High RPM right after start should be avoided because the engine is cold and will not be properly lubricated, resulting in greater wear and tear. 100% power is only available at sea level and with full throttle. For takeoff, pilots should gradually move the throttle from idle to full (about 3 seconds). The engine is seldom held at maximum power for more that 2 minutes, and usually not that long. Once the aircraft is at a safe altitude above the runway (about 1000 feet), the power is reduced to a setting that is used for climbing and can be maintained for longer periods of time. After the aircraft has climbed to cruising altitude, the power of the engine is further reduced to a cruise setting which can be maintained for the duration of the flight. For aircraft equipped with constant speed propellers, power should be reduced by first decreasing the manifold pressure with the throttle, then decreasing the RPMs with the propeller control. When increasing power the opposite sequence should be employed: increase RPMs first, then increase manifold pressure—this will help to avoid placing undue stress on the engine. Operating at high RPMs and low power (as well as rapid opening and closing of the throttle) can cause detuning of the counterweights on a balance weight-equipped crankshaft and over-stress it. The counterweights are designed to position themselves by the inertia forces generated during crankshaft rotation and to absorb and dampen crankshaft vibration effectively. If the counterweights are detuned (allowed to slam on mounts), the vibrations are not properly dampened and crankshaft failure can occur. Operating the engine at very low RPMs for long periods can cause deposits on spark plugs that lead to misfiring. 

The throttle controls the total volume of fuel and air entering the combustion chamber. With full throttle, power decreases with altitude until an altitude is reached where 75% power is the maximum power available—This usually occurs at altitudes of 6000 to 10,000 feet, depending on the amount of friction loss in the intake system and the RPM selected by the pilot. During climb in a normally aspirated engine, ambient pressure decreases at a rate of about one inch of Mercury per 1000 feet of altitude gained. To maintain climb power, the pilot must adjust the throttle every few hundred feet of climb until the throttle is fully open. Critical altitude is the term used to describe the altitude where the throttle is fully open in order to achieve the desired power setting. The desired power setting cannot be maintained above this altitude. Most normally aspirated reciprocating engines in general aviation aircraft are designed to produce continuous power with economical operation and long life if operated at or below 75% of their sea level rated power. Full power (takeoff) should be used when needed but not for continuous operation.

Pilots should remember that power is taken from the engine to drive the alternator only when the alternator is pumping electrons. The power taken from the engine is proportional to the current being produced by the alternator. Power taken from the engine is subtracted from the power available at the propeller. So, if someday you have to depart a short field with an obstacle with a heavy airplane on a hot, humid day with high density altitude, perhaps it would make sense to pull the alternator field circuit breaker, or open the alternator switch, and let the battery handle the electrical system until you have cleared the obstacle.

Water vapor in the air will cause a significant reduction of power output from reciprocating engines. If humidity is high, water molecules will displace oxygen molecules in a cylinder full of air, producing two power-reducing effects: 1) Less oxygen is available to burn fuel to produce power; and 2) the fuel:air ratio, already richened for full power production on takeoff, becomes even richer and causes a further decrease in power production. Power losses of 5% are not unusual in warm air having 100% humidity; but a 5% decrease in power can cause a much greater decrease in performance. Since warm air holds more water as vapor, the effect of humidity on power decreases as temperature decreases. Also for this reason, humidity effects become inconsequential at high altitudes. More power can be expected from cold air. As a general rule, for each 6 degrees C below standard temperature, one percent more horsepower is produced than at standard temperature and one percent should be subtracted for each 6 degrees C the temperature is above standard. Very cold temperatures introduce another effect on engine power—poor fuel vaporization in the carburetor which creates poor fuel distribution to the cylinders.

Any change in the power setting results in a change in the airspeed or altitude of the airplane. If the power setting is increased in straight-and-level flight and the airspeed held constant, the airplane climbs. If power is decreased while the airspeed is held constant, the airplane descends. If altitude is held constant, power applied will determine the airspeed. Altitude and airspeed are interchangeable; altitude can be converted to airspeed by lowering the nose, or convert airspeed to altitude by raising the nose. An aircraft is trimmed for a specific airspeed, not pitch attitude or altitude. Both low speed and high speed require high thrust (to overcome parasite drag at high speeds and induced drag at low speeds)

During descent, power should be reduced by increments to allow the engine to cool slowly and prevent thermal shock. During a prolonged power off glide, such as is done when practicing forced landings from altitude, the engine should be cleared periodically by advancing the throttle to a medium power position for a few seconds until the engine runs smoothly, then power can be reduced. This helps keep the bottom spark plugs from fouling and warms the engine and exhaust so that carburetor heat is available.