contains a smaller mass of air. In other words, the density is  decreased. In fact, density is directly proportional to pressure.  If the pressure is doubled, the density is doubled, and if the  pressure is lowered, so is the density. This statement is true  only at a constant temperature.  Increasing the temperature of a substance decreases its  density. Conversely, decreasing the temperature increases  the density. Thus, the density of air varies inversely with  temperature. This statement is true only at a constant pressure.  In the atmosphere, both temperature and pressure decrease  with altitude and have conflicting effects upon density.  However, the fairly rapid drop in pressure as altitude is  increased usually has the dominant effect. Hence, pilots can  expect the density to decrease with altitude.  The preceding paragraphs are based on the presupposition of  perfectly dry air. In reality, it is never completely dry. The  small amount of water vapor suspended in the atmosphere  may be negligible under certain conditions, but in other  conditions humidity may become an important factor in the  performance of an aircraft. Water vapor is lighter than air;  consequently, moist air is lighter than dry air. Therefore, as the  water content of the air increases, the air becomes less dense,  increasing density altitude and decreasing performance. It is  lightest or least dense when, in a given set of conditions, it  contains the maximum amount of water vapor.  Humidity, also called relative humidity, refers to the amount  of water vapor contained in the atmosphere and is expressed  as a percentage of the maximum amount of water vapor  the air can hold. This amount varies with the temperature;  warm air can hold more water vapor, while colder air can  hold less. Perfectly dry air that contains no water vapor has  a relative humidity of zero percent, while saturated air that  cannot hold any more water vapor has a relative humidity  of 100 percent. Humidity alone is usually not considered an  essential factor in calculating density altitude and aircraft  performance; however, it does contribute.  The higher the temperature, the greater amount of water  vapor that the air can hold. When comparing two separate air  masses, the first warm and moist (both qualities making air  lighter) and the second cold and dry (both qualities making  it heavier), the first must be less dense than the second.  Pressure, temperature, and humidity have a great influence  on aircraft performance because of their effect upon density.  There is no rule-of-thumb or chart used to compute the effects  of humidity on density altitude, but it must be taken into  All of the principal components of flight performance involve  steady-state flight conditions and equilibrium of the aircraft.  For the aircraft to remain in steady, level flight, equilibrium  must be obtained by a lift equal to the aircraft weight and a  powerplant thrust equal to the aircraft drag. Thus, the aircraft  drag defines the thrust required to maintain steady, level  flight. As presented in Chapter 4, Aerodynamics of Flight,  all parts of an aircraft contribute to the drag, either induced  (from lifting surfaces) or parasite drag.  While parasite drag predominates at high speed, induced drag  predominates at low speed.  [Figure 11-5]  For example, if  an aircraft in a steady flight condition at 100 knots is then  accelerated to 200 knots, the parasite drag becomes four  times as great, but the power required to overcome that  drag is eight times the original value. Conversely, when the  The various items of aircraft performance result from the  combination of aircraft and powerplant characteristics. The  aerodynamic characteristics of the aircraft generally define  the power and thrust requirements at various conditions of  flight, while powerplant characteristics generally define the  power and thrust available at various conditions of flight.  The matching of the aerodynamic configuration with the  powerplant is accomplished by the manufacturer to provide  maximum performance at the specific design condition (e.g.,  range, endurance, and climb).  The primary factors most affected by performance are the  takeoff and landing distance, rate of climb, ceiling, payload,  range, speed, maneuverability, stability, and fuel economy.  Some of these factors are often directly opposed: for example,  high speed versus short landing distance, long range versus  great payload, and high rate of climb versus fuel economy.  It is the preeminence of one or more of these factors that  dictates differences between aircraft and explains the high  degree of specialization found in modern aircraft.  Performance is a term used to describe the ability of an  aircraft to accomplish certain things that make it useful for  certain purposes. For example, the ability of an aircraft to land  and take off in a very short distance is an important factor  to the pilot who operates in and out of short, unimproved  airfields. The ability to carry heavy loads, fly at high altitudes  at fast speeds, and/or travel long distances is essential for the  performance of airline and executive type aircraft.  consideration. Expect a decrease in overall performance in  high humidity conditions.  Effects of Temperature on Density  Effects of Humidity (Moisture) on Density  Performance  Straight-and-Level Flight  11-5 