- 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.
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.
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.
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.
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.
I imagine that the mental adjustment from sphere to almost but not quite a sphere must have been harder than a switch from flat earth to spherical. You just expect reality, if it matches a certain mathematical principle, to match perfectly.
This has not been the case since 1889, when the metre was formally defined as the length of a prototype metre bar. So the metric system has not suffered the same error for more than a century.
Of course, the metric system has not been in use since 1960, but this is also true for the SI.
Oh, I think I didn't even read that correctly since it seems like it's just a mistake. A knot is, for historic reasons, a term for NM per hour. An NM is defined by latitude.
I believe that the term knots originates from a method of measuring speed at sea by use of a twisted rope and a sea anchor, but perhaps that's apocryphal. Presumably though the definition in terms of NM was a later improvement.
While I don't have any real evidence for this, I would speculate that part of the reason for the enduring use of knots in aviation is that, much like the old twisted rope, it links the unit to the instrument. The speeds usually quoted for aircraft are in KIAS, Knots Indicated Airspeed, a measurement which is basically defined by what the instrument shows. It is intentionally uncorrected for various errors in the instrument design. This happens to be convenient because many of the same factors that affect the instrument calibration also affect the aircraft handling, so handling at 100 KIAS tends to feel similar even though the actual speed varies by altitude etc. KCAS and KTAS, Calibrated and True Airspeed, are both KIAs after correction for various errors and are useful for certain calculations but not nearly as often as KIAS. My point is that it might be helpful to use Kts rather than MPH or KPH because it helps keep these factors in mind and helps to reduce ambiguity about whether a number is indicated or true, although people very frequently just write knots when meaning KIAS.
Of course for ground speed none of this matters but ground speed isn't really a "flight" concern per se, more just used for navigation calculations.
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.
> 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?
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.
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.
- 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.