Wouldn’t it be great if you could drive between two points by simply making your own direct highway? You can (with some limitations) in the sky. You can fly between points using a navigation system called dead reckoning.
It’s an unfortunate term, dead reckoning. The “dead” part probably came from the word “deduced,” because dead reckoning means navigating an airplane based on calculations. If you fly in a straight line at 100 knots for 2 hours you have traveled 200 nautical miles. You then turn to a compass heading of, say, 180 degrees (south), and fly another hour. By planning your path in advance (called a flight plan) and sketching it out on your sectional chart you know which direction you’ll fly and for how long to get to your intended destination.
Dead reckoning is navigation using computations based on airspeed, course, heading, wind direction, wind speed, ground speed, and elapsed time.
Of course, flying isn’t quite that easy. In your car you know that if your odometer says you’ve driven 100 miles, you have. The wheels, in contact with the ground, have rolled 100 miles. As you’ve learned, there are other factors that add or subtract from the total distance traveled in an aircraft. For example, a tailwind (one from behind the aircraft) of 10 knots increases your speed over the ground. If the plane is flying at an airspeed of 100 knots, your ground speed is 110 knots (100 + 10). Likewise, a headwind (one coming toward you from directly in front of the aircraft) subtracts from your airspeed of 100 knots and gives you a ground speed of 90 knots (100 [nd] 10). As the ground speed changes, so does your time and fuel needed to get to you destination.
To make things more confusing, most winds don’t come at the plane directly from in front or the rear. There is what is called a crosswind component. The crosswind affects your ground speed and it also affects the direction you must point the plane to get from point A to point B. Part of your training will include learning to use an E6B or other computer to figure the crosswind components on a flight. (The E6B really isn’t an electronic computer but a type of slide ruler, although electronic calculator versions are available.)
Dead reckoning is a navigation system that calculates your flight path based on …
- Course (the path you’ve marked on the chart).
- Heading (the direction you fly on your compass).
- Track (the actual path you fly over the ground; you hope it matches your planned course).
- Wind direction (how the wind will affect your course).
- Indicated airspeed (what the airspeed indicator reports).
- True airspeed (the indicated airspeed adjust for altitude and temperature corrections).
- Elapsed time (how long it has been since you started or since you encountered a specific checkpoint).
The challenge of dead reckoning navigation is that there are so many corrections required to get an accurate answer. To name a few:
- The crosswind component changes your groundspeed and your track.
- The compass heading might be challenging to maintain in a plane that is encountering turbulence.
- Drifting off your heading can result in a change in your track and you will not maintain your planned course.
Fortunately, the E6B computer and its calculator cousins make recalculating heading and true airspeed (TAS) relatively easy — once you understand how to use these tools.
You will be asked questions on the FAA private-pilot knowledge and practical tests about dead reckoning navigation, so be ready. Buy an E6B computer (under $20) or E6B or flight calculator (under $100) and read the instructions to know how to use it.