If everything is computerized, the costs to maintain the old system can be hidden when the computer can be programmed to display in any combination of units. With computers, the cost to change could be zero as all that would be required would be a few seconds to change a setting to prefer metric units.

Consider that through the 1990s as shops upgraded their weighing scales to dual as they needed to be replaced anyway, that on 2000-01-01 there would be no cost to switch the scale to metric mode simply by pressing a switch or changing a setting. After that date as scales wear out or when they are naturally upgraded they can be replaced with metric only scales. Thus the costs can be zero if planned properly. Resistance and refusal to change is what adds costs.

The fact is, once everyone is working in one standard, supported system, the costs and errors (errors are costly too), become zero.

Spherical trigonometry is complex, however, the basic triangle solved is from the point of origin to destination, and the nearer pole. The three sides are the two co-latitudes and the oblique great circle cutting origin and destination. The three angles are the meredian difference, and the forward and reverse initial bearings. All the sides are measured as arcs, and the distance from origin to destination is calculated as an arc, seen from the center of the earth. It may be specified in degrees and minutes, or all minutes. In “all minutes” it is nautical miles (recognizing that spherical earth is an approximation).

I do not understand what you mean by inferring nautical miles and degrees are only consistent in a north-south direction. (other than circles of latitude being small circles except for the equator). You do have to solve the spherical triangle, not just take differences in latitude and longitude.

But, if you want navigators to love distances in kilometers, change the angular units of latitude and longitude to grads, and reprint all the maps.

[I think we can all understand (in principle) the trigonometry behind the argument (we all learned about Pythagoras at school). But it STILL doesn't explain why the unit of distance has to correspond with minutes of a great circle. It sounds like special pleading from a profession that wants to stick to what it is used to - rather than use the same system as everybody else - Editor]

I think it depends on whether we retain degrees and minutes as the primary angular notation for latitude and longitude on maps. If we do, nautical miles play nicely with these units.

If all the maps are changed to lat/long either in radians, or in grads, navigators would probably prefer to change. Grads of arc would approximate 100 km, and centigrads, 1 km. (New reduction tables in those units would be required)

It is a specialty unit used in a narrow field and the CIPM seems to have accepted it. If they can’t accept it, they should demand absolute SI compliance of all disciplines, no astronomical units, liters, tonnes, hectares, barns, Angstroms, minutes, hours, days, angular degrees, minutes, and seconds, bars, mm Hg, etc.

I admit there are several of those I can do without. but I assume they listen to how hard the experts in the field whine, and have some process for deciding. There is a rationale for both the value and continued use of the nautical mile. Even if it is not rock solid, it is more solid than any argument for the continued use of the International Mile (or in the US, in many States, the Survey Mile)

However, I would point out the CIPM does accept it and Table 8, footnote d gives a rationale approximating mine. I don’t think we on a forum will have much impact if people who actually navigate (and make maps) oppose the change. [This is all very interesting, but we seem to be talking past each other. What nobody has explained is why the co-ordinates used for global navigation (which are only dimensionally consistent in the special case of north-south direction) should determine the units of length used for measuring distance and speed. They are independent. - Editor]

The debate is about whether the nautical mile should be retained as a primary unit of distance for aerial and maritime navigation.

The “sailings” method used in exceptional circumstances may invoke some reckoning of distance in the manner described, but does that justify the use of the nautical mile instead of meters in routine navigation?

The circumference of an average great circle on Earth does happen to be very close to 40 000 km. Perhaps that too should be considered as having some merit.

Since the earth is ellipsoidal, some approximation is unavoidable. Past nautical miles have either been 1 minute average on a meredian (slightly over 1852 m) or 1 minute on a spherical earth with the same surface area as ellipsoidal earth (slightly over 1853 m). Rounding to either integer value (or switching) leads to minor errors compared to speed and bearing instrumentation on historical ships and airplanes (and navigating in a moving fluid).

Great circle sailing consists of finding some intermediate points on the great circle route and sailing between them using plane sailing or mid-latitude sailing. The three together are known as “The Sailings” to navigators. The result is an arc specified by its central angle in degrees. It is easy to multiply by 60 for nautical miles, almost as easy by 111.12 for kilometers, or 69.05 for statute miles (with a calculator). All three are slightly wrong because the earth is an ellipsoid.

Prior to electronic navigation, course holding and speed of both planes and ships had larger errors, and needed intermediate fixes and course corrections. The basic method is to assume a position, calculate an altitude of celestrial body and compare to an observed sextant altitude. The error in minutes leads to a Sumner line of position, whose perpendicular distance to the assumed position in nautical miles is the altitude error in minutes. Two or more Sumner lines of position lead to a fix.

Obviously, modern GPS navigation is far better. The question is whether this is a useful backup method in the event of electronic failure. Small boats (crossing oceans) routinely carry a sextant and navigation tables to carry this out if required. The Navy used to on ships, I’m not sure if they still do. Is it important or a way to haze midshipmen? Debatable. Still, with a cheap plastic sextant and Table 35 (Ageton’s method) I could navigate a lifeboat to a port.

However, I think those who criticize the unit should learn “The Sailings” and how to reduce a sextant altitude to a line of position before they decide the unit is useless.
(All of this has no bearing on feet or meters for plane altitude; I think that is a matter of the ATC equipment installed in the country.)

By the way, great circle sailing can be updated to great ellipsoid sailing using the series expansion known as Vincenty’s equations, widely used in geodesy and in GPS. I would suggest they are tractable on a computer or programmable calculator, intractable on a hand calculator or by tabular methods. You would either be there or miss before you determined the solution by hand.

However tha SI system does still have its problems because it doesn’t recognise Angle as a distinct Dimension. Thus we have N.m and m.N for both torque and work, yet one must be multipled by Angle to get the other! Perhaps a topic for a new thread? http://www.iso.org/iso/keeping.pdf [The SI unit for plane angle is the radian (rad). As it is defined in terms of pi, it is arguable a better unit than the degree, which is purely arbitrary, and hence the nautical mile, knot etc, which are based on dividing the Earth's circumference by 2160, are equally arbitrary - Editor]

Description
Flight level or altitude confusion occurs when a pilot is cleared to fly at a particular level and correctly acknowledges this clearance, yet levels at a different flight level or altitude.
Flight level or altitude confusion is usually the result of the combination of two or more of the following factors:
Read-back/hear-back error because of similar sounding phrases;
Non-standard phraseology;
Mindset tending to focus on two digits, e.g. “one zero” and thus to understand more easily “FLIGHT LEVEL ONE ZERO ZERO” when the clearance was to FL 110;
Failing to question the unusual (e.g. bias of expectation on a familiar standard terminal arrival (STAR); and/or,
Subconsciously interpreting a request to slow down to 250 kt as a clearance to descend to FL 100.

“Flight level” actually means the altimeter is NOT corrected for local variation for sea level pressure. Below a transition altitude, a correction (called QNH) is used, such that on the ground, the altimeter would indicate height above sea level. The pressure/altuitude relationship is based on a “standard atmosphere” true at about 45° latitude, and is NOT corrected for variation at the poles, equator, lapse rate, etc. Aircraft fly pressure contours, not real altitude. However, with QNH correction, it is referred to as altitude, but is still assigned in hectofeet by air traffic control. The transition rules for altitude vs flight level vary by country; for the US it is 18000 ft.

In countries which fly metric (Russia, China, Mongolia, CIS States), 300 m separation is used in the same sense that 1000 ft separation is used in foot flight countries.

Regional jets do not typically have “glass cockpits” and sophisticated, switchable instrumentation, long-haul jets do, and can accomodate the switch to metric altitude when flying into or over the above countries.

A bigger barrier is the ATC equipment on the ground, which is often decades old and not so sophisticated. If it is set up for feet, I suspect it would have to be replaced with new equipment that could switch or be metric only. This is probably a major barrier to Europe or other metric countries switching if they have been using feet for aviation. More pilots would need additional training, but long-haul jets from the US could handle the transition better than local ground facilities. (We fly to Russia, China, etc and obey ATC in their airspace.)