3 ms·

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. -

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.
  - mulmen 5y ago
    > So that when kph/mph is heard a pilot knows they're referencing ground speed, whereas knots is air speed. Who is using MPH/KPH in the course of flying a plane? Why would this ambiguity resolve to “that must be ground speed”? Why would there be ambiguity in a speed readout at all? Surely context and procedure should disambiguate this rather than units?
    - jcrawfordor 5y ago
      I'm not actually sure where that quote came from, but yes, there's not likely to be much confusion. Ground speed is usually displayed by some kind of computer, could be a glass panel or just an aviation GPS unit, and is usually marked "GS" or "Kts Ground." And it's sort of considered, philosophically, to be more a navigation thing than an actual flight instrument, since you only really pay attention to it in flight if you're doing navigation calculations.
      - mulmen 5y ago
        That quote came from the post I replied to originally. Beyond that I’m unsure of the source.
leetcrew 5y ago
seems slightly odd to use an angular unit for airspeed; the linear equivalent would technically depend on altitude. I guess airplanes don't actually fly high enough for that to matter.
- jcrawfordor 5y ago
  It was only ever approximately accurate anyway, a NM is no longer an arc minute because that is inconsistent even along lines of latitude, so it had to be pinned to 1.852km. Aviation, broadly speaking, inherited all of its navigation techniques from the maritime world. In particular, prior to radio methods and to some extent today aviation navigation is based primarily on dead reckoning periodically corrected by observations, which is a technique that was pioneered in marine navigation. Since early pilots learned to navigate from seafarers, their units and conventions became strongly embedded in the field. It is, after all, still not unusual to refer to an aircraft as a ship, especially informally.
  - deleted 5y ago
    [deleted]
  - usrusr 5y ago
    Ironically the metric system suffers the same theoretical "error" as the not-an-arc-minute-anymore NM because the meter was originally defined by taking the distance from pole to equator (to the best of their knowledge at the time) and then shifting the decimal point to get something human scale.
    - mulmen 5y ago
      > to the best of their knowledge at the time I think it was actually extrapolated from a survey of the length of France. No idea how they did that extrapolation.
      - kergonath 5y ago
        Initially, yes. The story itself is quite fascinating, and the Wikipedia page on the metre is a good starting point. This was a hot topic during the 19th century as different improvements in the techniques and devices used to measure the Earth showed that the first measurements were imperfect, however good they had been at the time, and that the whole idea is not workable because the Earth is not a sphere. This lead to the formal definition of the metre as the length of a prototype in 1889.
_s 5y ago
Pilot here, all distance and speeds are in nautical miles / hours. Mach numbers are also used often for faster / higher aircraft. Weight / volume changes from gallons to litres to cubic meters to cubic feet depending on US/RoW. Height is always in feet.

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.
- mulmen 5y ago
  > So that when kph/mph is heard a pilot knows they're referencing ground speed, whereas knots is air speed. Who is using MPH/KPH in the course of flying a plane? Why would this ambiguity resolve to “that must be ground speed”? Why would there be ambiguity in a speed readout at all? Surely context and procedure should disambiguate this rather than units?
  - jcrawfordor 5y ago
    I'm not actually sure where that quote came from, but yes, there's not likely to be much confusion. Ground speed is usually displayed by some kind of computer, could be a glass panel or just an aviation GPS unit, and is usually marked "GS" or "Kts Ground." And it's sort of considered, philosophically, to be more a navigation thing than an actual flight instrument, since you only really pay attention to it in flight if you're doing navigation calculations.
    - mulmen 5y ago
      That quote came from the post I replied to originally. Beyond that I’m unsure of the source.

leetcrew 5y ago

seems slightly odd to use an angular unit for airspeed; the linear equivalent would technically depend on altitude. I guess airplanes don't actually fly high enough for that to matter.

jcrawfordor 5y ago
It was only ever approximately accurate anyway, a NM is no longer an arc minute because that is inconsistent even along lines of latitude, so it had to be pinned to 1.852km. Aviation, broadly speaking, inherited all of its navigation techniques from the maritime world. In particular, prior to radio methods and to some extent today aviation navigation is based primarily on dead reckoning periodically corrected by observations, which is a technique that was pioneered in marine navigation. Since early pilots learned to navigate from seafarers, their units and conventions became strongly embedded in the field. It is, after all, still not unusual to refer to an aircraft as a ship, especially informally.
- deleted 5y ago
  [deleted]
- usrusr 5y ago
  Ironically the metric system suffers the same theoretical "error" as the not-an-arc-minute-anymore NM because the meter was originally defined by taking the distance from pole to equator (to the best of their knowledge at the time) and then shifting the decimal point to get something human scale.
  - mulmen 5y ago
    > to the best of their knowledge at the time I think it was actually extrapolated from a survey of the length of France. No idea how they did that extrapolation.
    - kergonath 5y ago
      Initially, yes. The story itself is quite fascinating, and the Wikipedia page on the metre is a good starting point. This was a hot topic during the 19th century as different improvements in the techniques and devices used to measure the Earth showed that the first measurements were imperfect, however good they had been at the time, and that the whole idea is not workable because the Earth is not a sphere. This lead to the formal definition of the metre as the length of a prototype in 1889.

_s 5y ago

Pilot here, all distance and speeds are in nautical miles / hours. Mach numbers are also used often for faster / higher aircraft. Weight / volume changes from gallons to litres to cubic meters to cubic feet depending on US/RoW. Height is always in feet.