Analog E6B — The Circular Slide Rule, Interactive

Try it now: Analog E6B Trainer →

Every pilot who has worked toward a certificate has wrestled with the E6B flight computer. The real thing is a cardboard or aluminium wheel — two concentric discs with logarithmic scales, a rotating compass ring on the back, and a sliding grid behind it. It solves time, speed, distance, fuel, and the wind triangle without a battery, and it has been doing that since World War II. The problem is that the technique is not obvious until you have turned the disc a few hundred times.

This trainer is a full simulation of both sides. The computer side has three modes: "Show me" steps through any problem with the dial animating to each position and a plain-English explanation of what is happening and why; "Free play" lets you spin the disc yourself and watch the readouts in real time; and "Practice" gives you randomised questions and checks your answer against the real result. The wind side animates the compass ring, sliding grid, and wind dot, and walks you through the five-step procedure for finding heading and ground speed from a given wind.

How the circular slide rule works

The outer scale is fixed. The inner disc turns. Both scales are logarithmic, which means that every pair of numbers aligned across the dial has the same ratio. That one fact is the entire engine. Set 60 (the hour index) on the inner scale under your ground speed on the outer scale, and the disc is now calibrated so that any time on the inner scale reads the matching distance on the outer scale — no arithmetic required, just read across. Change a number and every answer on the wheel updates instantly.

The catch that trips up new users is the decimal point. The disc shows digits; it does not track magnitude. If the answer reads "1 5 3" it could mean 1.53, 15.3, 153, or 1530 depending on what the numbers were. A quick sanity check — the kind the tool walks you through after every read — puts the decimal in the right place. Once you have done it a few times, the estimate becomes automatic.

The wind side

The back of the real E6B is a different instrument. The rotating compass ring sets the wind or the course under a fixed true-index marker. The transparent slide moves up and down over a printed grid of speed arcs and drift lines. You mark a wind dot, rotate the card to align your course, then slide the grid until the dot lands on your airspeed arc. Ground speed reads under the grommet (the centre hole) and the wind correction angle reads from the drift line the dot is sitting on.

The trainer draws the speed arcs and drift lines as SVG geometry so they recompute live as you drag, shows where the wind dot should be, and confirms whether it is sitting on the right arc. The guided walkthrough animates each of the five steps with a description of what you are doing and why. Enter your own course, airspeed, wind direction, and wind speed to practice the exact numbers you need for a real flight.

Why build this alongside the digital E6B?

The digital calculator I posted earlier is the faster tool for getting a number. This one is the better tool for understanding where the number comes from. The written knowledge test for a private certificate includes E6B problems, and those questions require recognising what to set and where to read — not just entering fields and pressing Calculate. This trainer builds the muscle memory for that. It also happens to be a satisfying thing to spin around even if you never fly; the log scale has a way of making proportions feel intuitive once you have played with one long enough.