12.36 Pressure Instruments

12.36.2 Identify the three basic instruments which rely on air pressure for their operation.

The three basic instruments in an aircraft which rely on air pressure for their operation are the airspeed indicator, the altimeter, and the vertical speed indicator. The airspeed indicator measures the speed of the aircraft relative to the surrounding air, which is determined by the difference in pressure between the pitot tube and the static port. The altimeter measures the altitude of the aircraft above sea level, which is determined by the difference in pressure between the static port and the aneroid wafer inside the altimeter. The vertical speed indicator measures the rate of climb or descent of the aircraft, which is determined by the rate of change of pressure in the aircraft's static system.

12.36.4 Describe static pressure and dynamic pressure, and the main factors which affect them.

Static pressure is the pressure of the air at rest, which is the same in all directions. Dynamic pressure is the pressure of the air in motion, which is caused by the kinetic energy of the air molecules. The main factors which affect static pressure are altitude, temperature, and humidity. As altitude increases, static pressure decreases due to the decrease in the density of the air. As temperature increases, static pressure decreases due to the decrease in the density of the air. As humidity increases, static pressure decreases due to the increase in the water vapor content of the air. The main factors which affect dynamic pressure are airspeed and air density. As airspeed increases, dynamic pressure increases due to the increase in the kinetic energy of the air molecules. As air density increases, dynamic pressure increases due to the increase in the number of air molecules in motion.