Compass Turning Error

[TopGun1234]

I'm having a hard time understanding this one and I can't find any material online that addresses this.

I know that when we turn from the north to south that our compass will lag.

And when we turn south to north our compass will lead.

In practice though, when flying on a east heading and then turning north, our compass will actually lag.

Why is this happening?

[digits_]

"Never see North, always see South"

If you are turning on a Northerly heading, the compass will lag. To start E and stop at a N heading, you'll have to roll out at 030 indicated (roughtly, depends on your exact location). If you are turning around a Southerly heading, the compass will lead. To roll out from E to South, you'll have to roll out around 210 indicated.

This only works properly in areas where the magnetic field is strong enough. In Europe for example, this works beautifully. In big parts of Canada, this won't work properly, and the compass could start turning in the opposite direction and quickly spin. A technique with timed rate one turns would be better suited here.

Regarding your question: the part that is important is the last part of your turn, as you approach your heading. So going from S to N (Via E), the only thing you should be concern about is the part from E to N, not the S to E part. Eventually you compass will go through E and you can start anticipating the lead or the lag.

Your confusion is probably caused by the fact that during any 180 degree turn, the compass will at some point lead and lag.

[youhavecontrol]

To simply help my students remember which is which, I explain it to my students by saying that the North is "cold", and the compass "slows down" (lags) around the North because it's cold and groggy. While that's just a silly simplification, it helps them remember. Alternately, turning through the South produces the opposite effect, because the compass is warm and happy to be South.

As to what's actually happening, here's a video describing in better detail: https://www.youtube.com/watch?v=lh9vaXWsBw0

[digits_]

That movie shows what happens, but is very brief on why it happens.

To fully understand what happens exactly, try to understand the image below:



There are 3 things:
- you have the compass with a bit of a dip. The dip is caused by the compass that is pointing towards North. North is (usually) below the airplane, so the N end of the needle will point downwards
- this causes the center of gravity of the compass construction (indicated by the circlue with black and white) to not be vertically aligned with the needle point on which the compass rests
- if the plane accelerates, the needle point is being moved initially, and the center of gravity (the circle) has, initially, the tendency to remain stationairy. This causes a turning moment in compass

See if you can figure it out. The accleration error will be mostly visible on an E or W acceleration.
The turning error (which your original post was about) wil be mostly visible by a turn that start N or S.

Remember, the plane only moves the compass by the needle. That's the only connection between the airframe and the compass rose.

[TopGun1234]

Thanks for clearing this up. My understanding wasn't correct at all.

[redlaser]

Turning N to S, lag in the turn, E is E, E to S lead. Now if you go S to N, it will lead, but W is W, W to N it will lag, When passing through either E or W the compass will read correctly with no error, Have your instructor demonstrate it to you in the air,

[trey kule]

I know you did not ask this, but,
This all can be demonstrated in a plane during a lesson and you can see it happening. Takes maybe 2 minutes out of a lesson.
And. What is the practical lesson here to learn?
The great little reliable mag compass should only be used when flying unaccelerated in fairly calm air. Not of much use in a turn.

Somehow the practical application of this knowledge gets lost in thetheory

[photofly]

It's kind of fun when you get the compass to stop, and turn backwards, during a steep turn