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

It makes sense that this would reduce or prevent north/south-facing turning errors, and would also create acceleration errors while facing east or west.

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.

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 professor 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 '19 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 '19 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 '19 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).

I do not have any reason to believe that the simple magnetic compass technology has changed significantly in the past 10 years, so it looks like it is true that the north-labeled side of the compass card is indeed heavier than the south-labeled side.

This weight is intended to mitigate the north/south turning errors, but is the cause of the acceleration errors.

Old FAA IFH

enter image description here

Current FAA IFH enter image description here

A third, yet less official, source that corroborates this information is here.

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Some compasses are counterweighted to decrease magnetic dip. Older and/or cheaper compasses for land navigation are this way. Many to most are not weighted, especially in aviation compasses. Weighted compasses would require different compasses for the North and South hemispheres. Instead, aviation wet compasses are built with the low friction mounting point above the CG and magnetic bar. This causes the CG to move to the South polarity end of the compass assembly when the magnetic bar dips downward to the North Pole and to the North polarity end when dipping towards the South Pole. I like this article that explains this phenomenon.

Several decades ago, my unit had the opportunity to train with the Australian 3rd Para Regiment in Australia. Many of us had been using personally owned GI lensatic compasses because the tritium markers worked better than the newer, issued lensatic compasses. We were warned that the older models may not be accurate down under due to them being Northern Hemisphere compasses instead of the universal ones. I did not understand the reasoning for this until many years later.

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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 '19 at 12:45
  • $\begingroup$ What people who say there is not weight are trying to tell me is that acceleration is causing the compass card to "teeter-totter" and tilt and that slight tilt causes a dip effect. While that would make sense if it were tilting, I don't think it does beyond a negligible amount. When you accelerate/decelerate, you see no significant tilting in the compass card, and the minor tilting you do see is not commensurate with the amount of card swing, so it stands to reason that its primarily because of a weight imbalance causing interial lag, not because of tilt-dip. $\endgroup$ – Ryan Mortensen Jun 14 at 17:24
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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 '19 at 6:55
  • $\begingroup$ Based on what I have learned on this subject over the past year, this answer is not correct. There is indeed a weighted side and it is not meant to be a perfectly balanced system. It is just a mitigation. $\endgroup$ – Ryan Mortensen Jun 14 at 17:17
  • $\begingroup$ There is a weight at the bottom so that it returns to a level position. This weight is subject to accelerations during the flight. There is no weight on one needle tip though, otherwise the compass would only work on one latitude exactly. $\endgroup$ – Jan Jun 16 at 6:17

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