The first sentence got me all psyched up to answer the question "What
horizontal force do the engines generate in order to accelerate it ?".
But the actual question, in the second sentence, turned out to be
a completely different one.
When the plane levels off and continues on at a constant altitude, it's
not accelerating up or down, so the net vertical force on it is zero.
The lift generated by the wings is exactly balancing the downward
force of gravity on the airplane.
