From calibrated to true airspeed
The airspeed indicator measures dynamic pressure, so it reads correctly only in sea-level standard air. Higher up the air is thinner, and you fly faster through it than the needle shows. True airspeed (TAS) is the speed that goes into the wind triangle and your flight-time planning.
The calculator takes calibrated airspeed (CAS), pressure altitude and outside air temperature:
- The CAS defines an impact pressure at sea-level standard conditions.
- At the static pressure of your pressure altitude, that impact pressure means a certain Mach number (compressible, subsonic flow).
- The speed of sound follows from the temperature, and TAS = Mach × speed of sound.
In the worked example, 150 knots calibrated at 10,000 feet and −5 °C gives Mach 0.273 and a true airspeed of 174 knots.
The E6B's density method
A mechanical E6B's altitude window divides CAS by the square root of the density ratio, which ignores compressibility. For the example that gives 174.5 knots, within half a knot. The result lists this value as "E6B density method" so you can check your wheel work, and the E6B simulator animates the setting.
Equivalent airspeed and Mach
Equivalent airspeed (EAS) is CAS corrected for compressibility; it matters for structural limits at high speed. It equals CAS at sea level and drifts below it as speed and altitude rise. Mach number is TAS over the local speed of sound: 638 knots at −5 °C, 661 knots on a standard sea-level day. These relations hold below Mach 1; the calculator refuses supersonic input rather than returning a wrong number.
The 2 percent rule
A quick check pilots use: TAS is about 2% above CAS per 1,000 feet. At 10,000 feet that predicts 180 knots for 150 knots calibrated. The example day is almost exactly standard, and the real increase there is about 16%, so the rule runs a few knots high. Use it as a sanity check and the exact value for planning.
From the airspeed indicator to CAS
The calculator starts from calibrated airspeed. Your airspeed indicator shows indicated airspeed, which differs from calibrated because of instrument and installation error. The POH's airspeed calibration table converts one to the other; the correction is largest at low speeds and with flaps extended, and usually a knot or two in cruise. Use the pressure altitude from the altimeter set to 29.92 and the outside air temperature gauge reading.
True airspeed on a mechanical E6B
- Convert indicated to calibrated airspeed with the airspeed calibration table in your POH.
- In the altitude window, set the pressure altitude over the outside air temperature.
- Find the calibrated airspeed on the inner scale and read true airspeed directly above it on the outer scale.
Questions pilots ask
What is the difference between indicated, calibrated and true airspeed?
Indicated airspeed is what the airspeed indicator shows. Calibrated airspeed corrects it for instrument and position errors using the POH table. True airspeed is the actual speed through the air, which grows with altitude and temperature because the air is thinner.
What is the rule of thumb for true airspeed?
True airspeed is roughly 2 percent more than calibrated airspeed for every 1,000 feet of altitude. At 10,000 feet, 150 knots calibrated is about 180 knots by the rule; the exact answer for a −5 °C day is 174 knots.
Why do the E6B and the calculator differ at high speed?
The mechanical E6B's altitude window uses air density only. At higher speeds the air compresses in the pitot tube, and the exact calculation accounts for that. Below about 200 knots the difference is under 1 percent.
How do I get Mach number?
Mach number is true airspeed divided by the local speed of sound, which depends only on temperature. The result lists both.