Why isn't the course fixed along an airway?
Isn't it supposed that an airway has always the same magnetic course between navaids?


No. The course depends on where you plot it from.



That's a random RNAV airway I picked, it has two headings, 114° and 298°, and the difference is not 180°, it's 184°, so over the course of 112 nm there is a 4° shift.

That's due to the curvature of the Earth.

Consider the following image carefully, notice the North-South needles, how they look on a 2D projection and on the actual globe.



Source: Wikipedia

That's the shortest path connecting those two points in South Africa and Australia.

If I'm in South Africa, looking at Australia, I'll look toward 141° (not 90°), as I get closer, this converges on 42°. If I look back from Australia, I'll face 222°. The difference between 222° and 141° isn't 180°.

If your airway exceeds 150 nm, this becomes noticeable. It becomes even more noticeable if you are flying between distant RNAV waypoints (e.g. NAT route), your heading (track to be more precise) will keep changing.

If you chase a true heading, and keep it constant over a long distance, this happens:



Source: Wikipedia

That's a very long turn, not straight-and-level flying.

Flying a great circle route, or staying on an airway between two navaids, requires flying a course, not a heading/track. The course's heading is only valid from where you plot it / start it. Along that course (i.e. route), the track keeps changing as the first image illustrates, and you're not turning.

An advanced autopilot follows the route computationally; a pilot hand flying would intercept the course, keep the selected course unchanged, and fly the deviation. If no factors (e.g. wind) push the plane off course, no turn inputs will be needed and the actual course will shift as the plane flies straight-and-level. The Course Deviation Indicator won't show any deviation because there isn't any.

Where It Starts

To further clarify why the course depends on where it starts, it's the fact that a VOR station—for example—transmits the radials in straight lines, those straight lines translate to a curved line on a 2D projection. They don't appear curved on a chart because the distances aren't big, even for the most powerful high altitude VOR station.

Hypothetically, if VHF had a huge range and wasn't limited by line-of-sight, VOR navigation would have become a bit confusing the farther out you are from a station.

Speaking of VOR

Just for the sake of completion, you'll notice I focused on RNAV waypoints. There's another factor affecting VOR stations. Two nearby VOR stations may have different alignments to the magnetic north. It depends on when they were built. One could be 20 years older than the other.

Once they're built and aligned, they are in most cases never re-aligned when the magnetic declination changes. It has to do with the cost and the redrawing of airways.

That can amplify the shift further.

So, think of a radial's heading as a highway marker.