Thrust is the force exerted by the engine and its propeller(s). It pushes
the air backward with the object of causing movement of the airplane in
the forward direction.
Weight is the downward force due to the weight of the airplane and its
load. It is directly opposed to lift
An aircraft is in a state of equilibrium when the thrust and drag are
equal and opposite. It will continue to move forward at the same uniform
speed. If thrust or drag becomes greater than the opposite force, the
aircraft loses its state of equilibrium. If thrust is greater than drag,
the aircraft will accelerate. If drag is greater than thrust, the aircraft
will lose speed and eventually descend.
When lift and weight are equal and opposite, the airplane is in a state
of equilibrium. If lift is greater than weight, the aircraft will climb.
If weight is greater than lift, the airplane will descend.