Why are there different ways of measuring an aircraft’s speed?

Depending on the phase of flight and the need, there are different ways of measuring aircraft speed, which can give different results.

We’ve already touched briefly on the subject of airplane speed, explaining why, with the help of the wind, a plane could cross the Atlantic at 1300 km/h without breaking the sound barrier, but some of you still had questions on the subject. In fact, as you watched the flight instructions broadcast by certain IFEs (in-flight entertainment systems), you noticed that several speed indications were sometimes given, and you wondered what they corresponded to.

This article takes a closer look at the subject.

Indicated airspeed (IAS)

It’s the easiest. Indicated Airspeed (IAS) is measured by the pitot tubes made infamous by flight AF447 from Rio to Paris.

To keep things simple, these tubes measure the pressure difference between ambient pressure (static pressure) and the pressure coming from the outside air flow (total pressure) which is nothing more or less than the wind you feel on your face when you’re riding your bike: it’s not the air moving around you, but you moving through a mass of air, and the higher your speed, the stronger this wind (called relative wind). … This difference is the dynamic pressure used to determine the aircraft’s speed.

And why is it done this way? It’s very simple. To measure the speed of a car, we measure the rotation of the wheels, which is of course impossible for a plane in the air. An aircraft is a body moving in a mass of air, which may itself be in motion. So if you know your speed in the mass of air and the speed of the mass, you can deduce your real speed.

True airspeed (TAS)

True Airspeed (TAS) is the speed of the aircraft within the air mass in which it is flying. It is not measured, but calculated according to the indicated speed and altitude of the aircraft.

It is generally 2% higher than the indicated speed for every 1000 feet of altitude.

It is not really of interest for piloting, but is used for performance calculations for take-off, landing or cruising. It is also used to calculate ground speed.

Ground speed (GS)

Groundspeed (GS) is the true speed to which a correction is applied according to the wind (headwind or tailwind).

In the absence of wind, it is equal to true speed.

It gives the horizontal speed of the aircraft in relation to the ground.

This is the speed available to air traffic controllers on their screens, but when they assign a speed to an aircraft it’s an indicated airspeed.

Calibrated speed (CAS)

Calibrated Airspeed (CAS) is the speed indicated on on-board instruments. This is the indicated speed corrected for instrumental errors.

Measurements can be affected by temperature, pressure and humidity, and the calibrated speed corrects for these errors. For an aircraft operating in near-sea-level conditions (15°C, pressure 1013 hPa, 0% humidity), it is equal to true airspeed.

This is the speed displayed on the dashboard.

Why is speed measured in knots?

The unit of measurement for aircraft speed is the knot (abbreviated kt), which corresponds to 1 nautical mile per hour (1 kt = 1.852 km/h).

Why use this unit? Simply because it was the standard in the maritime transport industry, and if you’re going to adopt a standard for the sector, you might as well adopt one that was already widely used.


Speed is also sometimes expressed in Mach, Mach 1 being the speed of sound (344 m/s, or 1,240 km/h at sea level). Mach speed is the ratio of true speed to the speed of sound.

Bottom line

An article that will undoubtedly be of interest to the more technically minded among you, and will enable others to shine at dinner parties….

Image : throttle control by DC Studio via Shutterstock

Bertrand Duperrin
Bertrand Duperrinhttp://www.duperrin.com
Compulsive traveler, present in the French #avgeek community since the late 2000s and passionate about (long) travel since his youth, Bertrand Duperrin co-founded Travel Guys with Olivier Delestre in March 2015.

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