The 5 Airspeeds: What Are the Differences?
Indicated Airspeed, Calibrated Airspeed, Equivalent Airspeed, True Airspeed, and Ground Speed
You’ve heard these terms being thrown around, but you might not know the real differences between them. Let’s dive into that in today’s article!
Indicated Airspeed (IAS)
The indicated airspeed of an aircraft is the airspeed measured by the aircraft’s pitot-static system using the pitot tubes. The IAS is the airspeed displayed by the airspeed indicator or the primary flight display.
The indicated airspeed is given by the equation below:
The IAS is considerably the most important airspeed to note when flying. This is because airspeeds published in performance manuals or flight manuals are all IASs. For example, the stall speed, in true airspeed terms, varies considerably with density and altitude. However, since the indicated airspeed recorded by the pitot tube uses air pressure and density in its calculations, an aircraft will always stall at the indicated airspeed published in the performance manual, regardless of air density or altitude.
Calibrated Airspeed (CAS)
The calibrated airspeed of an aircraft is the indicated airspeed after correcting for known instrument and position errors. At sea level and International Standard Atmosphere conditions (15°C and 1013hPa or 1 atm; note: this is not STP!), the CAS should read the same as true airspeed.
The impact pressure is the pressure difference between the total pressure and the static pressure, which is also used in the equation for IAS.
Equivalent Airspeed (EAS)
The equivalent airspeed of an aircraft is the calibrated airspeed of the aircraft corrected for the compressibility of air at high Mach numbers. In low-speed flight, the EAS is equivalent to the IAS.
Calculations for EAS at high Mach numbers are much more complicated, and it will not be covered in this article.
True Airspeed (TAS)
The true airspeed of an aircraft is the speed of the aircraft relative to the air around it.
The cruise speed of an aircraft (in flight plans or documentation) is written in true airspeed, not accounting for wind factors. This is why your flight could arrive earlier than planned with a tailwind.
TAS and EAS are related by the equation below:
Ground Speed (GS)
The ground speed of an aircraft is the horizontal speed of the aircraft relative to the surface of the earth. If the aircraft is cruising at a steady altitude, the GS would be the vector sum of TAS + wind speed (vector addition since wind can blow in any direction).
If the aircraft is either climbing or descending, TAScos(θ) (where θ is the angle of ascent or descent) is used in place of TAS since the ground speed of an aircraft is the horizontal speed only.
Conclusion
Even though some of these airspeeds are less used than others, all 5 of these are essential to conducting a safe flight. From documentation to operation, from calculating V speeds to calculating flight times, a combination of these airspeeds is used in everyday operations. Have any other questions? Leave a response below :)
