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At least every 15 minutes we set the Heading Indicator's number to match the number of the magnetic compass.

In this situation, should I consider the deviation card below the compass card or not?

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  • $\begingroup$ Welcome to aviation.SE! $\endgroup$ – Pondlife Jan 10 at 1:16
  • $\begingroup$ If you have an electronic DG like the Garmin G5, you specifically don't want to do this because it's already compensated, but then again you don't need to set it every 15 minutes either. $\endgroup$ – Ron Beyer Jan 10 at 3:29
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Yes the DG is supposed to be set to the corrected magnetic indication whenever you set it, so you should mentally add or subtract the degree or two or three based on the closest segment on the card as a standard practice.

I agree with Michael however that it's not really that big a deal for VFR flying in the real world, especially in the age of GPS where you can just adjust heading based on GPS ground track on your phone or a tablet to go from A to B and you don't even really need to look at your compass (although, you might suddenly find you need it, so don't neglect it).

If you were doing old fashioned dead reckoning navigation, precision in holding a heading is important. Or in the IFR world where you have to fly headings in a clearance. Even there though, the allowable calibration error on a compensated (that is, WITH the deviation accounted for) compass is +/- 10 deg (!), so the little 2 or 3 degree calibration card can be quite a bit off anyway and still be legal.

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Personally I never bother, but then when I fly an airplane with a Directional Gyro that needs adjustment I am always VFR, and generally know right where I am going so I am not relying on a super accurate compass. (I know the Puget Sound area where I typically fly very well, and there are lots of excellent visual reference points) Also, plus or minus a few degrees is a little tough to even see on those small dials.

However, if you are working on an instrument rating, or already have one and are getting ready to shoot an approach, or are navigating on an airway, I would definitely get in the habit of accounting for the deviation. At least acknowledge the issue and make an attempt, especially if flying with a CFI or DPE.

Just remember the adjustment is temporary because it will drift away from your compensation shortly, so don't focus on dialing in that extra one or two degrees of accuracy at the expense of other essential tasks if you are overloaded.

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  • $\begingroup$ Even in the VFR world you should reset your DG from time to time. That’s especially true if you are doing training maneuvers like steep turns or turns around a point which make your DG precess more than normal. Then, if you are on flight following and ATC gives you a heading to fly to avoid other traffic you are flying in the right direction. $\endgroup$ – JScarry Jan 10 at 2:48
  • $\begingroup$ I agree, and I do. I just don't take the extra time to fine tune it to the compass deviation card. $\endgroup$ – Michael Hall Jan 10 at 3:05

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