Hundreds of years ago, our ancestors started exploration through sea voyages. They quickly learned that navigation in the sea presented unique challenges as it was quite easy to get lost within the vastness of the oceans. Traditional techniques and units of measure failed at sea as there were no landmarks to identify, and no way of knowing the distance traveled. That was when “nautical miles” were defined as a unit of distance at sea and it is the same unit of measure which was later adopted in aviation.
With that said, with the advent of modern navigation and positioning systems, we are now able to navigate easily through air, sea, or land and can alternate between any units of measurement we like. So, why is the speed of aircraft still measured in knots?
Airplanes use ‘knots’ not only because it makes air navigation easier, but also because it is recommended as a unit of airspeed measurement by the International Civil Aviation Organization (ICAO).
This unit for airspeed in uniformly accepted around the globe as ICAO recommends all aircraft manufacturers to use knots for the airspeed indicators. Owing to its traditional value and global acceptability, the unit ‘knots’ has made a permanent space for itself in the aviation sector.
With that said, let’s learn more about knots, why they are still used, and how they came about in the first place!
Table of Contents
How much is one knot (1 kt)? : Nautical miles vs knots
Historically, one nautical mile was defined as the distance equal to one minute of latitude.
To put this in perspective, if the earth is vertically sliced into 360 equidistant degrees, each degree would contain 60 minutes. It was approximated that one-sixtieth (1/60) distance between two latitudes had been traveled in one nautical mile.
A knot, in turn, is a unit of speed measurement, defined as one nautical mile per hour.
Here is a video that might be of use to you if you struggle to grasp this concept!
Historical Vs modern knot distances
In the good old times, one knot (1kt) was a slight approximation, as one nautical mile did not represent a standard fixed distance.
This was because the distance between latitudes varies on the equator (1843 meters) as compared to that on the poles (1861 meters), so the value of the nautical mile used to be different as well.
Due to a lack of precise navigational aids, nautical miles and knots remained the best approximation for distance speed for hundreds of years.
This approximation of knots has now been replaced by a precise value and we will read about that; but first, let us see how the term ‘knots’ came about!
The Modern Definition of one knot
The precise value of one knot (1kt) in terms of SI units is 1.852km/h.
With the advent of GPS and precise navigational aids, the need to precisely define and standardize the value of 1 knot (kt) arose.
In 1906, France became the first country to define one nautical mile (1Nm) as equal to exactly 1852m which was later adopted by the rest of the world.
Knots Vs Other Standard Units of Speed
With the precise definition of knots, we are now able to accurately compare one knot (1kt) against other units of speed:
1kt = 1.852km/h = 0.514m/s = 1.1508miles/h = 1.688feet/s
Why are knots called “knots”?
You may be wondering why a speed of one nautical mile per hour is termed as ‘knots’? After all, it’s a quite strange name.
To understand this better, we need to take a history trip to see how sailors measured the speed of ships centuries ago.
To estimate the speed of a ship during long voyages, sailors used a “dutchman’s log”. A dutchman’s log was a wooden device having fixed weight and dimensions, attached to a rope with knots on it.
This log was thrown into the sea by sailors while letting the knotted rope slide through their hands for 30 seconds. The number of knots which would pass through a sailor’s hand during 30 seconds gave an approximation of the speed of the vessel in, you guessed it, ‘knots’.
While techniques for speed measurement of ships evolved with time, the unit of measure ‘knots’ was retained.
But why do we still use knots when it comes to aviation?
Why are Knots still used in aviation?
Most countries of the world have adopted the International System of Units (SI) where speed is measured in meters per second (m/s) or kilometers per hour (km/h) but the ICAO recommendation for utilization of that knots (kt) has not been altered.
The standards and recommendations given by ICAO are based on their acceptability across the globe and followed internationally by everyone connected to air navigation, including pilots and air traffic controllers.
Advantages of using Knots
The key advantage of using ‘knots’ as we have discussed above, is to simplify navigation in air.
In aviation, the air routes are defined in terms of waypoints (latitude, longitude) and their distance is expressed in terms of nautical miles, hence, the utilization of knots provides a quick estimation of time and speed requirements for aviators.
Additionally, aviators feel comfortable using ‘knots’ as this unit of measure as it ranges within nominal values of 0kt -400kt for commercial airliners. Whereas, in comparison, the value of the SI unit, kilometers per hour (km/h) would be quite much larger.
Airspeed represented in terms of knots also indicates that it is fundamentally different from the Ground Speed. As the airspeed of an aircraft is calculated directly from the pressure of the airflow hitting the aircraft, it may be significantly different from the speed it actually moves over the ground. Hence, using ‘knots’ for airspeed helps maintain the distinction between airspeed and ground speed.
Disadvantages of using Knots
The SI units of measurement like km/h, have established themselves within land-based transportation systems and vehicles. So, whenever a comparison in the speed of aircraft versus ground vehicles is needed, we are presented with a challenge.
Owing to its advantages, the airspeed of aircraft is always represented in ‘knots’. But it is important to note that there are three distinct types of airspeeds:
- Knots Indicated Airspeed (KIAS)
- Knots Calibrated Airspeed (KCAS)
- Knots True Airspeed (KTAS).
Did you notice that “knots’ is common in all of them?
Let’s have a closer look it!
What are KIAS, KCAS and KTAS?
Well, as we discussed above, the airspeed of aircraft is always indicated and described in ‘knots’.
But for a quick review, let us summarize the three types of airspeeds for you:
- Knots Indicated Airspeed: This is the airspeed (in knots) directly calculated from the air pressure inside the pitot probe of aircraft and it represents the speed of airflow as the aircraft travels through the air
- Knots Calibrated Airspeed: By correcting for the minor instrumental and positional errors of the pitot probe we get the Knots Calibrated Airspeed.
- Knots True Airspeed: True airspeed is the speed of the aircraft, relative to the stationary air around it. The values of indicated and true airspeed diverge as we move to higher altitudes owing to a difference in static air pressure.
If you want to learn more about the differences between these three types of airspeed, we recommend looking closer at our article about KIAS, KCAS, and KTAS.
Let us now summarize what we have discussed above.
Summary
- The units, Nautical Mile for distance and Knots for speed, were derived by sailors in the 17th century. These units of measurement are directly related to the distance between two latitude lines and therefore, they made navigation in the sea a lot easier.
- As the aviation sector developed and progressed in the 20th century, it borrowed a lot of existing terminologies and units of measure from the marine sector. The unit of speed ‘knots’ has been the standard unit of speed in aviation since its beginning.
- One knot (1kt) is equal to one nautical mile per hour (1Nm/h) and it has been defined to be equal to 1.852km/h in terms of SI units. The utilization of ‘knots’ has been recommended by ICAO and is therefore accepted and understood in aviation around the globe.
- Although there are three distinct types of airspeeds, they are all measured in ‘knots’. Due to the ease in usage, understandability, and its history, ‘knots’ are expected to stay as a standard measurement of the speed of aircraft in the foreseeable future.
- So, the next time, we hear the speed of aircraft being expressed in terms of ‘knots’, we will precisely know why.
Happy Flying!
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