The airspeed indicator (ASI) is a standard flight instrument of aircraft that enables pilots to monitor their airspeed in the form of knots, kilometers per hour, miles per hour, or meters per second. Determining forward speed is crucial for many flight procedures such as maintaining a set speed for optimal fuel burning or ensuring speed is safely reduced for a turn or landing. As the airspeed indicator is paramount for the safe operation of an aircraft, having a basic understanding of its functionality and how to read one is important for any pilot or aircraft engineer.
Whether a particular speed indicator is based on mechanical gauge technology or is a part of an advanced glass cockpit display, such instruments always rely on the pitot-static system. The pitot-static system consists of the pitot tube and the static port, both of which capture and measure different air pressure types. The pitot tube assembly is often installed near the front of the fuselage within the flow of air so that it can capture ram air. The static port, meanwhile, is situated in an area of the fuselage that is relatively undisturbed so that it may capture static pressure.
Within the airspeed indicator instrument case, a diaphragm assembly is placed with a connected line feeding ram air from the pitot tube into its interior. On the outside of the diaphragm, a static air line fills the instrument interior with static pressure. Based on the difference in pressures, the diaphragm will either expand or contract in accordance. As the flexing of the diaphragm occurs, attached mechanical linkages will move a pointer across the dial of the instrument to provide a reading. When pressures are equal to one another while on the ground, the dial will provide a reading of zero. As the aircraft moves forward, the pitot tube pressure will begin to surpass the static port pressure, thus causing the pointer to rise.
As the airspeed indicator relies on the pitot-static system for its measurements, it is important that such assemblies remain unblocked and functional each flight. Insects, icing, dirt, and other objects or substances can all cause a blockage of the static port or pitot tube, and any hindrance will affect the accuracy of readings. To maintain safety and the operability of all instruments, the pitot-static system should always be inspected for blockages before each flight.
When readings are provided by the airspeed indicator, the measurement is known as the indicated airspeed (IAS). While this speed is very useful for many operations and procedures, it does not account for any instrument or installation errors and air density variations. When the IAS is corrected, the pilot can find what is known as the calibrated airspeed (CAS). This value can further be refined with an airspeed calibration chart, and accounting for altitude and nonstandard temperatures will result in the true airspeed (TAS). TAS is important for flight planning, and it can sometimes be determined by a flight computer depending on the model.
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