5 ms·

Why are knots used in aviation over kph?

by Daishiman 5y ago

cmurf 5y ago
From nautical roots, hence aeronautical. Quite a lot is borrowed by aviation from maritime.
NikolaeVarius 5y ago
Convention, smaller numbers, everyone that it matters to knows it
- pc86 5y ago
  I can't imagine "smaller numbers" plays any role at all.
ghaff 5y ago
Historical reasons mostly. For the same reason that various units are used in all sorts of different industries. And guess what? If you changed you'd probably end up with some accidents in the transition as a result.
realityking 5y ago
Both aviation and marine navigation generally use nautical miles for as it‘s a little easier to use for long distance travel (plus tradition and inertia). Measuring speed in knots make things like dead heading calculations a lot easier.
nradov 5y ago
Using knots made navigation calculations easier with paper maps marked in degrees and minutes. A nautical mile is the same as a minute of latitude, and a knot is one nautical mile per hour. Now with electronic navigation systems kph would be just as easy, but aviators are all used to thinking in knots so changing would be difficult.
- jillesvangurp 5y ago
  It's partly true. However, a lot of planes outside of the US would have used kilometers per hour historically. For example German WW II planes did that. Likewise a lot of Russian cold war era planes did the same. And lots of modern planes (e.g. French ones) still do. And some planes in the US actually use miles per hour instead of knots; especially in general aviation. It's just that post war, the US aviation industry was very dominant and the metric system never was popular there. For the same reason feet are used instead of meters for measuring altitude. And unlike with nautical miles, there's not much of an advantage for that. However, the units for altimeter settings are metric (i.e. millibar instead of inches of mercury) in much of the world except in the US. For flight controllers and procedures they had to pick a unit for flight levels that everybody understands in the same way and since the US aviation industry after WW II pretty much dominated, a lot of planes would have used feet. Internationally, flight levels are used everywhere above certain altitudes. Those are basically feet divided by 1000 (kilo feet?, LOL). So, FL 29 would be 29000 feet at 29.92 inches of mercury in the US and 1013.2 millibars in the rest of the world. The Russians actually used meters until very recently apparently and only switched to feet a few years ago after of course first supporting flight levels. And apparently, the Chinese still use meters: https://ivao.aero/flightops/divprocedures.php?id=CN https://ivao.aero/flightops/divprocedures.php?id=CN and planes must actually be capable to switch their altimeters to using meters to fly in China. Likewise, North Korea uses meters apparently.
supernewton 5y ago
A nautical mile is equal to (about) one minute of latitude, which probably made calculations easier back in the day, with the tradition carrying forward.
jleahy 5y ago
History is surely one thing, but another key point is that these speeds are always KIAS (indicated air speed). This is a measurement that doesn’t correspond to either true ground speed or true air speed (but is far more useful for flying than either of these). Certainly I would find kph/mph slightly confusing as you might expect it to correspond to some actual velocity, rather than something to do with air pressure. Maybe we should measure airspeed in mbar instead? (ofc navigation is a different matter, and there just history prevails)
jcrawfordor 5y ago
Other commenters say it's historical are, well, mostly right, but there is a distinct advantage to knots: a knot is 1 NM per hour, and 1 NM was historically defined as one arc minute latitude. So if you are looking at a chart and doing calculations by hand or in your head, NM and kts are very convenient. It lets you mostly do chart -> distance -> time and fuel measurements/calculations in your head. Of course this is not perfectly accurate in several ways but given chart projections it's usually Close Enough. The speeds given in the article are a little ambiguous in the details, the number given in the text is probably KIAS (but maybe not?) and the numbers from FlightAware will be ground speed.
thinkingkong 5y ago
When you translate maps with vast distances down to units, a nautical mile equals a minute of latitude. With long distances, you refer to lat/long over following a road.
Someone1234 5y ago
From my understanding there are two advantages: - So that when kph/mph is heard a pilot knows they're referencing ground speed, whereas knots is air speed. - Pilots use latitude/longitude, and a knot is "exactly equal to 1/60th of a degree of latitude. A 1/60th of a degree of latitude is known as a minute of latitude. Therefore, a knot is equal to one minute of distance. There are 90 degrees from the equator to the pole." But reality is that changing international standards is very hard, and nobody wants to have the battle when knots works "well enough." It likely started because existing nautical maps were used for over-ocean flight planning and stuck.
- mulmen 5y ago
  Knots is used for ground speed as well I thought?
  - adamm255 5y ago
    I’ve never heard Knots used on land. Sea and Air yes!
    - V-eHGsd_ 5y ago
      the ground speed of an aircraft is reported in kts.
    - anamexis 5y ago
      Knots are indeed used when taxiing at airports.
    - mulmen 5y ago
      Ground speed is speed over the ground. This includes when the aircraft is in the air. Airspeed is the speed of the plane through the air. These are not the same thing. Both are reported in knots.
  - jcrawfordor 5y ago
    Really depends on the context. Usually things that were designed for pilots to use will display ground speed in knots since it's likely the pilot will be comparing against calculations done in knots. Things that were designed for the general public usually show mph since most people don't have a good sense of how fast a knot is. Taxi speeds are always written in knots, so I suppose you could say it is "official" (in terms of aircraft handbooks, airfield directories, etc) to use knots for ground speed. Of course the situation of an aircraft's ground speed while on the ground is sort of an edge case. On the topic of odd units, the operating handbooks for jet aircraft often use mach number for limits, which has some of the same properties as KIAS as far as being density dependent but is a little more precisely defined.
chrisseaton 5y ago
Because the Earth is a non-Euclidean surface, and knots make more sense for navigating a sphere.
mulmen 5y ago
Maritime navigation techniques were adapted to aviation. For a similar reason airplanes have captains. A few decades ago they also had navigators. With a dedicated position in the cockpit. It’s also customary to measure altitude in feet. Except in China (except Hong Kong), Kyrgyzstan, Kazakhstan, Tajikistan, Uzbekistan and Mongolia. Russia is actually moving to feet at some altitudes. Turkmenistan similarly uses feet or meters depending on altitude. The units themselves are irrelevant as long as everyone uses the same ones. Switching costs are significant both in terms of training and equipment.
inglor_cz 5y ago
At the very beginning, aviation had to solve the very same problems that sailors used to encounter at sea (navigation, precise position), so they naturally used the same algorithms and equipment. That is how naval terminology entered the lexicon. By now, it is so deeply ingrained that switching the entire world (well, there are exceptions) to the metric standard would likely cause some preventable loss of life. At least the knot/mile system is fairly internally regular. We tolerate a much less regular one, with a lot of weird exceptions (the Gregorian calendar) out of inertia.

Why are knots used in aviation over kph?

cmurf 5y ago

From nautical roots, hence aeronautical. Quite a lot is borrowed by aviation from maritime.

NikolaeVarius 5y ago

Convention, smaller numbers, everyone that it matters to knows it

pc86 5y ago
I can't imagine "smaller numbers" plays any role at all.

ghaff 5y ago

Historical reasons mostly. For the same reason that various units are used in all sorts of different industries. And guess what? If you changed you'd probably end up with some accidents in the transition as a result.

realityking 5y ago

Both aviation and marine navigation generally use nautical miles for as it‘s a little easier to use for long distance travel (plus tradition and inertia). Measuring speed in knots make things like dead heading calculations a lot easier.

nradov 5y ago

Using knots made navigation calculations easier with paper maps marked in degrees and minutes. A nautical mile is the same as a minute of latitude, and a knot is one nautical mile per hour. Now with electronic navigation systems kph would be just as easy, but aviators are all used to thinking in knots so changing would be difficult.

jillesvangurp 5y ago
It's partly true. However, a lot of planes outside of the US would have used kilometers per hour historically. For example German WW II planes did that. Likewise a lot of Russian cold war era planes did the same. And lots of modern planes (e.g. French ones) still do. And some planes in the US actually use miles per hour instead of knots; especially in general aviation. It's just that post war, the US aviation industry was very dominant and the metric system never was popular there. For the same reason feet are used instead of meters for measuring altitude. And unlike with nautical miles, there's not much of an advantage for that. However, the units for altimeter settings are metric (i.e. millibar instead of inches of mercury) in much of the world except in the US. For flight controllers and procedures they had to pick a unit for flight levels that everybody understands in the same way and since the US aviation industry after WW II pretty much dominated, a lot of planes would have used feet. Internationally, flight levels are used everywhere above certain altitudes. Those are basically feet divided by 1000 (kilo feet?, LOL). So, FL 29 would be 29000 feet at 29.92 inches of mercury in the US and 1013.2 millibars in the rest of the world. The Russians actually used meters until very recently apparently and only switched to feet a few years ago after of course first supporting flight levels. And apparently, the Chinese still use meters: https://ivao.aero/flightops/divprocedures.php?id=CN https://ivao.aero/flightops/divprocedures.php?id=CN and planes must actually be capable to switch their altimeters to using meters to fly in China. Likewise, North Korea uses meters apparently.

supernewton 5y ago

A nautical mile is equal to (about) one minute of latitude, which probably made calculations easier back in the day, with the tradition carrying forward.

jleahy 5y ago

History is surely one thing, but another key point is that these speeds are always KIAS (indicated air speed). This is a measurement that doesn’t correspond to either true ground speed or true air speed (but is far more useful for flying than either of these). Certainly I would find kph/mph slightly confusing as you might expect it to correspond to some actual velocity, rather than something to do with air pressure. Maybe we should measure airspeed in mbar instead? (ofc navigation is a different matter, and there just history prevails)

jcrawfordor 5y ago

Other commenters say it's historical are, well, mostly right, but there is a distinct advantage to knots: a knot is 1 NM per hour, and 1 NM was historically defined as one arc minute latitude. So if you are looking at a chart and doing calculations by hand or in your head, NM and kts are very convenient. It lets you mostly do chart -> distance -> time and fuel measurements/calculations in your head. Of course this is not perfectly accurate in several ways but given chart projections it's usually Close Enough. The speeds given in the article are a little ambiguous in the details, the number given in the text is probably KIAS (but maybe not?) and the numbers from FlightAware will be ground speed.

thinkingkong 5y ago

When you translate maps with vast distances down to units, a nautical mile equals a minute of latitude. With long distances, you refer to lat/long over following a road.

Someone1234 5y ago

From my understanding there are two advantages: - So that when kph/mph is heard a pilot knows they're referencing ground speed, whereas knots is air speed. - Pilots use latitude/longitude, and a knot is "exactly equal to 1/60th of a degree of latitude. A 1/60th of a degree of latitude is known as a minute of latitude. Therefore, a knot is equal to one minute of distance. There are 90 degrees from the equator to the pole." But reality is that changing international standards is very hard, and nobody wants to have the battle when knots works "well enough." It likely started because existing nautical maps were used for over-ocean flight planning and stuck.

mulmen 5y ago
Knots is used for ground speed as well I thought?
- adamm255 5y ago
  I’ve never heard Knots used on land. Sea and Air yes!
  - V-eHGsd_ 5y ago
    the ground speed of an aircraft is reported in kts.
  - anamexis 5y ago
    Knots are indeed used when taxiing at airports.
  - mulmen 5y ago
    Ground speed is speed over the ground. This includes when the aircraft is in the air. Airspeed is the speed of the plane through the air. These are not the same thing. Both are reported in knots.
- jcrawfordor 5y ago
  Really depends on the context. Usually things that were designed for pilots to use will display ground speed in knots since it's likely the pilot will be comparing against calculations done in knots. Things that were designed for the general public usually show mph since most people don't have a good sense of how fast a knot is. Taxi speeds are always written in knots, so I suppose you could say it is "official" (in terms of aircraft handbooks, airfield directories, etc) to use knots for ground speed. Of course the situation of an aircraft's ground speed while on the ground is sort of an edge case. On the topic of odd units, the operating handbooks for jet aircraft often use mach number for limits, which has some of the same properties as KIAS as far as being density dependent but is a little more precisely defined.

chrisseaton 5y ago

Because the Earth is a non-Euclidean surface, and knots make more sense for navigating a sphere.

mulmen 5y ago

Maritime navigation techniques were adapted to aviation. For a similar reason airplanes have captains. A few decades ago they also had navigators. With a dedicated position in the cockpit. It’s also customary to measure altitude in feet. Except in China (except Hong Kong), Kyrgyzstan, Kazakhstan, Tajikistan, Uzbekistan and Mongolia. Russia is actually moving to feet at some altitudes. Turkmenistan similarly uses feet or meters depending on altitude. The units themselves are irrelevant as long as everyone uses the same ones. Switching costs are significant both in terms of training and equipment.

inglor_cz 5y ago

At the very beginning, aviation had to solve the very same problems that sailors used to encounter at sea (navigation, precise position), so they naturally used the same algorithms and equipment. That is how naval terminology entered the lexicon. By now, it is so deeply ingrained that switching the entire world (well, there are exceptions) to the metric standard would likely cause some preventable loss of life. At least the knot/mile system is fairly internally regular. We tolerate a much less regular one, with a lot of weird exceptions (the Gregorian calendar) out of inertia.