Aircraft Systems

2Pressure Instruments

In this chapter, the properties of the atmosphere will be described and the precise meanings of sea level, elevation, height, altitude, and flight level (FL) will be defined. The instruments that determine altitude, rate of climb or descent, airspeed, and Mach number by measuring static and dynamic air pressure will be described. Initially, the operation of traditional direct‐reading instruments will be explained as these are still used in the majority of General Aviation aircraft and some commercial aircraft. In addition, the fundamental measurements that they make are the same as the ones performed by transducers used in glass cockpits. Finally, the operation of the Air Data Computer (ADC), used in the majority of commercial airliners, and how it processes and displays the relevant output data in the cockpit will be described.

2.1 Layers of the Atmosphere

The Earth’s atmosphere is continuous from the ground to the border with space, but it can be described in terms of layers at different altitudes (Figure 2.1a), each having distinct properties. One of the properties that distinguishes the different layers is how the temperature varies with altitude, which is shown in Figure 2.1b for the International Standard Atmosphere (ISA: see the following section). In the troposphere, which is the lowest layer next to the ground, the temperature decreases linearly with height up to the tropopause, which marks the boundary between the troposphere and the next ...