August 2009Features

Dead Reckoning

Deducing your way from point A to point B


Did you know that it's possible to fly from one place to another without using a GPS, referring to ground-based radio navigation aids, or relying on vectors from air traffic control? All it requires is a sectional chart, a pencil, and a compass. An E6B flight computer makes it easier, as does a plotter, but even those aren't required. It may not be as easy or seem as useful, but using dead reckoning and pilotage as a backup to your GPS means never having to say "I'm lost" if the batteries die.

In its purest form, dead reckoning'short for deduced reckoning'involves picking a compass heading and holding it until you reach the next waypoint, while using elapsed time to calculate your position. If you're lucky, you have some interim information that allows you to adjust your compass heading to correct for wind-sailors used a rock on a rope to gauge drift, but we can call flight service and get the winds aloft along our route. Even without the corrections, however, by holding the compass heading, you'll come out somewhere in the general neighborhood of where you want to be. If there are no major wind changes and the leg isn't too long, you may even hit it on the nose.

If you monitor your progress across the ground below, and don't keep your nose pressed to the GPS screen, you're actually navigating with a combination of dead reckoning and pilotage. Here, ground references help to verify the accuracy of your dead reckoning.

Navigating by pilotage and dead reckoning offers myriad benefits. Planning every trip as if the GPS won't be along develops a pilotage and dead reckoning mind set. In effect, you've flown the trip in your mind in great detail during the initial planning. This puts a general outline of flight times, airports, and the topography along the route in your mind's eye so you'll automatically know if something is going astray.

This kind of approach also prepares you for the changes that are bound to happen along the way, especially on a longer trip. As part of the planning you'll get the winds aloft at select points along your route. Since you'll be expecting wind, you'll factor its anticipated speed and direction into your calculations, and if appropriate, the heading you'll fly will include a wind correction angle.

The attention paid to the compass heading is probably the most important safety factor to come out of planning via dead reckoning and pilotage as it relates to flying by GPS. The compass heading is all-important with dead reckoning and pilotage, so you're constantly aware of it when flying. Then, if your GPS takes a break, it's only a minor inconvenience because you've already fine-tuned the compass heading and all you have to do is hold it. And regardless of the navigation method, holding a heading is a basic flying skill that must be mastered.

It starts with a line

Whether pure dead reckoning or with pilotage sprinkled in, the planning starts the same way'with a line on a map. On each of the legs you'll pick checkpoints which, depending on the speed of the airplane, could be anywhere from 10 to 20 miles apart. Remember that the farther apart they are, the less certain you'll be of your progress.

When selecting checkpoints make them absolutely unmistakable, even if you can't find one at the exact increment you'd like. Don't choose a bend in a river, for instance, unless the river is the size of the Mississippi and the bend forms your initials'or it's the only bend for miles. Do chose something unique, such as two railroad tracks side-by-side crossing a river next to a racetrack. Don't pick a highway interchange in the middle of a city because there will be dozens of them. In other words, don't pick a checkpoint that has any chance of being mistaken for something else. It's amazing how in the heat of thinking you're lost your mind will "identify" a checkpoint and from there on, everything goes downhill.

The line that is drawn on the map has a direction as measured from north. The problem is there are two norths: true north and magnetic north. The lines running true north converge on the poles (lines or longitude), but the magnetic lines don't. They converge on magnetic north, which is offset from the poles. Picture an orange with a black spot a half-inch off the navel on each end. Now draw lines from black spot to black spot and you can clearly see that they cross the true north and south lines at an angle. But the angle changes as you go around the orange. That is the magnetic variation that has to be worked into the course planning.

In the United States magnetic variation runs from 10 degrees west in New York City to 15 degrees east in San Francisco, a difference of 25 degrees. That's a lot. It's easy to see why you don't just draw a line on a chart and take off on a long cross-country. On shorter cross-countries, however, the difference isn't really a factor.

If you don't want to worry about figuring out local variation (it's marked on each sectional), or you don't have a plotter, roll a pencil parallel to the course line until it lays across a VOR compass rose. Those are marked in magnetic degrees so you can get a direct reading of your magnetic course.

Just matching your compass heading to the magnetic course line doesn't work either because every compass installation is different, and the compass error'the magnetic deviation'is also different. The steel and electronics installed in the general neighborhood of the compass pull it off center, requiring you to add or subtract a few degrees. By regulation, there's a deviation card on the panel of every airplane that shows how far out of whack that particular compass is at the cardinal headings. Once you apply the deviation of that particular compass to the magnetic course, you have your compass heading. This is assuming, of course, you're flying in a vacuum with no wind.

Welcome, wind

Wind is the cross-country pilot's bogeyman. You can't feel the wind when you're in the airplane; it could be dead calm or blowing 20 knots right across the intended course and you'd never know the difference. More important, you wouldn't know how much it has changed since you took off. It's a safe assumption that any cross-country longer than about three miles is going to have you crossing into different winds than you had on takeoff. During the planning session we religiously call flight service and check the winds aloft at different locations for the times we expect to cross the reporting points. And we make very clear notes on the flight planning card about the wind forecasts and how many degrees you'll be required to add or subtract at each of those points.

If you don't have a radio to call flight service to check the winds while en route, you can, at the very least, watch for indications on the ground. Winds aloft are almost always different (sometimes quite different) from what you see on the surface, but indicators such as smoke, water, and open fields will give you a general indication.

The truth is that everything you do in the planning can be corrected by using an old-fashion airmail pilot approach to the first couple of checkpoints, which should be in sight of each other. The first checkpoint should be within easy sight of the departure airport. To try this, take off and climb for a couple of minutes in the opposite direction. As you hit cruising altitude, turn and go across the airport, aiming right at the first checkpoint while holding the calculated compass heading. If it is right, it will take you right to the first checkpoint. If it's not, you'll see you're getting off course and can turn just a few degrees to correct.

When you hit the first checkpoint, you should be able to see the second and you'll repeat the process. By the time you cross the second checkpoint, you should have a compass heading that exactly matches the wind conditions at that altitude and in that location. From that point on, the checkpoints will be further apart, and if you see yourself going left or right of them, you can change the compass heading to match the existing conditions. It may not be the most direct route, but it works.

Incidentally, don't waste space on the sectional chart after drawing your course and finding your checkpoints. For my money, I like to see as much of the time and distance information as possible put right on the chart. Yes, all of it is on a navigation log that's on a clipboard, but I like to see it represented front and center on the sectional chart. If it's put directly on the sectional in a very visible color with fine-tip marker, you don't have to keep looking back and forth from the sectional to the navigation log, which makes it much easier to hold the sectional in one hand and the controls in the other.

Many pilots put this information on the sectional drawing a Z-shape format, which puts the information in an easily understandable graphic style. Some folks put a Z on each checkpoint, others every second or third one. That's up to the individual and how long he is willing to wait for information updates. If there is a GPS in the mix, the Zs can be spread farther apart; it's fun to see how close your calculations come to what the GPS shows. If a GPS isn't being used, don't spread the Zs out too far.

Even if you own a GPS, you might try taking a cross-country using only pilotage and dead reckoning. It'll do two things for you: It'll make you appreciate the miracle of the GPS that much more, but it'll also give you important tools that will prevent you from getting lost if the GPS lets you down.

Budd Davisson is an aviation writer/photographer and magazine editor who has written approximately 2,200 articles and has flown more than 300 different types of aircraft. A CFI since 1967, he teaches about 30 hours a month in his Pitts S-2A Special. Visit his Web site.

True north

Magnetic north

There's no right or wrong way to approach the planning for a cross-country flight involving dead reckoning. But follow the simple steps below to get you on your way. The Earth shows one of the more abstract concepts of navigation'that the Earth and sectional charts may be based on true north, but our compasses follow a different path.

Follow these simple steps to plan your course.

1. Lay out course line (true course).

2. Determine magnetic course (add or subtract magnetic variation).

3. Apply the airplane's compass deviation.

4. Work out wind effect for actual compass heading.

Remember to calculate these important things at each checkpoint during the planning process.

  • Distance traveled
  • Distance to go
  • Estimated time en route
  • Estimated time of arrival at the checkpoint (added just after takeoff)
  • Ground speed (estimated)
  • Compass heading
  • Distance between checkpoints

Do your planned figures equal the real numbers? Check the following at each checkpoint during the flight.

  • Actual time en route
  • Actual time of arrival
  • Ground speed calculated from above
  • Compass heading to note if it has changed from last one to hold course


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