ‘steer’ line, go to the ‘for’ line, set the HI. So using your example above, if you are setting the HI on the ground or in the air, note the compass card value (let’s say it is 005), go to the ‘steer’ line and find 005, go to the ‘for’ line and find 0 or N, set the HI to N. You now have the HI, the nose of the airplane, and the 0 degree magnetic line on the ground below the airplane all in alignment.

    Once the HI is set to the correct magnetic heading and assuming it is working correctly and doesn’t change from precession, it doesn’t have to be touched again. It started out on the MH and stays there.

    The nav log is actually used in two different ways (I have never seen this explained in any handbook).

    1. If flying an airplane with no HI, turn the airplane so that the compass card is on the CH calculated on the nav log.

    2. If flying with a HI, turn the airplane so that the HI is on the MH on your nav log, not the CH.

    The sequence for calculations on a nav log is left to right TC – TH – MH – CH because we start by drawing a TC line on the chart. However the sequence for setting the HI is right to left because we start by getting the position of the compass card (CH, MH, set HI).

    If you set the HI as above, you can turn to any heading and it will remain on the correct MH without any additional adjustment, so it will reduce workload. But the truth is, most deviation amounts are quite low so even if you don’t set the HI exactly right, you’re usually not too far off, but overall it will result in resetting the HI more times in any given flight and flying hopefully only briefly on an incorrect heading.

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