In aircraft, pilot's rely on three primary pressure-based instruments for pressure readings. These instruments comprise the airspeed indicator, the altimeter, and the vertical speed indicator, all of which function within the pitot static system. To understand in general how this system works, it is useful to understand the relationships between pressure and velocity as they relate to static pressure, total pressure, and dynamic pressure. This blog will offer a brief overview of the aircraft pitot static system and the functions of its main instruments.
While gyroscopic principles are used for many important instruments, pressure is used to determine airspeed, altitude, and the rate at which altitude is changing (vertical speed). An aircraft’s pitot-static system measures total pressure and static pressure separately, from which dynamic pressure can be easily calculated by applying Bernoulli’s equation. In order to receive the information needed for this equation, the pitot system includes a pitot probe, generally located under the wing, and a static port, generally located on the forward fuselage. These instruments measure total pressure and static pressure respectively. The dynamic pressure is determined by subtracting the static pressure from the total pressure. The aircraft’s velocity is a function of the dynamic pressure and can be determined by rearranging Bernoulli’s equation and solving for the velocity.
Specifically, the total pressure value of the aircraft is found by using incoming airflow from the forward motion of the aircraft to determine the pressure of air outside the aircraft. This measured total pressure is the sum of the static and dynamic pressures acting on the aircraft at the location of the probe. Meanwhile, static pressure is a measure of the ambient atmospheric pressure in which the aircraft is flying. Aircraft use vents specifically located in positions where they are free from dynamic pressure. It is also important to keep in mind the difference between indicated airspeed, that of which is seen on the airspeed indicator, and calibrated airspeed, that of which is calculated by the pilot to avoid any errors.
Each pressure-related instrument works differently using measured total and static pressure, as well as calculated dynamic pressure. For example, an altimeter directly displays altitude through the relationship between static pressure and altitude gained. For every 30 feet of altitude gained, altitude alters by approximately 1 hPa (hectopascal), so the static pressure information gained from the vent can be used to determine the altitude of the plane. Meanwhile, the rate of change of the static pressure is used by the vertical speed indicator to determine the climb rate or descent of the aircraft. Lastly, the airspeed indicator uses velocity and air density to determine the speed of the aircraft.
As it is vital to the safe operation of the aircraft that the instantaneous static pressure be available to the pilot at all times, aircraft will often rely on backup static ports so pressure readings are never incorrect.
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