Why Do Pilots Measure Airspeed In Knots?

While most of us on the ground are used to measuring speed in kilometers or miles per hour, pilots use a different unit of measurement: Nautical miles per hour – also known as knots. Knots are also how the speed of boats is measured. But why is this unit the standard rather than what we’re used to seeing when we’re driving?

Why Do Pilots Measure Airspeed In Knots?
A Boeing 767 cruises at about 460 knots. This translates to roughly 960 kilometers per hour. Photo: Vincenzo Pace | Simple Flying

One common unit

Anyone who travels any kind of distance – internationally or even across one large country –  knows that various things are done differently depending on the region. Of course, when crossing international borders, those differences are all the more noticeable.

We mostly encounter differences like language, currency, and social norms in our daily (non-aviator) lives. But for those in the cockpit, who have to deal with overseas air traffic controllers and other authorities, having set standards for critical data like speed and altitude ensures relatively smooth operations across borders and oceans. This has been made possible through the work of the International Civil Aviation Organization (ICAO).

In 1947, the first assembly of ICAO adopted a resolution that recommended a standardized unit system. Known as Annex 5, the system was adopted in 1948, although it would take a few more decades to get everyone on the same page in terms of common units.

The ICAO introduced the International System of Units, known as SI from the “Système International d’Unités,” as the basic standardized system to be used in civil aviation. The meter was the base unit of all SI measurements dealing with length.

However, it was recognized that some non-SI units have a special place in aviation and had to be retained – “at least temporarily,” the ICAO notes.

“These are the nautical mile and the knot, as well as the foot when it is used in the measurement of altitude, elevation or height only. Some practical problems arise in the termination of the use of these units and it has not yet been possible to fix a termination date.” – ANNEX 5 to the Convention on International Civil Aviation, ICAO

Why Do Pilots Measure Airspeed In Knots?
The knot is based on the nautical mile and is a non-SI unit. Photo: Vincenzo Pace | Simple Flying

Why knots?

According to Scandinavian Traveler, the use of knots (kt) makes air and nautical navigation easier because it’s based on the nautical mile.

The nautical mile is closely related to the longitude/latitude geographic coordinate system and 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 into 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.” -First Officer Jimisola Laursen, SAS Pilot via Scandinavian Traveler

HighSkyFlying points out that In aviation, air routes are defined in terms of waypoints (latitude, longitude), and their distance is expressed in terms of nautical miles. Therefore, the use of knots provides a quick estimation of time and speed requirements for pilots.

Additionally, it is noted that using knots is simpler as the numbers are within a smaller range when it comes to the speed of commercial aircraft– between 0kt and 400kt.

Why Do Pilots Measure Airspeed In Knots?
Aviation further divides knots into several different types: Indicated-Airspeed is shown on an aircraft’s standard pitot-static airspeed indicator. True Airspeed (TAS)- the speed relative to undisturbed air, and Groundspeed (GS)-  the speed relative to the ground. At higher altitudes, Mach (based on the speed of sound) is used. Photo: Vincenzo Pace | Simple Flying

The bottom line

At the end of the day, knots were essentially carried over from the maritime sector and thus gained prominence as the aviation industry became more prominent.

Even though it is a non-SI unit, the ICAO has recognized that using knots is too prevalent to terminate its use. The unit’s ease in usage, understandability, and history means that it will be in use for the foreseeable future.

Do you think this needs to change? Or should the aviation sector continue to use knots as a measure of speed? Let us know in the comments.