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When I was in college, I had an instructor who taught me that the turning errors in a magnetic compass were due to a counter-weight installed on the south side of the compass that was intended to counter the magnetic dip on the northern side.

This makes sense for both acceleration and turning errors.

For example, flying east the weight would be on the right, south-facing side, of the compass, which is the side labeled as 'N'. Now imagine a straight line acceleration...

The heavier, right side, would move rearward in relation to the aircraft due to its inertial lag, bringing the, weighted, southern side of the compass (which reads north) closer to the compass window causing the viewed indication be a number closer to north, making the "accelerate north" error, true.

This same idea also holds true for the north and south (Lead North, Lag South) turning errors because the weighted side of the compass would basically fall to the direction of bank.

Edit: (This is false, and the weighted side would not 'fall')

For example, flying due north, the south side of the dial, marked north, is facing you and is weighted to counteract the downward pull of the north pole on the north-facing side of the dial. If you turn right, the side facing the pilot would fall to the right, making the dial spin counter-clockwise, and a right turn would ultimately make the dial spin in that direction anyway to indicate east causing the "lead north" phenomenon because the weight made it swing around premature to the actual turn of the aircraft.


The thing is, I have never seen this explanation on paper, just that it is "magnetic dip" error. Is it true, and can you find a source that says so, or was my instructor wrong?

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  • $\begingroup$ None of my private pilot training, or instrument training, mentioned having weights. They did train us on leading and lagging turning effects, and the effects of acceleration and deceleration. You can see these yourself by taking a normal camping compass, which is clearly not weighted, and play with turning and accelerating in your car. en.wikipedia.org/wiki/Magnetic_dip en.wikipedia.org/wiki/Compass#Compass_balancing_(magnetic_dip) $\endgroup$ – CrossRoads Apr 8 at 2:07
  • $\begingroup$ "This same idea also holds true for the north and south (Lead North, Lag South) turning errors because the weighted side of the compass would basically fall to the direction of bank." - In a coordinated turn, objects don't fall toward the lowered wing, because gravity and centrifugal force balance each other out. (You still feel a vertical force from gravity and centrifugal force, just not a lateral force.) $\endgroup$ – Terran Swett Apr 8 at 3:20
  • $\begingroup$ @TannerSwett yeah, I realized that during my researching, and my answer makes a comment to the assumption about the question being false. I went ahead and edited the question to point that out. $\endgroup$ – Ryan Mortensen Apr 8 at 4:29
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I found a source in the old version of the Instrument Flying Handbook, page 3-14 that says, "In acceleration error, the dip-correction weight causes the end of the float and card marked N (the south-seeking end) to be heavier than the opposite end" (ASA-8083-15A).

However, the current copy hosted by the FAA does not have this same wording under the acceleration error section (ASA-8083-15B).

Now I wonder why they changed this in the new edition.

Old

enter image description here

New enter image description here

From what I have gathered there, it appears that dip error and inertial error are separate phenomena. I mistakenly assumed the weighted side would "fall" due to gravity, but that north and south error is indeed magnetic dip.

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There are no weights added. But the key piece of knowledge is that your compass needle (or the sensing magnets inside the @compass') does not lie on the horizontal, except at the Equator.

Due to the dip of the Earth's magnetic field, your compass/sensing magnets will be slightly nose-down in the Northern hemisphere, and slightly tail-down in the Southern hemisphere. This offsets the CG of the needle/magnets, which gives the appearance of having weights added to a horizontal needle (that isn't horizontal!).

If you now accelerate/decelerate on an Easterly/Eesterly heading, then the offset CG will lag slightly, giving the appearance of a turn. And on a Northerly/Southerly heading, if you bank the airplane the needle/magnets can align themselves with the dip in the Magnetic Field.

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  • $\begingroup$ The old copy of the handbook I mentioned definitely says there are weights. $\endgroup$ – Ryan Mortensen Apr 8 at 12:45
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If that were true you could only use your compass for one specific latitude and only for the northern hemisphere, which means there would have to be label on the compass warning pilots only to navigate between latitude x and y. And international flights would need dozens of compasses in their cockpit, one for each band of latitude where the compass needle is balanced enough. So no, there is no counter weight that balances your compass needle to be precisely horizontal.

The reason why your compass is showing compass errors is that your compass card pivot point is above the needle and mass center of the compass card; and that the earth magnetic lines are not parallel to the horizon but hit the surface at an angle.

As you accelerate your compass card swings back, then your needle is trying to align itself again with earth's magnetic field lines (that are at an angle to the horizon), which causes the indication error. This happens whenever the compass is tilted, e.g. acceleration or deceleration, climbing, descending, banking.

As soon as you imagine the earth magnetic field lines you can understand why it is rotating either left or right.

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  • $\begingroup$ Why would it swing from acceleration in level flight if it is balanced though? $\endgroup$ – Ryan Mortensen Apr 8 at 6:55

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