But it is also experiencing a constant change in momentum in the
vertical direction. That's what a force is: a change in momentum over
time.
No. The airplane is in level flight at constant speed.

I'm with fredfighter here, but it is primarily a semantic argument
having to do with frame of reference. Looking from an accelerated frame
of reference (a standstill in the earth's gravitational field) one gets
one answer. Looking from an unaccelerated frame of reference
(freefall), one gets a different answer.

To reconcile them, it is important to treat =everything= from the same
frame of reference, and make appropriate adjustments.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.