How to Calculate True Air Speed

An airplane cockpit has many gauges that show the pilot the conditions of the plane and the nature of its movement. One of the gauges will show the indicated airspeed (IAS)--usually measured in knots (kn)--of the plane. An airplane's IAS is determined by the difference between the static and dynamic air pressures that the plane senses as it flies. It is not an accurate reflection of the plane's speed relative to the ground--the true airspeed (TAS)--and it becomes even less accurate as the plane flies at higher and higher altitudes. TAS can be calculated from the IAS given the altitude of the plane in feet (ft) above sea level.

Instructions

1. Write the equation for TAS:

   TAS = IAS + 0.02 x IAS x altitude / 1,000

   Essentially what you are doing here is increasing the value of the IAS by 2 percent for every 1,000 feet in altitude.

2. Study the gauges on your plane to find your IAS and altitude above sea level.

   Example: IAS = 300 kn, altitude = 20,000 ft

3. Plug the numbers into the equation to find the TAS.

   Example: TAS = 300 kn + 300 kn x 0.02 x 20,000 ft / 1,000 = 420 kn

   The TAS value of 420 kn shows you just how different TAS can be from IAS.

References

• Aerospaceweb: Types of Airspeed
• CGS Network: True Airspeed Calculator