Of all flight instruments, as well as among navigation instruments in general, the magnetic compass has been with us the longest. It's self-contained, nearly unfailing, and requires no external power. To use it correctly, however, one must understand it. Simple though it is, there is more to the compass--and the Earth's magnetic field--than the basic information that most of us learned in ground school. We learn quickly about magnetic variation and deviation, but many find two other concepts--turning errors and acceleration/deceleration errors--less than intuitive. Most flight schools and the FAA define them, but they don't really explain them. Let's remedy that!
Simple, but effective
The compass is considerably modernized compared to the lodestones first used in ancient Greece and China 2,000 years ago. A magnet, attached to a scaled and labeled ring and prevailed upon by the Earth's magnetic field, swings on a central low-friction pivot and floats in a sealed case filled with a clear liquid such as white kerosene. Because of the fluid our aircraft compasses are often called a "wet compass" or occasionally a "whiskey compass." We look behind a reference line marked on a transparent portion of its case to project or find our direction of travel. But let's start with the Earth, rather than the compass.
Texts depict magnetic lines of force sprouting from one pole, shooting down around the Earth along every meridian--pretty much paralleling the planet's surface--and then diving back in through the opposite pole. We learn that those two types of compass errors arise because the lines of magnetic force "dip" earthward near the poles, and we nod our heads in firm resolve never to trust our compasses the next time we fly up to the Yukon...and then we pretty much forget about it.
The truth is that the magnetic field doesn't just dip earthward above the Arctic Circle. Figure 1 (see p. 40) is a typical representation, as misleading as it is artfully drawn; Figure 2 is a more factual one. Thus, if your magnetic compass were built to pivot freely in three dimensions, instead of primarily just horizontally in two dimensions, it probably wouldn't be called a compass anymore; it would be called a dip needle or something similar. It would no longer just "point North" but, strictly speaking, it would align with the local magnetic field.
Go for a dip
An aircraft's compass is designed to be a bit less fickle than a mere seesaw, however, and it possesses some of the stability of a pendulum by being "under-slung," whereby its center of gravity is beneath the pivot point. Also, its motion is damped within a fluid, and it is also built to have enough inertia not to be excessively influenced by the vertical component of the Earth's magnetic field.
One thing that is qualitatively true about any such diagram is that the magnetic lines of force are closer together near the poles than at the equator, and the Earth's magnetic field actually gets stronger as it dives earthward; in fact it is about twice as strong at the poles as at the equator.
Take a wild stab at just what the dip angle is, even at a fairly temperate latitude of 30 degrees North: 5 degrees down from the horizontal, perhaps? Would you believe 50?! Yessiree. And at 60 degrees North, the dip angle is something like 75 degrees down. Of course, the magnetic North Pole doesn't coincide with the geographic North Pole (which explains variation, but that's another part of the story); it lurks somewhere off in Canada's Northwest Territories. Over the past few decades it's been slowly drifting across the Canadian Arctic at about 40 kilometers per year, and is presently at about 83 degrees North, 114 degrees West. So there is a longitudinal dependency as well.
Now we'll look inside our airplane compass. For starters, it's not like some hiker's compass that you look down upon in the palm of your hand. In most airplanes (except those with vertical compass cards) we look at the compass from the side. This means that they have to label the cardinal directions backwards. If you popped open a standard horizontally oriented compass, and viewed it from above in a "North up" perspective, you would see something like Figure 3. Of course, if you want to pick nits, I'd be remiss not to mention that the end of the actual compass needle that's dutifully pointed northward is also actually pointing toward an attracting opposite polarity. Either the North pole is really a "South" pole, or else the end of everyone's compass with an "N" on it might actually be more properly labeled with an "S."
The oblique perspective shown in Figure 4 reveals an example where we see an "N" staring us in the face. Unlike the case of a pocket compass, the "N" itself isn't pointing northward. The direction we must face in order to read it is what that letter represents.
Lagging, then leading
Now let's first summarize those dip errors we learned about in ground school. After we've done that comes the fun: finally getting them explained and understood!
First, the "what": With turns from the north, the compass lags the actual heading change (and in fact can initially turn in the opposite direction), but with turns initiated from south, the compass leads the change. When turning on easterly or westerly headings, turning errors are almost zero. However, on those same east-west headings, acceleration causes a northerly indication, and deceleration causes the compass to "swing South"--even though the actual heading does not change. (In a north-south direction, acceleration errors vanish.)
Now, here's the "why?": In these illustrations, imagine that the entire disk of the compass card is doing what the slim needle of a hiker's compass would be doing; in an aircraft, that's how it works, anyway. While the card is in a banked attitude, the vertical component of the Earth's magnetic field causes the "attracted end" of the compass to dip (rotate) to the low side of the turn. As you've now learned, "North" is actually more of a "down" than an "over" (unless you're near the equator). Say you're flying North, then you turn to the West (as in Figure 5). The compass needle "wants" to point North (and down) but in doing so, we see an eastward swing. (Imagine that "N" arrow rotating downward, and you'll see why the "E" would move into the window.)
Acceleration errors are explained in a similar way. Imagine that you're headed West, and you pour on the coal. Because of its pendulum type of pivot design, the compass will "tilt" much as is shown in Figure 5, and what happens is that the compass rotates counterclockwise as the needle dips earthward. For accelerations when you are already heading on or near 360 or 180, the compass is already "in line," and so no swing is induced. As far as memory aids for acceleration errors (at least in the northern hemisphere), it's hard to beat the old "ANDS": accelerate North, decelerate South.
Keep these ground rules in mind. (These aren't just academic minutiae. If your vacuum system fails in instrument meteorological conditions, you're going to be really interested.)
- Accurate compass-only turns require that the maximum bank angle be no greater than about 15 degrees.
- You need know only the approximate lead or lag angle to apply, then wait for the compass to settle and fine tune it, say with standard-rate turns.
- With acceleration/deceleration, the compass will settle down, and you don't really need to do anything, other than treat any apparent heading changes with caution.
- For northerly turns, since the compass lags, you'll actually have turned more than what you'll see. At U.S. mid-latitudes and about 100 kt, a good approximation is that your actual direction change will be about twice what you see (e.g., turning from 090 to 030, it'll show 060, etc.), so start out by undershooting by about half.
- For southerly turns, since the compass leads, you need to turn until it's beyond what you wanted, because it won't really be that much.
- At mid-latitudes, a good approximation is to turn to one-third more than what you're after (e.g., from 090 to 150, or a 60-degree turn, stop at about 170, or another 20 degrees of overshoot). Another rule, a bit rougher but simpler, is to lead by your latitude.
- If absolutely everything goes south in instrument flight conditions and the only instrument you have left is the compass, the best way for you to go (depending perhaps on intervening weather) could be South, as well. Because the compass indicates turns correctly (albeit with a little too much enthusiasm) on a southerly heading, and there are no acceleration errors, arguments have been made that attitude control could theoretically be maintained (well, at least prolonged) by use of the compass alone.
- Lastly, always lead your rollout heading by half the bank angle.
- These tips, combined with an understanding of how the compass works, will help to smooth your flight if you should find yourself relying on the compass alone.
Jeff Pardo is an aviation writer in Maryland with a commercial pilot certificate for airplanes, and instrument, helicopter, and glider ratings. He has logged about 1,250 hours since 1989. An Angel Flight mission pilot, Pardo has also flown for the Civil Air Patrol.
Want to know more?
Links to additional resources about the topics discussed in this article are available at AOPA Flight Training Online.
Related Articles
