Why is Airspeed measured in knots?

Knots, or nautical miles, are a unit of measurement used in aviation; however, the use of this system originates from early sea voyages. At sea, as well as in the air, there are no landmarks or significant indicators of distance. Of course, as technology advanced, other units of measurement became available to non-land-based travel, but knots are still used when measuring aircraft speed for a handful of reasons. On top of being an easy and familiar way to navigate in flight, the International Civil Aviation Organization (ICAO), recommends knots as the preferred unit of airspeed measurement. Today, the knot is a much more precise unit of measurement than it was at the time of its advent, but this blog will cover a brief history of airspeed knots as it provides insight to current flight practices.

When nautical miles were first used at sea hundreds of years ago, they represented the distance between two latitude points on the globe. Assuming a ship could travel this distance in 60 minutes, travel speed would be one nautical mile per hour. The distance in meters between two latitude points varies, so one knot was equal to multiple measurements (1kt ~ 1843 meters).  Therefore, in 1902, France assigned a specific distance to the unit and one knot became equal to 1852 meters (1kt = 1852 meters). Despite being interchangeable, the term “knot” is not an abbreviated version of “nautical mile,” but refers to actual knots that sailors would tie into rope as a means of measuring distance. During travel, sailors would release a rope off the back of the ship at a steady pace and count the amount of knots that passed through their fingers over the span of 30 seconds. Knots were tied at a measured distance apart from one another to mark one nautical mile, and this practice allowed sailors to manually calculate distance.

The main reason that this unit of measurement is still used today is because the ICAO finds that knots are easily accepted across the globe. Similar to Latin terminology, the use of knots is convenient because it is relatively universal and provides an international standard. Additionally, this measurement system is useful for air travel because air routes rely on waypoints (latitude and longitude). By measuring distance based on aircraft’s relation to the ground, knots account for the factor of air pressure as it affects speed, which is not possible through units of Ground Speed measurement, such as kilometers. Meanwhile, this creates the challenge of comparing aircraft speeds to those of ground vehicles because the two rely on different systems. The accepted solution to these issues is the use of three different types of airspeeds, each of which is used for different purposes.

The first type of airspeed measurement is referred to as Knots Indicated Airspeed (KIAS). This is the speed of the aircraft as directly indicated by the pitot probe which measures air pressure and represents the speed of airflow as the aircraft travels through the air. Knots Calibrated Airspeed (KCAS) is calculated manually by correcting these instruments for error. The last way to measure airspeed is through Knots True Airspeed (KTAS), which boasts the speed of the aircraft relative to stationary air. The use of these three measurements enable pilots to understand their speed in relation to both the ground below as well as in relation to the unique factors of air travel.

When operating aircraft, it is important to understand both speed and location. Whether at sea or in flight, this challenge is unique when there is no nearby land, so it is helpful to understand knots and their applications. Aviation Gamut is your parts procurement partner for premium aviation parts, such as dial knots, aircraft bearings and aircraft fasteners; and our team of experts is here to ensure your operations go smoothly. Owned and operated by ASAP Semiconductor, we have access to an inventory of over 2 billion new, used, obsolete, and hard-to-find parts. Get started by submitting an RFQ today and we will provide you with a custom quote within 15 minutes. Rely on us at Aviation Gamut to offer you premium, efficient service so that you can continue to focus on what you do best!


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