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What is a simple way to understand the difference between the nautical air mile (NAM) and the nautical ground mile (NGM)?

How does the wind affect the net displacement of the aircraft as that is the only differentiating factor between NGM and NAM? (NAM by definition does not include the vertical displacement or the altitude gained by the aircraft.)


For further reference to mathematical formulae defining the above terms: https://www.theairlinepilots.com/forum/viewtopic.php?t=320

From my deep dive in the quest to unravel the mystery of NAM and NGM, with inspiration from the BELOW answers I'm able to conclude with a couple of points:

  • NAM and NGM are same if wind is still

  • NAM measurement is independent of the wind and depends solely on the movement of the aircraft relative to the air around it. NGM is a dependent quantity which solely depends on the wind ie. With a Headwind component NGM is lower than NAM and vice versa for tailwind condition.

  • The usage of NAM was most prevalent in older flight planning systems for long range cruise before the age of FMCs which do the same thing electronically, NAM provides a database of performance of an aircraft in various phases of flight independent of the wind, to which the wind correction maybe applied to determine the practical value.

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    $\begingroup$ Is this a question about the distance covered through the airmass versus the distance made good over the ground? I hate to see a question about elementary physical concepts turned into a question about the definitions of certain phrases, which are arbitary. In a typical ground school course you won't be taught anything about a thing called an "air mile" or a thing called a "ground mile". Ask a pilot about the meaning of a "NAM" or a "NGM" and many will likely give you a blank stare. These sets of initials aren't necessarily widely used in aviation, though I'm sure they have their niches. $\endgroup$ – quiet flyer Jun 19 '20 at 12:39
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    $\begingroup$ Re "(NAM by definition does not include the vertical displacement or the altitude gained by the aircraft.)"-- where did you get this definition? And what does that same source say is the definition of a NGM and a NAM, and how are they different? There's your answer. If we are flying in a horizontal path over Mt Everest, would the NGM distance include the effect of the height gain and loss that we would experience if we were hiking on the ground rather than flying? Hope to see that cleared up in a self-answer by the OP, as only he knows what source is he is relying on for these definitions. $\endgroup$ – quiet flyer Jun 19 '20 at 12:43
  • $\begingroup$ I think the key is in learning to draw the vector triangle showing the effect of wind; see my anwer. $\endgroup$ – quiet flyer Jun 19 '20 at 14:02
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    $\begingroup$ Regarding your edit, I'm not sure what kind of "deep dive" you may have performed to unravel this "mystery", but the effect of wind is pretty basic when you think about it, and is something every beginning student pilot is expected to know. $\endgroup$ – Michael Hall Jun 20 '20 at 15:16
  • $\begingroup$ I didn't intend to be toxic, it's just that the effect of wind on ground speed and track can be instinctively understood and is well documented, here and elsewhere. Your question didn't show much evidence of any prior research, and questions are often closed for this reason. $\endgroup$ – Michael Hall Jun 20 '20 at 18:00
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A nautical ground mile is one nautical mile over the ground. A nautical air mile is one nautical mile through the air mass. This distance can be different because of wind.

For example:

If you are flying at 100 knots true airspeed in a zero wind condition, nautical ground miles will equal nautical air miles.

However, if you are flying at 100 knots true airspeed into a 100 knot headwind, your nautical ground miles will alway be zero, regardless of how many nautical air miles you fly for (at 100 knots).

Anything in between will result in some variance affecting the “net displacement of the aircraft”. This variance can be calculated using the basic wind triangle.

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    $\begingroup$ would it be different due to altitude? i understand that it's small effect (421 vs 420 miles on 3 mile altitude) but still $\endgroup$ – Noone AtAll Jun 20 '20 at 2:34
  • $\begingroup$ Historically a nautical mile was 1 minute of latitude along any line of longitude, but note that the distance corresponding to 1 minute of an arc depends on the radius.... The modern definition has 1 nautical mile be exactly 1852m. So if the nautical air mile inherits the older definition then altitude matters (slightly), if using the modern one then it does not. $\endgroup$ – Dan Mills Jun 20 '20 at 12:43
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    $\begingroup$ Why would "nautical air mile" be infinite? If your TAS is X NM/hour, then the airmass has moved X NM in one hour relative to the aircraft, regardless of winds. Stable winds are only a consideration when ground is concerned. $\endgroup$ – JZYL Jun 20 '20 at 16:32
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    $\begingroup$ @JZYL: The answer is saying that some journey (over some ground distance) will take infinite air miles, because the ground speed is zero. Not infinite air speed, but rather infinite total time. The usefulness of making the point this way is questionable, but it doesn't look like a technical error. (Except that no specific ground distance was ever mentioned.) $\endgroup$ – Peter Cordes Jun 20 '20 at 18:28
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    $\begingroup$ Right, ground distance will always be zero in that case, that wasn't the problem. Nothing about that implies infinite time. You're unnecessarily introducing the complication of an infinity into your example. I left a suggested edit that avoids that but still makes the same point. It should fix the problem @JZYL and I are pointing out. $\endgroup$ – Peter Cordes Jun 20 '20 at 20:29
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How does the wind affect the net displacement of the aircraft...?

Before we worry about the definitions of certain phrases such as "nautical air mile" and "nautical ground mile", we should learn to draw the vector triangle of (true) airspeed, wind, and groundspeed, assuming that the flight path, the earth's surface, and the wind are all horizontal. The ratio of miles covered through the airmass to miles covered over the ground is identical to the ratio of (true) airspeed to groundspeed.

In the vector triangle of airspeed, wind, and groundspeed-- overlooking for the moment the difference between "true" and "indicated" airspeed-- we can say that the airspeed vector represents the "inherent" performance of the plane, given the position of the throttle or thrust lever, and the position of the elevator control which determines the wing's angle-of-attack. For a given thrust and angle-of-attack, the wind has no effect on the airspeed vector. The plane is flying "inside" the moving airmass and not directly "feeling" the wind. The plane's "inherent" performance won't be enhanced or diminished by the wind. But when we add the wind vector as illustrated in the link above, then we can find the groundspeed vector which shows the actual achieved performance over the ground.

As for your implied question about the difference between a "NGM" and a "NAM", the most logical meaning of the "NGM" distance would be the distance coverered over the ground, assuming flat ground, expressed in nautical miles, while the most logical meaning of the "NAM" distance would be the miles covered through the airmass, assuming a horizontal flight path and a horizontal, uniform motion of the airmass, expressed in nautical miles. However, we should be clear that there is no actual difference between the length of a "Nautical Ground Mile" and a "Nautical Air Mile". To make it clear that we are talking about two different quantities of interest rather than two different units of measure, it would be better if we spoke of "distance covered through the airmass" and "distance covered over the ground", rather than "nautical air miles" and "nautical ground miles".

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  • $\begingroup$ Thank you for the reply! I'm familiar with the concept of wind triangle, and the explanation makes perfect sense. The only part that I'm still not completely convinced is the NGM and NAM explanation as these are distinct terms used in flight planning and hold different mathematical formulae per my observation. $\endgroup$ – Darshan Patil Jun 19 '20 at 14:03
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    $\begingroup$ If you understand the wind triangle then why are you asking this question? The difference is the distance covered over the ground vs through the airmass. That’s it. And I disagree that these terms are used in flight planning. Not commonly anyway. Not by people I have been flying with. Ground miles are simply called miles, and air miles are only important as they relate to ground speed and time. Nobody cares how many miles you flew through the airmass. It isn’t measurable like time and distance are. $\endgroup$ – Michael Hall Jun 19 '20 at 16:21
  • $\begingroup$ @MichaelHall -- have you ever seen a photo of an "air distance computer"? I would provide a link but I can't find one right now; more later. $\endgroup$ – quiet flyer Jun 19 '20 at 19:57
  • $\begingroup$ Do you mean air data computer? $\endgroup$ – Michael Hall Jun 19 '20 at 20:14
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These are very strange terminologies. As far as aeronautics is concerned, we care about these things:

  • Airspeed: speed of the aircraft relative to the local air-stream; this is important for keeping the airplane afloat
  • Ground speed: speed of the aircraft relative to the ground; this is important for seeing how fast you may be getting from point A to B
  • Maybe, ground distance: total distance moved along a [curved] trajectory

Ground distance, which could be expressed in Nautical mile, would be an integral of the ground speed. Maybe you can call that unit Nautical ground mile.

The integral of airspeed, however, is more or less meaningless. At best, it states how much distance a parcel of air has traveled relative to the aircraft. But since winds change over the trajectory, it wouldn't even be the same parcel of air. The unit of that distance could be what you called Nautical air mile.

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  • $\begingroup$ Meaningless seems overstated as is not fuel usage based closer on integral of airspeed than integral of ground speed? $\endgroup$ – chux - Reinstate Monica Jun 20 '20 at 10:35
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    $\begingroup$ @chux Not really. Neither is a good gauge of fuel usage. If you're climbing at 5000ft/min at any particular airspeed, the fuel usage would be drastically different than descending at idle throttle at the same airspeed. $\endgroup$ – JZYL Jun 20 '20 at 11:25
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Although, I have never heard the term Nautical Air Mile, it does make sense. Based on your attached link, an NAM would be the distance from a particular parcel of air. Airspeed would be the speed at which the aircraft is moving through, towards or away from particular parcels of air relative to those parcels. Airspeed is also the speed at which parcels of air are moving towards or away from a particular relative point or object.

Ground distance can be measured directly. So can time. Therefore, groundspeed can be calculated. Groundspeed and airspeed can also be measured through its affect on our instruments relative to the instruments themselves. Measuring NAMs directly seems impractical if not improbable. The most direct way that comes to mind is the movement of radiosonde balloons. Otherwise, NAM can only be calculated from other metrics. I find it difficult to think of any useful application of nautical air miles in practical use.

Also of note, in aviation, nautical miles is generally in reference to the distance between two point over which we are flying. Slant range in nautical miles takes into account the straight line distance between the aircraft and a point on the ground taking into consideration the aircraft’s altitude (the hypotenuse of the right triangle). When an airplane’s ground track is affected by wind, its measurement of airspeed vs wind speed is still relative to the ground. But, sense a nautical miles is defined as 1 minute of arc distance of latitude, it makes sense that it would be slightly greater in the air than on the surface of the Earth.

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  • $\begingroup$ So this answer is saying that the higher the altitude, the larger the number of feet or meters contained in a Nautical Air Mile? I understand the rationale for such a suggestion, but I doubt that this answer is harmonized to the definition of a NAM in whatever source the OP is relying on for definitions of these concepts. $\endgroup$ – quiet flyer Jun 19 '20 at 13:03
  • $\begingroup$ This is a good answer, but the OP specifically excluded any altitude effects. Also, if you click on the link that spawned his question they are discussing the wind triangle. $\endgroup$ – Michael Hall Jun 19 '20 at 16:14
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    $\begingroup$ @MichaelHall - Thanks. The link was added after my answer. That helps to bring context to the question. Although, I still question the usefulness or relevance of knowing your distance in reference to a particular parcel of air. It seems more academic than practical. However, using the wind triangle to determine other metrics like groundspeed, ground distance, relative motion across the ground, heading vs track, and time of flight are very useful, relative, and important. $\endgroup$ – Dean F. Jun 19 '20 at 16:23

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