Atmospheric Pressure

Though there are various kinds of pressure, pilots are mainly concerned with atmospheric pressure. It is one of the basic factors in weather changes, helps to lift the aircraft, and actuates some of the most important flight instruments in the aircraft. These instruments often include the altimeter, the airspeed indicator (ASI), the vertical speed indicator (VSI), and the manifold pressure gauge. Though air is very light, it has mass and is affected by the attraction of gravity. Therefore, like any other substance, it has weight; because it has weight, it has force. Since fluid substance, this force is exerted equally in all directions, and its effect on bodies within the air is called pressure. Under standard conditions at sea level, the average pressure exerted by the weight of the atmosphere is approximately 14.7 pounds per square inch (psi). The density of air has significant effects on the aircraft’s performance. As air becomes less dense, it reduces: • Power, because the engine takes in less air • Thrust, because the propeller is less efficient in thin air • Lift, because the thin air exerts less force on the airfoils The pressure of the atmosphere may vary with time but more importantly, it varies with altitude and temperature. Due to the changing atmospheric pressure, a standard reference was developed. The standard atmosphere at sea level has a surface temperature of 59 degrees Fahrenheit (°F) or 15 degrees Celsius (°C) and a surface pressure inches of mercury ("Hg) or 1013.2 millibars (mb). [Figure 11-1] A standard temperature lapse rate is one in which the temperature decreases at the rate of approximately 3.5 °F or 2 °C per thousand feet 000 feet. Above this point, the temperature is considered constant 000 feet. A standard pressure lapse rate is one in which pressure decreases at a rate of approximately 1 "Hg per 1,000 feet of altitude gain to 10,000 feet. [Figure 11-2] The International Civil Aviation Organization (ICAO) has established this as a worldwide standard, and it is often referred to as International Standard Atmosphere (ISA) or ICAO Standard Atmosphere. Any temperature or pressure that differs from the standard lapse rates is considered nonstandard temperature and pressure. Adjustments for nonstandard temperatures and pressures are provided on the manufacturer’s performance charts. Inches of Standard Millibars Standard Mercury Sea Level Sea Level Pressure 30 1016 Pressure 29.92" 1013 Hg 25 847 mb 20 677 15 508 10 339 5 170 Temperature Pressure Altitude (ft) ("Hg) (°C) (°F) 000 000 000 000 000 000 000 000 22.22 −0.9 000 21.38 −2.8 000 20.57 −4.8 000 19.79 −6.8 000 19.02 −8.8 000 18.29 −10.8 000 17.57 −12.7 000 16.88 −14.7 000 16.21 −16.7 000 15.56 −18.7 −1.6 18,000 14.94 −20.7 −5.2 19,000 14.33 −22.6 −8.8 20,000 13.74 −24.6 −12.3 Since all aircraft performance is compared and evaluated using the standard atmosphere, all aircraft instruments are calibrated for the standard atmosphere. Thus, certain corrections must apply to the instrumentation, as well as the aircraft performance, if the actual operating conditions do not fit the standard atmosphere. In order to account properly for the nonstandard atmosphere, certain related terms must be defined.