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In Defense Of The Compass

It May Be Old-Fashioned, But It Works

The advertisement could read, "Fantastic navigational aid. No batteries needed! Always tells you where you are, where you've been, and where you're going. Guaranteed to never be wrong (well, almost never). Only $139.95 new or $85 rebuilt. Minor training for use required." Sound too good to be true? Some wild new GPS implant that is injected into your bloodstream when you are issued your pilot's certificate? Hardly. In fact, it's about the most old-fashioned thing in aviation. It's the compass.

In recent years, the old compass has been largely ignored. In fact, when was the last time you actually looked at your compass for anything other than to set your directional gyro (DG)? In this day and age, when all you have to do is punch the To button on your moving map to get a display of all of the information about your route, your progress, and the restaurant at your destination airport, it's hard to get excited about something that requires both thinking and understanding.

Why make life difficult? Why even think about going places with nothing but a compass when there is GPS? The primary reason to become a compass jockey is that there is a basic problem with the almighty GPS: It's a complex manmade system. This is another way of saying it has failure built into it. Like all machines, especially those that use electricity, it's just waiting to let you down at the least opportune moment. The compass, on the other hand, is always sitting there like some omnipresent Cyclops that sees all and knows all. Yes, even a simple compass can spring a leak and fail, but such failures are extremely rare. The downside of using the compass isn't in its reliability. It's that you need to do a little planning to make it work for you. That's also one of the upsides to a compass - it forces you to do the planning you were supposed to do in the first place. And there is a way to do that planning without getting all bogged down in forms, wind triangles, and all the other stuff that the GPS has helped us avoid. In fact, there's a way of using the compass that requires only a chart and a brightly colored pencil.

What I'm going to do is take the entire student pilot navigation course and boil it down to the bare essentials so it can be put on your sectional and flown using nothing but your compass. Of course, the exact same method works with a GPS. So, if you do this basic planning, you'll be prepared for the day you lose your satellite signal, the batteries in your handheld die, or the electrical system powering your panel-mounted unit shuts down.

First, think about what the compass tells us that the GPS doesn't. For one thing, a GPS doesn't actually tell us in what direction the airplane's nose is pointed because it doesn't really care. It only cares about the ground track and your position relative to the destination you have programmed in. So, you have to use the DG and mess with heading changes to hold that exact track. But, of course, the DG is set by the compass. So, there you are, back at the compass again.

The compass, once calibrated, tells you in which direction the nose is pointed, but only in a stable situation. In turns, the heading shown on the compass will lead your actual heading or lag behind it, depending on which direction you are turning. The mechanics of that process are a subject for ground school. For now, don't worry about which it is doing. Just make gentle turns followed by subtle corrections initiated after you have leveled the wings. The compass may also be inaccurate if you are accelerating or decelerating. Again, don't worry about how this works for now.

I said the compass works once it's calibrated. A compass can't "find" itself like a GPS will. It's up to the operator to get it set up right. It takes only a few minutes on a compass rose (that circle with compass headings found painted on a ramp or taxiway at many airports) with a plastic or brass and aluminum screwdriver to tune it up. (The type of screwdriver is important because compasses are magnetic and some metal screwdrivers send them spinning.) Once you've made the adjustments, you need to write the deviations for various headings on the card below the compass.

If you don't have a compass rose, you can do the same adjustments on a runway. First line up with the runway, then set the compass, turn 180 degrees, set it again, and turn 90 degrees to the runway in both directions, checking to see how far off the compass is. With that information you can guesstimate the deviation points in between those points. If you use a runway, check to see what the actual heading of the runway is before you start since the number has probably been rounded off.

However, even if your compass is totally out of whack and a huge number of degrees off, you may still be able to use it. As long as it is free-swinging and reflects your heading changes, it is still a reliable navigation aid because it's only important that you figure out which number is the one that points you where you need to be pointed. I'll show you a way to work with a really sick compass in a minute.

The various compass errors should be understood to make your planning easier and keep confusion from setting in once you are in the cockpit (this is all early ground school stuff). Just know that the line you draw on a chart has a true direction (degrees as measured from true north, the black longitudinal lines on the chart) and it has a magnetic direction (degrees as measured from magnetic north). The difference between the two is the magnetic variation, and it changes for different parts of the country and different locations. It's marked on charts, but sometimes the numbers are a real bear to find. If you're ever in doubt, there's a simple way to figure it out. Just draw a line parallel to your course that goes through a VOR rose marked on the chart. VOR degrees are magnetic, not true, so you can read your magnetic course for that location right from the VOR rose. Again, the difference between the true course you've drawn on your chart and the magnetic course that runs through the VOR rose is the magnetic variation.

If you had a perfect compass in a totally nonmagnetic airplane and there was no wind pushing you around, your compass heading would be the same as your magnetic course. That never happens, however, because of the deviation caused by the fact that every compass is slightly different and reacts to the airplane it's installed in differently. Airplanes are full of metal and stray electronic and magnetic currents that coax the compass into saying things that aren't entirely true. That's why a compass deviation card is necessary for each airplane, and that's what you come up with from "swinging" your compass on a compass rose. The process tells you which compass indication actually points you toward magnetic north, magnetic east, etc. The compass deviation card tells you, "For 270 degrees, steer 272 degrees," and so forth.

Now that we've explored magnetic and true courses and headings, you might be confused. The good news is that none of this makes any difference. That statement ought to get the attention of a few traditionalists. So will this one: Wind correction isn't critical either.

Let me explain. There are lots of ways to work out all these details (compass heading is true course corrected for magnetic variation and installation deviation and then corrected for wind drift). But most of them are too confusing. Yes, you absolutely should add up all the plusses and minuses and work your wind triangle on your trusty E6B. However, once you are in the air, the correct heading is the one that will take you from one checkpoint to another, and that almost never exactly matches your calculations because your speed and the wind are never exactly as projected. So, when planning your course, set up two checkpoints right at the beginning of the course that are close enough together that you can see them both at the same time. Then just line up on the first one and keep correcting the compass heading until your ground track points at the second checkpoint. It's a little crude, but nothing beats getting right up there, where you aren't working with projected winds and generalized magnetic variation information. Even if your compass is so screwed up that it says you're going south when you know you're going west, this method will pick out a number on the compass that is good for that particular course in those specific winds. All you have to do is hold that compass heading.

A variation on the same idea is to take off, circle around the airport in the opposite direction of the desired course while climbing to cruise altitude, then fly across the airport while heading toward the first checkpoint. Keep changing your heading as necessary to hit the checkpoint, and that'll give you the right compass heading.

Now, back to the paperwork or, more correctly, the elimination of paperwork. There are certain facts that we all want to know when flying cross-country, but the most important ones (once the proper heading has been determined) are, in this order, where are we and how long will it take to get where we are going? Any additional information is just gravy. First, understand that what you're going to do is mix dead reckoning (using nothing but heading, time, and speed to determine your position) with pilotage (looking out the window and determining position by landmarks).

When you draw the course line on your chart, you're going to make the assumption that, at some point in the flight, every piece of avionics is going to die. Your only tools will be the compass, a chart, a pencil (which is sort of optional), and a wristwatch (also nice, but optional). So, everything having to do with this flight will be written on the chart before takeoff. It'll all be included on the course line.

First, draw the line in some color that stands out from the background. You want to be able to see everything you write on the chart even in the worst lighting conditions. Then, make little cross-hatches on the line at given increments. If you fly a 120-mph airplane, do them every 10 miles; for faster birds, you might want to do them every 15 or 20 miles. It's your call, but you need them there so you can simply count them up between any combination of checkpoints and mentally come up with a distance without using a plotter to measure it. Measuring is a pain in the empennage in a crowded or busy cockpit.

Getting to the final destination isn't as important as making it to the next fuel stop, so imagine this flight as a series of legs from gas pump to gas pump and plan accordingly. Then, pick out really obvious checkpoints. Really obvious. Again, if you're flying a slower airplane, put them closer together. For faster airplanes, spread them out more. However, be warned that the farther apart they are, the higher the likelihood that any navigation error is going to get you lost. Ten to 15 minutes between major checkpoints is plenty, depending on how conservative you are as a pilot.

On those charted checkpoints, draw a big Z-shaped mark with the Z lying on its side with the right leg pointing in the direction you're going and the left leg pointing back the way you came. On top of the right leg, ahead of the arrow formed by the corner of the Z, boldly print the remaining distance to the next fuel stop (checkpoint). On the bottom of the left leg, below the arrow, print the distance you've come so far. At a glance you know how far you've traveled and how far you have to go. On the top left corner of the Z, put the estimated time en route (ETE) it will take to get to that point. On the bottom right corner, under the distance-to-go-arrow, put the ETE to the next fuel stop (checkpoint). At a glance you can estimate how long it should have taken to get where you are and how long it's going to take to reach the next destination.

From this point on, start mentally tracking trends, both in terms of heading and in terms of time. When you take off, mark the time at the bottom of the course line. During preflight planning, you estimated how many minutes it would take to reach the first checkpoint and wrote it on the chart. By mentally adding those minutes to the takeoff time, you should know about what time you expect to hit the checkpoint. Of course this only works if the forecast wind is the same as the actual wind, and usually it won't be. If you want, you can write that estimate time of arrival next to the ETE on the left side of the Z as you fly, or you can just remember it. However, when you do hit the first checkpoint, write down the actual number of minutes it took to get there under the number of minutes you had estimated it would take. Any difference between the two tells you how accurate your projected winds and groundspeed were. After you've done that for about three checkpoints, you're going to see a trend developing that clearly shows how many minutes ahead or behind your estimates you are. You can use that information to extrapolate ahead for an arrival time at your destination. Without even figuring your actual groundspeed, you'll be able to guesstimate how long it's going to take to get to the next fuel stop. If you want an exact figure, you can work out your groundspeed and use that to project your arrival time. That's a good way to go if timing is critical; otherwise guesstimating will put you within a few minutes of your actual time.

While all this is going on, you'll be doing your level best to hold exactly the compass heading you established while flying inbound to the first checkpoint. Once you decide what that compass number should be, write it next to the checkpoint Z. If you don't hit a checkpoint dead center, correct a degree or two until you're hitting them like zipping up a zipper.

Incidentally, you don't need to stare at the compass to keep it steady. Pick out something on the horizon and line it up with something on the cowling as if you're using a gun sight. If the reference doesn't move, the compass won't either. You will have to peek at the compass every minute or so to make sure.

If you take the time to do a little planning and you put your trust in your compass, you'll always get where you're going - even if all of your electronic navigation aids quit. And one day, they probably will.

Budd Davisson

Budd Davisson is an aviation writer/photographer and magazine editor. A CFI since 1967, he teaches about 30 hours a month in his Pitts S–2A.

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