When the power is reduced during straight and level flight, the thrust needed to balance the airplane's drag is no longer adequate. Due to the unbalanced condition, the drag causes a reduction in airspeed. This decrease in speed, in turn, results in a corresponding decrease in the wing's lift. The weight of the airplane now exceeds the force of lift so the resulting flight path is downward as well as forward. Since the flightpath is inclined downward, the force of gravity is providing the forward thrust. In effect, the airplane is actually going "down hill."
As in entering a climb, the forces acting on an airplane again go through definite changes when transitioning from level cruising flight to a descent. When forward pressure is applied to the elevator control or the airplane's pitch attitude is allowed to lower, the wing's angle of attack is decreased, the lift is reduced, and the flightpath starts downward. This change in the flightpath is the result of the lift becoming less than the weight of the plane as the angle of attack is reduced. This unbalance of lift and weight causes the airplane to descend with respect to the horizontal path of level flight. The initial reduction of lift which starts the airplane downward is momentary. When the flightpath stabilizes, the angle of attack again approaches the original value, and lift and weight stabilize.
Just as in climbing flight, the downward force on the horizontal stabilizer becomes less as the airspeed decreases. This produces an unbalanced condition which results in a tendency for the airplane to nose down. In order to maintain the desired descent attitude, it is usually necessary to apply slight back pressure on the elevator control.
As the descent is started, the airspeed may gradually increase, due to a component of weight now acting forward along the flight path. The overall effect is that of increased thrust, which in turn would cause the airspeed to increase if the power were allowed to remain the same as that used for level cruise flight. For descent at the same airspeed as flown in level cruise flight, the power must be reduced as the descent begins.
The component of weight acting forward along the flightpath increases as the descent attitude is steepened and, conversely, will decrease as the descent attitude is shallowed. Therefore, the amount of power reduction for a descent at cruising speed will be determined by the rate of descent desired.