E6B Slide Rule
A circular E6B slide rule for the Apple Watch
The calculator side of an E6B, for Apple Watch, iPhone and iPad. Turn the outer ring until your rate meets the index at twelve o’clock, and the whole problem is on the face at once.
A slide rule hands you the digits. The decimal point is still your job.
One setting answers the whole problem.
-
Set the rate
Turn the outer ring with a finger, or with the Digital Crown on Apple Watch, until your figure meets the amber index at the top of the dial. 120 knots. 9.5 gallons an hour. Whatever the rate is, it goes there once.
-
Read it anywhere on the face
With 120 against the index, 30 on the minutes scale sits opposite 60 on the outer ring — half an hour, sixty miles. So does every other pair, in both directions, all the way round. Distance from time, time from distance, fuel burned in the hour you have been flying: they were all there the moment you set the ring.
-
Leave it where you put it
Let go mid-turn and the ring carries on and coasts to a stop. Turn slowly and the crown gets finer under your finger, so you can settle on the graduation you meant. A click marks each numbered graduation passing the index — feedback, not a detent, so the dial never pulls you onto a mark you did not choose. Come back later and the setting is still there.
Three scales, drawn to the proportions of the real thing.
Rate, distance, fuel, quantity — and the ring that turns. One revolution is one decade, 10 through 100, so a single graduation reads 12, 120 or 1,200. You supply the magnitude.
Fixed, with the rate index locked at twelve o’clock. The one pointer you read everything against never moves, which is exactly what rate and time work wants.
1:00 to 10:00, beneath the minutes, so a long leg reads as 2:30 instead of 150 minutes.
Amber on 60, red on 10, green on 36 — the seconds mark, for a rate given per second rather than per hour. All three are on the fixed ring and are read against the outer scale.
Built for the watch first.
A complication puts the dial one tap from the watch face, and it opens where you left it. On iPhone and iPad it is the same instrument at a size that has room for the numerals the watch has to leave off — same graduations, same geometry, more type.
There is no equals key.
That is the point rather than an omission. A slide rule shows you the whole family of answers at once and lets you pick the one you need; a calculator shows you one number and forgets it. If you want to type digits into a watch, this is the wrong app.
- No wind side. This is the calculator face only. Headings and wind correction are not part of it.
- No typed entry and no computed readout. You set the ring and read the scales.
- No account, no analytics, no advertising. Nothing is collected, and nothing leaves your device.
Two things do get remembered on the device and nowhere else: the angle you last left the dial at, and when. Both are discarded after a day. See the privacy policy.
An instrument that never needed a battery.
The E6B is an analogue flight computer worked out in the 1930s by Philip Dalton, a lieutenant in the U.S. Navy, for training and for figuring problems in the air. Generations of pilots learned on one, and the circular slide rule on its front has barely changed since: multiplication and division, time, speed and distance, and fuel burn, all on one face.
This app is that face and nothing else. If you wore an aviator watch and miss reaching for its bezel, the bezel is what this is.
Getting in touch
Bugs, questions and suggestions all go to one place: [email protected]. Tell me which device you were on and what the dial was showing, and I will get back to you.
Why doesn’t the clock go away on my watch? watchOS will not let an app hide it. The face is laid out to sit clear of it.