Excess power is the available power over and above that  required to maintain horizontal flight at a given speed.  Although the terms power and thrust are sometimes  used interchangeably (erroneously implying they are  synonymous), distinguishing between the two is important  when considering climb performance. Work is the product of  a force moving through a distance and is usually independent  of time. Power implies work rate or units of work per unit  of time, and as such is a function of the speed at which the  force is developed. Thrust, also a function of work, means  the force which imparts a change in the velocity of a mass. During takeoff, the aircraft does not stall even though it  may be in a climb near the stall speed. The reason is that  excess power (used to produce thrust) is used during this  flight regime. Therefore, it is important if an engine fails  after takeoff, to compensate the loss of thrust with pitch  and airspeed. For a given weight of the aircraft, the angle of climb depends  on the difference between thrust and drag, or the excess  thrust. When the excess thrust is zero, the inclination of the  flightpath is zero, and the aircraft is in steady, level flight.  When thrust is greater than drag, the excess thrust allows a  climb angle depending on the amount of excess thrust. When  thrust is less than drag, the deficiency of thrust induces an  angle of descent. Aircraft accelerate in level flight because of an excess of  power over what is required to maintain a steady speed. This  is the same excess power used to climb. Upon reaching the  desired altitude with pitch being lowered to maintain that  altitude, the excess power now accelerates the aircraft to its  cruise speed. However, reducing power too soon after level  off results in a longer period of time to accelerate. Like any moving object, an aircraft requires a sideward force  to make it turn. In a normal turn, this force is supplied by  banking the aircraft in order to exert lift inward, as well as  upward. The force of lift is separated into two components  at right angles to each other.  [Figure 4-14] The upward  acting lift together with the opposing weight becomes the  vertical lift component. The horizontally acting lift and its  opposing centrifugal force are the horizontal lift component,  or centripetal force. This horizontal lift component is the  sideward force that causes an aircraft to turn. The equal and  opposite reaction to this sideward force is centrifugal force,  which is merely an apparent force as a result of inertia. If allowed to slow several knots, the airplane could enter  the region of reversed command. At this point, the airplane  could develop an unsafe sink rate and continue to lose speed  unless the pilot takes a prompt corrective action. Proper pitch  and power coordination is critical in this region due to speed  instability and the tendency of increased divergence from  the desired speed. Pilots of larger airplanes with higher stall speeds may find the  speed they maintain on the instrument approach is near 1.3  V$_{SO}$, putting them near point C  [Figure 4-11]  the entire time  the airplane is on the final approach segment. In this case,  precise speed control is necessary throughout the approach. It  may be necessary to temporarily select excessive, or deficient  thrust in relation to the target thrust setting in order to quickly  correct for airspeed deviations. For example, a pilot is on an instrument approach at 1.3  V$_{SO}$, a speed near L/D$_{MAX}$, and knows that a certain power  setting maintains that speed. The airplane slows several knots  below the desired speed because of a slight reduction in the  power setting. The pilot increases the power slightly, and the  airplane begins to accelerate, but at a slow rate. Because the  airplane is still in the “flat part” of the drag curve, this slight  increase in power will not cause a rapid return to the desired  speed. The pilot may need to increase the power higher  than normally needed to maintain the new speed, allow the  airplane to accelerate, then reduce the power to the setting  that maintains the desired speed. The ability for an aircraft to climb depends upon an excess  power or thrust over what it takes to maintain equilibrium.  Large Airplanes Acceleration in Cruise Flight Turns Climbs Figure 4-14.  Forces in a turn. Horizontal component of lift Resultant lift Vertical component of lift Weight  L  L  L  W  4-10