Ask the pilot: Why is airspeed on planes measured in knots?

Even the most experienced travelers have many questions about the inner workings of airplanes.  How fast can they go? What do all the noises mean? How do they take off and land?  The experts with all the answers are, of course, the pilots.

Jimisola Laursen       

Age: 41
Career: Joined SAS in 2014. Has flown CRJ900s and 737NGs. Started flying the ­Airbus A320 in 2016. A former track athlete and 400m indoor Nordic record holder.
Home base: CPH
Flies: Airbus 319/320/321
Flight hours: 5,400

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Why is airspeed on planes measured in knots?


Hi Tom,

The short version is that it makes air and nautical navigation easier.

The knot is based on the nautical mile. Although the unit knot is not an SI base unit, (the meter is the SI base unit for length) its use in nautical navigation and aviation is important because the length of a nautical mile is closely -related to the longitude/latitude geographic coordinate system. The nautical mile is based on the circumference of the Earth. Imagine that the equator is a circle divided into 360 degrees (like a compass). Each degree can be split in to 60 equal parts called minutes. The length of each such -minute is equal to approximately 1 nautical mile. One knot is equal to 1 nautical mile per hour or 1.85 km/h.

In aviation, there are several different airspeeds (in knots) that we use. Indicated -Airspeed (IAS) is the speed shown on an aircraft’s standard pitot-static airspeed indicator. True Airspeed (TAS) is the speed of an aircraft relative to undisturbed air, while Groundspeed (GS) is the speed of an aircraft relative to the ground (true airspeed corrected for wind).

In addition, Mach, which is relative to the local speed of sound, is used for airspeed at higher altitudes. By definition, Mach 1.0 equals the speed of sound and Mach 0.65 is 65% of the speed of sound. Aircraft used by SAS normally cruise at Mach 0.70-0.80.

First Officer Jimisola Laursen


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