What the E6B does and why pilots carry one

The E6B flight computer is a mechanical calculator designed to solve the math problems that come up before and during a flight. It performs multiplication, division, fuel burn calculations, time-distance-speed problems, and unit conversions without batteries or electricity. Pilots use it to plan routes, calculate how long a flight will take, figure out how much fuel they need, and convert between different measurement systems — all tasks that come up on every flight.

The device has two sides. The front is a circular slide rule with scales printed around the edge and a rotating disc in the middle. The back is a wind triangle — a small protractor-like tool for calculating how wind affects your ground speed and heading. Both sides solve real problems you face in the cockpit or at the flight planning table.

The E6B is not required by regulation, but it is the standard tool in general aviation because it is fast, reliable, and works when your battery is dead. Many flight schools still teach it, and many pilots keep one in their flight bag as a backup to electronic calculators.

Key Takeaways

  • The front side of the E6B is a circular slide rule that solves multiplication, division, and unit conversion problems by aligning numbers on the outer and inner scales.
  • The most common flight problem — time, distance, and speed — uses the inner wheel: set your ground speed under the index mark, then read time and distance across from each other on the outer scales.
  • Fuel burn calculations work the same way: set your fuel burn rate under the index, then read how much fuel you need for any distance or time.
  • The back side is a wind triangle used to find your true heading and ground speed when wind is present, by drawing lines on the grid and reading the result.
  • The E6B requires practice to use smoothly, but the basic method — align one number, read the answer — stays the same for nearly every calculation.

The front side: setting up the slide rule

The front of the E6B has a fixed outer ring with numbers and labels printed around the edge, and a rotating inner disc that spins freely. The outer ring holds the main scales you read from; the inner disc holds the scales you align to set up your problem.

At the top of the outer ring, you will see a small triangle or arrow called the index mark. This is your reference point. Every calculation starts by rotating the inner disc until a number on one of the inner scales lines up with the index mark. Once that number is set, you read your answer by finding a second number on the inner scales and looking straight across to the outer scales.

The scales are labeled with their purpose. You will see "MPH" (miles per hour), "KNOTS" (nautical miles per hour), "MIN" (minutes), "HOURS", and others. The numbers repeat around the circle because the scales are logarithmic — the same physical distance on the slide rule can represent 10, 100, or 1000 depending on where you are on the scale. This is why you must pay attention to the decimal point in your answer; the E6B gives you the digits, but you supply the magnitude.

Solving time-distance-speed problems

The most frequent calculation in flight planning is: "If I fly 120 knots for 45 minutes, how far will I go?" or the reverse: "If I need to cover 180 nautical miles and I fly 120 knots, how long will it take?" The E6B solves both with the same setup.

Locate the speed scale on the inner disc — it is usually labeled "MPH" or "KNOTS" depending on which unit you are using. Rotate the inner disc until your ground speed (the speed you actually move over the ground, accounting for wind) lines up with the index mark at the top. For this example, rotate until 120 appears under the index mark.

Now look at the outer ring. You will see a scale labeled "MIN" (minutes) and another labeled with distance units. Find 45 on the "MIN" scale on the outer ring. Look straight across from 45 to the distance scale directly below it — that is your answer in nautical miles. The E6B will show you roughly 90 nautical miles.

To reverse the problem: if you know the distance is 180 nautical miles and your speed is 120 knots, find 180 on the distance scale on the outer ring, look straight up to the "MIN" scale, and read the time. You will find 90 minutes. The same setup answers both questions because time, distance, and speed are linked mathematically.

Calculating fuel burn and fuel needed

Fuel planning uses the same method as time-distance-speed. Your aircraft burns fuel at a known rate — for example, 8 gallons per hour. You need to know how much fuel you will burn over a certain distance or time.

Find the fuel burn rate on the inner disc. This is usually on a scale labeled "FUEL FLOW" or "GAL/HR" (gallons per hour). Rotate the inner disc until your fuel burn rate lines up with the index mark. For this example, set 8 under the index.

Now the outer scales show fuel consumption. If you want to know how much fuel you need for a 3-hour flight, find 3 on the "HOURS" scale on the outer ring and look straight across to the fuel scale — you will read 24 gallons. If you want to know how long you can fly on 40 gallons, find 40 on the fuel scale and look up to the "HOURS" scale — you will read 5 hours.

This calculation is critical for flight safety. You must know your fuel burn rate from your aircraft manual or from experience, and you must account for the fact that fuel burn changes with altitude, temperature, and engine settings. The E6B does the arithmetic; you supply the real numbers from your aircraft.

Unit conversions: nautical miles, statute miles, and kilometers

Charts, weather reports, and navigation systems use different distance units. Nautical miles are standard in aviation, but some charts show statute miles (regular miles), and some older documents use kilometers. The E6B converts between them without calculation.

The outer ring has multiple distance scales printed around it, usually labeled "NAUT MI" (nautical miles), "STAT MI" (statute miles), and "KM" (kilometers). These scales are aligned so that equivalent distances line up vertically.

To convert, rotate the inner disc until any number on one distance scale lines up with the index mark — it does not matter which number you choose. Then find your starting distance on one of the outer distance scales and read the equivalent distance on a different outer scale directly across from it. For example, if you want to know how many statute miles equal 100 nautical miles, set any number under the index, find 100 on the "NAUT MI" scale, and read across to the "STAT MI" scale — you will see 115 statute miles.

The back side: the wind triangle for heading and ground speed

Wind changes your heading and your ground speed. If you want to fly to a destination that is due north and the wind is blowing from the west, you must point your aircraft slightly west of north to stay on course. The wind triangle on the back of the E6B solves this problem graphically.

The back side has a grid with a center point and concentric circles. You draw three lines on this grid: one for your desired track (the direction you want to go over the ground), one for the wind direction and speed, and one for your true airspeed (the speed your aircraft flies through the air). The triangle these lines form gives you your true heading (the direction to point the aircraft) and your ground speed (the speed you actually move over the ground).

Start by drawing a line from the center point in the direction of your desired track, extending it as far as the grid allows. Mark a point on this line at a distance that represents your ground speed — you will not know this yet, so you estimate or use a previous flight. From that point, draw a second line in the direction the wind is blowing, with a length that represents wind speed. From the end of that wind line, draw a third line back to the center point. The length of this third line is your true airspeed, and the direction it points is your true heading.

In practice, most pilots use the wind triangle only when wind is strong or when they need high precision. For routine flights, they estimate the effect of wind or use electronic flight planning tools. But the wind triangle is always available on the back of the E6B, and it requires no power or internet.

Common mistakes and how to avoid them

The most common error is misreading the decimal point. The E6B gives you the digits of your answer, but the scales repeat, so 12 could mean 1.2, 12, 120, or 1200. Always estimate your answer before you look at the E6B — if you are flying 120 knots for one hour, you know the answer is roughly 120 nautical miles, so if the E6B shows 12, you know to read it as 120, not 12 or 1200.

A second mistake is forgetting to account for wind when calculating ground speed. The time-distance-speed calculation uses ground speed, not true airspeed. If you use your aircraft's airspeed instead of your actual speed over the ground, your time estimates will be wrong. Use the wind triangle first to find ground speed, then use that number in the time-distance-speed calculation.

A third mistake is setting the wrong number under the index mark. Always double-check that the number you are setting is the one you intended. The scales are small and close together, and it is straightforward to align the wrong number by accident.

Frequently Asked Questions

Do I need an E6B if I have a calculator on my phone?

No, but many pilots carry one as a backup. A phone calculator is faster and more accurate for routine math, but it requires power and a signal. The E6B works in any weather, at any altitude, and never runs out of battery. Many flight schools still teach it because it builds understanding of the relationships between speed, distance, and time.

Which side of the E6B do I use most often?

The front side. Most flight planning involves time-distance-speed and fuel calculations, both of which use the circular slide rule. The wind triangle on the back is important but used less frequently, especially on calm days or when electronic flight planning tools are available.

Can I use the E6B for problems other than aviation?

Yes. The front side is a general-purpose circular slide rule, so it solves any multiplication, division, or unit conversion problem. The back side is specific to aviation wind calculations, but the front side works for cooking, construction, or any field that needs quick math without a calculator.

What is the difference between true airspeed and ground speed?

True airspeed is how fast your aircraft moves through the air. Ground speed is how fast you move over the ground. Wind is the difference. If you fly at 100 knots true airspeed into a 20-knot headwind, your ground speed is 80 knots. The E6B's wind triangle converts between them.

How do I know my aircraft's fuel burn rate?

Check your aircraft's pilot operating handbook, which lists fuel consumption at different altitudes and power settings. You can also calculate it from experience: fly for a known time at a known power setting, measure how much fuel you burned, and divide by the time. Always use conservative numbers for flight planning — assume higher fuel burn than you expect.