Dead reckoning is navigation solely by means of computations based on four variables: time, airspeed, distance, and direction. A pilot starts from a known position, applies these calculations, and predicts where the aircraft should be at any given moment, without looking out the window or relying on GPS. It remains one of the foundational navigation skills in aviation, tested on pilot exams and genuinely useful when electronic navigation fails.
How Dead Reckoning Works
The core idea is straightforward. If you know where you started, how fast you’re going, which direction you’re pointed, and how long you’ve been flying, you can calculate where you are now. In practice, a pilot draws a course line on a sectional chart before the flight, measures the distance and direction, then uses estimated groundspeed to figure out how long each leg should take. During the flight, the clock becomes the primary instrument. If your calculated groundspeed is 120 knots and a checkpoint is 30 nautical miles away, you should reach it in 15 minutes.
The tools are surprisingly simple. As AOPA notes, all you truly need is a sectional chart, a pencil, and a compass. A plotter helps you measure the true course angle off the chart, and an E6B flight computer (a circular slide rule that dates back decades and still sits in most flight bags) handles the math for airspeed corrections and wind calculations. But even without those, you can roll a pencil parallel to your course line until it crosses a VOR compass rose printed on the chart, giving you a direct magnetic course reading.
The Wind Triangle
Wind is the biggest complication in dead reckoning, and solving it is where most of the math lives. An aircraft moves through a mass of air that is itself moving over the ground. This creates three vectors that form what pilots call the wind triangle.
- Air vector: the direction the airplane is pointed (true heading) and its speed through the air (true airspeed).
- Wind vector: the direction and speed the surrounding air mass is moving over the ground.
- Ground vector: the actual path the airplane traces over the earth (ground track) and its speed relative to the ground (groundspeed).
The relationship is simple in concept: the ground vector equals the air vector plus the wind vector. If you’re flying north at 100 knots through air that’s blowing from the west at 20 knots, you’ll drift east and your groundspeed will differ from your airspeed. To fly a straight course to your destination, you need to point the airplane slightly into the wind, a correction angle that the wind triangle solves. Pilots work this out on the back of an E6B or with a manual wind triangle drawn on paper, plugging in the forecast winds for their altitude to determine the heading correction and the resulting groundspeed.
Converting True Course to Compass Heading
A course line drawn on a chart gives you a true course, referenced to the geographic North Pole. But the compass in an airplane points toward magnetic north, which is not the same place. The angular difference between true north and magnetic north at any given location is called magnetic variation, and it changes depending on where you are. Sectional charts print isogonic lines showing the local variation so you don’t have to memorize it.
On top of variation, the metal and electrical systems in the airplane itself pull the compass needle slightly off magnetic north. This additional error is called deviation, and it’s specific to each aircraft. A deviation card, usually mounted near the compass, lists the correction for various headings.
The conversion chain works like this: start with true course from the chart, apply the wind correction angle to get true heading, subtract or add magnetic variation to get magnetic heading, then apply deviation to arrive at compass heading. Pilots remember the variation step with a rhyme: “variation east, magnetic least” (subtract) and “variation west, magnetic best” (add). Each step is small, often just a few degrees, but skipping any of them compounds the error over distance.
Building a Navigation Log
Before a dead reckoning flight, pilots fill out a navigation log (sometimes called a PLOG) that breaks the route into legs between visual checkpoints. For each leg, the log captures the true course, distance, winds, heading corrections, estimated groundspeed, estimated time en route (ETE), estimated time of arrival (ETA) at each checkpoint, and fuel burn rate in gallons per hour. The total fuel on board goes at the top.
This log becomes a living document once airborne. At each checkpoint, the pilot notes the actual arrival time and compares it to the estimate. If you arrive two minutes early, your groundspeed is higher than planned, possibly because the winds aloft differ from the forecast. You can then revise your estimates for the remaining legs. This constant comparison between predicted and actual position is what makes dead reckoning a dynamic process rather than a one-time calculation.
Dead Reckoning vs. Pilotage
Pilotage is navigation by visual reference to landmarks: rivers, highways, towns, airports. You look outside, identify a feature on the ground, find it on the chart, and confirm your position. Dead reckoning, by contrast, is entirely computational. You could fly it with the windows painted over (in theory) because it depends on instruments and math, not eyesight.
In real-world VFR flying, pilots use both methods together. Dead reckoning tells you where to look and when to look for the next checkpoint. Pilotage confirms whether you’re actually there. If the two disagree, something needs recalculating, either the wind correction, the compass deviation, or the groundspeed estimate. Used together, they form a cross-check that catches errors before they compound into being genuinely lost.
Why It Still Matters
GPS has made dead reckoning feel obsolete to many pilots, and the FAA itself acknowledges that the mention of dead reckoning often triggers uncomfortable memories of written exams and E6B confusers. But electronic navigation can and does fail. Batteries die, screens go dark, satellite signals get lost. The FAA’s own safety literature poses the question directly: are you ready to fall back to compass and clock if your electronic aids fail?
Beyond emergency backup, dead reckoning builds a kind of situational awareness that GPS doesn’t. When you’ve calculated your heading, timed your legs, and predicted your checkpoints, you develop an intuitive sense for how wind, speed, and distance relate to each other. You notice when something feels wrong, when the groundspeed seems too fast, or the heading doesn’t match the landscape, because you’ve internalized the math. That awareness persists even when the GPS is working perfectly, and it’s a large part of why dead reckoning remains a tested skill for pilot certification.
The FAA also recommends periodic compass swings, a calibration check for your magnetic compass, even though they aren’t formally required on a set schedule. Pilots who rely on dead reckoning as a backup skill would be wise to verify their compass accuracy at least annually, since the entire method depends on that instrument telling the truth.