There are several different airspeeds used in aviation, each with a specific purpose. Here are some of the most important airspeeds and how they are calculated for general aviation aircraft:
- Indicated Airspeed (IAS): This is the airspeed that is displayed on the aircraft’s airspeed indicator, and it represents the speed of the aircraft relative to the surrounding air. IAS is calculated using the difference between the static pressure (measured by the pitot-static system) and the ambient pressure.
- True Airspeed (TAS): This is the actual speed of the aircraft relative to the surrounding air, and it is calculated by correcting IAS for factors such as altitude, temperature, and pressure. TAS is important because it affects the aircraft’s performance and fuel consumption.
- Groundspeed (GS): This is the speed of the aircraft relative to the ground, and it is affected by the aircraft’s TAS and the speed and direction of the wind. GS is important because it determines how long it will take to reach a destination and how much fuel will be consumed.
- Vertical Airspeed (VS): This is the rate of change of altitude, and it is calculated using the aircraft’s altimeter. VS is important for maintaining a steady rate of climb or descent.
- Calibrated Airspeed (CAS): This is the IAS corrected for instrument and installation errors. CAS is used for calculating the aircraft’s stall speed and other critical performance parameters.
To calculate these airspeeds, the aircraft’s instruments must be calibrated and regularly checked for accuracy. Pilots must also take into account factors such as altitude, temperature, pressure, and wind when calculating TAS and GS.
True airspeed is probably the most complex calculation.
The formula for calculating TAS is as follows:
TAS = IAS / (1 + (IAS x p / (T x k)))
where: IAS = Indicated Airspeed p = Pressure altitude (inches of mercury) T = Temperature (in degrees Kelvin) k = A constant (0.00002896).
NOTE : Convert Celsius to Kelvin by adding 273.15 to your temperature.
The pressure altitude is the standard pressure borometer reading, which is the
Altimeter
theoretical level at which the atmospheric pressure is 29.92 inches of mercury (1013.25 millibars) and the temperature is 15 degrees Celsius. To calculate the pressure altitude, pilots can use a standard atmospheric chart or an altimeter setting that has been adjusted to the local atmospheric pressure.
The temperature used in the formula should be the outside air temperature (OAT) at the altitude where the TAS is being calculated. Pilots can obtain the OAT from a thermometer or a temperature sensor on the aircraft.
The constant “k” is a correction factor that accounts for the density of the air. It is a small number that is based on the molecular weight of dry air.
Yup, can’t see myself doing that too often. Thank goodness for the CX-3.
The use of the formula, to get good answers that are even half correct took 18 minutes. Say you fly at 6500 feet PA and use the E6B at standard temperature of 15C at that altitude you get the answer far sooner. 2 min. 100KTS indicated. TAS of 113 kts. On some airspeed indicators there is a rotatable wheel that is a calculator to do TAS on the fly. Set 29.92 on the altimeter and get your PA. Look at your outside air temperature and adjust accordinly. The airspeed readout will indicate the proper true airspeed. Failing that the E6B does the same thing.
Thank you Bruce for the great insight. It is always just good to know how these different airspeeds are calculated. Everyone should check out Bruces site : http://nORTHOF49TECH.COM
He has a wealth of aviation knowledge.
Stay Tuned. Bruce and I will be launching a podcast soon talking about all things aviation in Canada
Terry