The answer is that the plane is, in fact,
accelerating (or, rather, decelerating) as it goes
north. The 300 mph number that the plane is
flying at is the relative difference in speed
between the plane and the air in which the plane
is flying. The fact that the plane is maintaining
the same longitude as it flies north, however,
does mean that the plane has to aim itself
slightly northwest instead of due north, for
exactly the reason you describe.
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