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The North Atlantic Tracks (NATs) are the airways generally used by airliners taking approximate great-circle routes between North America and Europe; due to the wild swings in wind strength and direction over the North Atlantic from day to day and place to place, the NATs are not fixed in position, but, rather, are plotted fresh every morning to allow transatlantic flights as much tailwind (and, thereby, as much groundspeed and as much per-flight fuel efficiency) as possible, and, then, sent to all the airlines preparing aircraft for transatlantic runs.

In contrast, the weather in the stratosphere - the layer of air lying above the tropopause (the upper boundary of the weather-prone troposphere; usually lying between 35 and 50 kilofeet at temperate and polar latitudes, but generally located at a much higher altitude over the tropics) and below the stratopause (the upper boundary of the stratosphere, and the lower boundary of the poorly-known mesosphere; usually located at around 165-180 kft) - is essentially uniform constant from day to day, changing (usually) only slowly, with the seasons. This removes the raison d’etre for the moving NATs, as no particular stratospheric route is objectively significantly better than any other of comparable length. For this reason, the Concorde (cruising at flight levels [FLs] 470-600, equating to an altitude of 47-60 kft, give or take a kilofoot or so)1 did not use the NATs, instead following, for open-ocean stratospheric cruise, one of a set of preplotted fixed airways, the locations of which did not change from day to day.

But the Concorde, even when it was still in service, wasn’t the only aircraft to cross the North Atlantic in the stratosphere; business jets (at least the many that are actually capable of doing so) routinely cruise at between FL 450 and FL 550 or so (the exact upper limit depending on the model of bizjet), and some smaller military jets routinely cruise above FL 600 in order to minimise aerodynamic drag on long-distance flights. These other high-altitude jets would stand to benefit just as much as the Concorde did from being able to use fixed stratospheric airways rather than having to follow the variable and non-stratospheric-aircraft-benefitting NATs; yet, I’ve only ever come across mention of immobile stratospheric routes with reference to the Concorde, and in language that makes it sound like the Concorde was unique in its use of these routes.

So do stratospheric bizjets and military jets have to use the NATs (and, if so, why?), or do they get to use fixed stratospheric routes, like the Concorde did?


1: An aircraft’s flight level is the altitude, in hectofeet, that its barometric altimeter (if properly calibrated, and assuming the aircraft’s pitot-static system is working properly) indicates when set to standard pressure (29.93 inHg).

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    $\begingroup$ Question is too longwinded with irrelevant asides. Just ask "Where do the NATs top out, and what routes are available above them?" $\endgroup$ – pericynthion May 6 at 5:38
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    $\begingroup$ @pericynthion, true, however the criteria for an up vote is "Shows research effort, is useful and clear". He definitely scores on the first 2, though could use some effort on the 3rd. OP's questions are a nice change from the dry, "Just the facts, Ma'am" questions we usually get 'round here. Gets my +1 for "kilofeet" alone. $\endgroup$ – FreeMan May 6 at 17:26
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    $\begingroup$ @pericynthion It looks like a great question to me. I don't see how your nitpicking comment helps. $\endgroup$ – curious_cat May 11 at 4:40
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Do high flying jets have to use the NAT, and if so, why?

They do not need to use the tracks when above the HLA (High Level Airspace), which is FL 285 to 420 (inclusive), but they need to be aware of the NAT procedures and have the current tracks. The why is in case they need to fly lower, say due to a loss of an engine.

This is highlighted in ICAO Doc 007 (North Atlantic Operations and Airspace Manual):

Although aircraft and pilots may fly above the NAT MNPSA without the requisite of an MNPS Approval, it is important that crews of such aircraft have both an understanding of the operational procedures and systems employed in the MNPSA and also specific knowledge of the location of any active OTS structure. This is important to help mitigate risk associated with any planned or unplanned penetration of the NAT MNPS Airspace (viz. emergency descents).

and

Crews of all NAT flights at or above FL290, even those that will transit the NAT above the NAT HLA (i.e. above FL420) or through the NAT HLA but are not planned to use the OTS, must be given both the organised track message and relevant amendments to it.

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The tropopause is in the low to mid 30s in the mid-north latitudes that the NATs reside (and is in the 20s in the arctic in winter) so an airliner crossing on a NAT at 55 DegN at FL370 will be well into in the stratosphere.

Concorde didn't use the NATs simply because the NATs top out at FL410. And in any case, the NATs are a large band of parallel tracks several hundred miles wide which are mostly just shifted more northerly or more southerly to accommodate weather patterns and the locations of polar jets etc. The routings aren't really customized much more than that.

Anyway, because they top out at 410, if your jet can cruise above 410, like a Global or Gulfsteam, you can avoid the NATs and can pretty much file for whatever custom routing suits you that doesn't conflict with someone else.

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