The E6B — the circular slide rule on one side and the wind face on the other — still shows up on the FAA knowledge test and checkride oral, even though most pilots now plan with an app. Understanding what the E6B is actually computing makes the electronic answers easier to check. This guide covers the two problems that matter most for cross-country planning: converting calibrated airspeed to true airspeed, and solving the wind triangle for heading and groundspeed.
The Airspeeds
- Indicated airspeed (IAS) is what the airspeed indicator shows.
- Calibrated airspeed (CAS) is IAS corrected for instrument and installation error, using the airspeed calibration table in your POH. The difference is usually small at cruise and larger at low speeds and high angles of attack.
- True airspeed (TAS) is your actual speed through the air mass. Because the airspeed indicator measures dynamic pressure, it reads low in thinner air; TAS corrects for that.
- Groundspeed (GS) is TAS combined with the wind.
True Airspeed
The quick rule: TAS increases about 2% per 1,000 ft above sea level over CAS. The more precise method uses air density:
δ = pressure ratio at pressure altitude · θ = (OAT in K) ÷ 288.15 · σ = density ratio. Valid for typical light-aircraft speeds, where compressibility is negligible.
Example: CAS 110 knots at 8,000 ft pressure altitude with an OAT of 0°C (ISA at 8,000 ft is about −1°C, so it's close to standard).
- Density-based TAS: about 124 knots
- 2% rule: 110 × (1 + 0.02 × 8) = about 128 knots
The shortcut lands within a few knots, which is fine for a sanity check, but the gap grows with altitude and with temperatures far from standard. On a manual E6B, you set OAT against pressure altitude in the airspeed window, then read TAS on the outer scale opposite CAS on the inner scale.
The Wind Triangle
The wind triangle has three vectors: your heading and TAS through the air, the wind's direction and speed, and the resulting track and groundspeed over the ground. Given true course, TAS, and the wind, you solve for the heading to fly and the groundspeed you'll get.
Heading = TC + WCA · GS = TAS × cos(WCA) − Wind speed × cos(Wind direction − TC)
A positive WCA means wind from the right and a correction to the right; negative means wind from the left.
Example: True course 090°, TAS 120 knots, winds aloft from 030° at 20 knots.
- Wind angle: 030 − 090 = −60° (wind from the left, ahead of the wing)
- WCA: arcsin(20 × sin(−60°) ÷ 120) ≈ −8.3°
- True heading: 090 − 8.3 ≈ 082°
- Groundspeed: 120 × cos(8.3°) − 20 × cos(60°) ≈ 118.7 − 10 = about 109 knots
From here, apply variation to get a magnetic heading — see True vs. Magnetic Heading.
Keep everything in true. Winds aloft forecasts are in degrees true, and so is the course you measure on the chart. Solve the wind triangle in true, then convert the resulting heading to magnetic. Mixing a magnetic course with a true wind is one of the most common nav log errors.
Time, Distance, and Fuel
With groundspeed in hand, the rest is arithmetic. Continuing the example, a 145 NM leg at 109 knots takes 145 ÷ 108.7 ≈ 1.33 hours, or about 1 hour 20 minutes. At 9 gal/hr, that's about 12 gallons. Feed that into your fuel plan along with climb fuel and reserves.
Before trusting any computed answer, estimate it. Wind from 60° off the nose at 20 knots: about half of it is headwind (10 knots) and most of it — roughly 17 knots — is crosswind. At 120 knots, every knot of crosswind is about half a degree of correction, so expect about 8–9° of WCA and a groundspeed near 110. If the calculator says otherwise, recheck the inputs.
Using the CockpitCalc E6B
- True Airspeed — enter CAS, pressure altitude, and OAT. The result shows TAS, density altitude, and the 2% rule estimate for comparison.
- Wind Correction — enter true course, TAS, and the wind (true). Returns WCA, true heading, and groundspeed.
- Time/Dist/Fuel — enter groundspeed, distance, and burn rate for time en route and fuel used.
Open the E6B Calculator
Compute true airspeed, wind correction angle, groundspeed, and time/distance/fuel.
Open the Calculator →