Why are knots used in aviation over kph?
- 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.
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.
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?
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.
I think it was actually extrapolated from a survey of the length of France. No idea how they did that extrapolation.
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.
Of course, the metric system has not been in use since 1960, but this is also true for the SI.
> a knot is "exactly equal to 1/60th of a degree of latitude. ...Therefore, a knot is equal to one minute of distance."
I'm not sure what that quote is from, but it's is akin to saying "A metre per second is equal to one metre." Um, no. The units don't match.
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.
Weight / volume changes from gallons to litres to cubic meters to cubic feet depending on US/RoW.
Height is always in feet.
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.
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.
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)
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 and planes must actually be capable to switch their altimeters to using meters to fly in China. Likewise, North Korea uses meters apparently.
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.