What Instrument is Used to Measure the Speed of an Aircraft?

The airspeed indicator (ASI) is an aircraft flight instrument in which the speed of the aircraft is measured and conveyed to the pilot in either km/h, kn, MPH, or m/s. Having accurate airspeed measurements is important for the safety of flight, as well as aids in conducting accurate navigation of the aircraft. In this blog, we will discuss what the airspeed indicator flight instrument is, as well as how it functions and provides accurate measurements.

Similar to many other flight instruments, aircraft speed instruments rely on the pitot-static system to obtain accurate measurements. The airspeed indicator, however, is the only flight instrument on an aircraft that utilizes both the pitot and static system for its measurements. The pitot-static system is composed of two main parts, which are the pitot tube and the static port. The pitot tube is most often located on the front area or wings or the aircraft, placed in a position in which it is exposed to relative wind. With the pitot tube, ram air pressure is forced into the tube to obtain total pressure. Meanwhile, the static port is placed in a location of the fuselage that is fairly undisturbed, allowing the instrument to also measure average pressure.

From the static port, static pressure enters the instrument through a line, surrounding a diaphragm and exerting pressure upon it. As the aircraft moves through the air and gains speed, ram air pressure begins to enter into the instrument within the diaphragm, slowly overtaking the pressure of the static air as speed increases. Once the ram air pressure begins to exceed that of the static air pressure, the diaphragm will begin to flex, causing a pointer on the instrument to move in a way that denotes the current aircraft speed for the pilot. As temperature and pressure variation may affect the readings, technologies such as flight computers and calibrated airspeed may be used to correct errors and find the true airspeed.

Airspeed indicator markings, or ASIs, are a standard color-coded marking system mandated by the FAA to allow the pilot to be aware of certain airspeed limitations to conduct safe operations. A white arc is a color coding that represents the flap operating range and is typically flown in during approaches and landings. The green arc denotes the standard operating range of the aircraft in which most cruise speeds fall under. The upper limit of the green arc is the maximum structural cruising speed which should not be surpassed unless the aircraft is operating within smooth air. Beyond the green arc, the yellow arc represents a cautionary range, allowing the pilot to know that speeds within this range should only be conducted with caution and during smooth air. The final color arc, the red line, is the limit in which should never be exceeded. Operating above the red mark of the airspeed indicator is dangerous and prohibited as it could cause damage or a structural failure.

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