E6B True Airspeed (TAS) Calculator
The E6B flight computer is an essential tool for pilots, used for a wide range of calculations from fuel burn to wind correction. One of its most critical functions is determining True Airspeed (TAS). Indicated Airspeed (IAS) is what your aircraft's airspeed indicator shows, but it doesn't account for changes in air density due to altitude and temperature. True Airspeed is the actual speed of the aircraft relative to the air mass it's flying through, and it's crucial for accurate navigation, flight planning, and performance calculations.
Understanding True Airspeed (TAS)
As an aircraft climbs, the air density decreases. At the same Indicated Airspeed, the aircraft is actually moving faster through the less dense air. Similarly, temperature affects air density; warmer air is less dense than colder air. Therefore, to get an accurate picture of your aircraft's speed relative to the air, you need to correct IAS for both pressure altitude and outside air temperature (OAT).
- Indicated Airspeed (IAS): The speed shown on the aircraft's airspeed indicator.
- Pressure Altitude: The altitude above the standard datum plane (the theoretical level where atmospheric pressure is 29.92 inHg and temperature is 15°C). It's obtained by setting your altimeter to 29.92 inHg and reading the altitude.
- Outside Air Temperature (OAT): The actual temperature of the air outside the aircraft, typically measured in Celsius for aviation calculations.
This calculator uses a common E6B approximation to convert your Indicated Airspeed into True Airspeed, taking into account the effects of pressure altitude and outside air temperature.
How the Calculation Works (E6B Approximation)
The calculator uses a widely accepted E6B approximation to determine True Airspeed (TAS). The core idea is that as you climb, and as the air gets warmer, the air becomes less dense. For a given Indicated Airspeed, the aircraft must move faster through this less dense air to generate the same dynamic pressure on the pitot tube. The calculation involves:
- Standard Temperature at Pressure Altitude (ISA Temp): First, the calculator determines the International Standard Atmosphere (ISA) temperature for your given pressure altitude. The standard lapse rate is 2°C per 1,000 feet of altitude decrease from the standard sea level temperature of 15°C.
- Temperature Deviation: It then calculates the difference between your actual Outside Air Temperature (OAT) and the ISA temperature at that altitude.
- Altitude Correction: An initial correction is applied to IAS based on pressure altitude, typically increasing TAS by approximately 1.7% for every 1,000 feet of pressure altitude.
- Temperature Correction: Finally, this altitude-corrected speed is further adjusted based on the temperature deviation, using a ratio involving absolute temperature. Warmer air (positive deviation) will result in a higher TAS, while colder air (negative deviation) will result in a lower TAS.
Example Scenarios
Let's look at a couple of examples to illustrate the calculator's use:
Example 1: Standard Conditions
- Indicated Airspeed (IAS): 100 knots
- Pressure Altitude: 5,000 feet
- Outside Air Temperature (OAT): 5°C
At 5,000 feet, the ISA temperature is 15°C – (5 * 2°C) = 5°C. Since OAT is also 5°C, there is no temperature deviation. The TAS will be primarily influenced by the altitude correction. The calculator would yield a TAS of approximately 108.5 knots.
Example 2: Cold Conditions at Altitude
- Indicated Airspeed (IAS): 120 knots
- Pressure Altitude: 8,000 feet
- Outside Air Temperature (OAT): -10°C
At 8,000 feet, the ISA temperature is 15°C – (8 * 2°C) = -1°C. The OAT is -10°C, meaning it's 9°C colder than standard (-10 – (-1) = -9°C). The altitude correction will increase TAS, but the colder-than-standard temperature will slightly reduce the final TAS compared to if it were at ISA temperature. The calculator would yield a TAS of approximately 131.9 knots.
Understanding and calculating True Airspeed is fundamental for accurate flight planning, estimating arrival times, and ensuring safe and efficient flight operations. Use this E6B calculator to quickly determine your TAS for various flight conditions.