writes:

If the vertical postion of an aircraft is constant, i.e. straight and
level flight, the first derivative of the vertical position is zero and
hence the second derivative is also zero.

Gravity is constantly trying to accelerate an aircraft downwards; something
else has to compensate for this, or it will descend.

An aircraft in straight and level flight is not accelerated.

Because a force is being applied to it that exactly counters gravity. Whence
comes this force?