6 Different Types of Airspeed: How to Calculate Each
There are numerous ways to measure your airspeed. We talk about 6 different ways below:
1. Indicated Airspeed (IAS)
The direct instrument reading obtained from the Airspeed Indicator (ASI), uncorrected for variations in atmospheric density, installation error, or instrument error.
The Airspeed Indicator (ASI) measures the difference between the static pressure from the aircraft’s static ports, and the ram pressure (dynamic + static) from the pitot tube. This difference is the dynamic pressure, which translates into a reading.
Manufacturers use this airspeed as the basis for determining aircraft performance.
Takeoff, landing, and stall speeds listed in the Aircraft Flight Manual (AFM) / Pilot’s Operating Handbook (POH) are IAS and do not normally vary with altitude or temperature.
2. Calibrated Airspeed (CAS)
Calibrated Airspeed is Indicated Airspeed (IAS) corrected for installation error and instrument error.
Although manufacturers attempt to keep airspeed errors to a minimum, it is not possible to eliminate all errors throughout the airspeed operating range.
At certain airspeeds and with certain flap settings, the installation and instrument errors may total several knots. This error is generally greatest at low airspeeds.
In the cruising and higher airspeed ranges, IAS and CAS are approximately the same.
You can find the calibrated airspeed for your airplane in the calibration chart of the aircraft manual.
Calibrated airspeed is the same as true airspeed when you are flying at sea level under International Standard Atmosphere (ISA) conditions.
3. True Airspeed (TAS)
True Airspeed is Calibrated Airspeed (CAS) corrected for altitude and nonstandard temperature.
Because air density decreases with an increase in altitude, an aircraft has to be flown faster at higher altitudes to cause the same pressure difference between pitot impact pressure and static pressure.
Therefore, for a given CAS, TAS increases as altitude increases; or for a given TAS, CAS decreases as altitude increases.
A pilot can find TAS by two methods. The most accurate method is to use a conventional or electronic flight computer.
A second method, which is a rule of thumb, provides the approximate TAS. Simply add 2 percent to the CAS for each 1,000 feet of altitude. At 10,000 feet, you are flying approximately 20% faster than your indicated airspeed.
The TAS is used for flight planning and when filing a flight plan.
4. Ground Speed (GS)
Groundspeed is the actual speed of the airplane over the ground. It is the True Airspeed (TAS) adjusted for wind.
While not an airspeed, GS is important for navigation and has a major impact on the time it takes to get to a destination.
GS decreases with a headwind and increases with a tailwind.
5. Equivalent Airspeed (EAS)
Equivalent Airspeed is Calibrated Airspeed (CAS) corrected for the compressibility of air at a non-trivial Mach number.
It is also the airspeed at sea level in the International Standard Atmosphere at which the dynamic pressure is the same as the dynamic pressure at the True Airspeed (TAS) and altitude at which the aircraft is flying.
It’s mainly used for structural calculations and testing.
6. Mach Number (M)
The Mach Number is the ratio of the True Airspeed (TAS) of the aircraft to Local Speed of Sound (LSS) displayed on the Machmeter.
M varies depending on atmospheric conditions, air temperature, and density.
It’s important for jet aircraft flying at high speeds.
The Mach Number is named after Austrian physicist and philosopher Ernst Mach.