Among the various instruments on an aircraft, the airspeed indicator is one of the most basic and most important. Without a functioning airspeed indicator, a pilot can only guess how fast their aircraft is going, making navigation or charting a flight-plan nearly impossible. The indicator measures the aircraft’s speed in knots or miles per hour and represents this speed with a basic needle and dial instrument. Each plane has its own specified airspeeds that a pilot needs to be aware of for things like takeoff, landing, stalling, and cruising.
The airspeed indicator works by comparing dynamic pressure (or ram air pressure) and static pressure in what is called the pitot-static system. A pitot-static system is a differential pressure system that compares dynamic air pressure from the pitot tube and static air pressure from a static port. Inside the casing of the instrument is a sealed diaphragm that receives both static and dynamic pressure from the pitot tube. Static pressure is also measured from inside the casing, but outside the diaphragm. The static pressures from both inside and outside the diaphragm cancel each other out, leaving a measurement of total dynamic pressure, or ram air pressure. As the aircraft accelerates, the dynamic pressure from the pitot tube increases, causing the diaphragm to expand. Through mechanical linkage, this measurement of increased airspeed is shown on the airspeed indicator’s needle.
There are various types of airspeed, however. Indicated airspeed denotes the airspeed read from the indicator. Calibrated airspeed is the actual speed of the aircraft, after adjusting for position and instrument errors. True airspeed is calibrated airspeed adjusted for nonstandard pressure and temperature, while equivalent airspeed is calibrated airspeed adjusted for compressibility errors. Finally, there is groundspeed, which is the actual speed of the aircraft over the ground. Groundspeed is true airspeed corrected for the effects of the wind and is most often used in flight-planning.
The airspeed indicator is separated into several different arcs, denoted by color. The white arc on the airspeed indicator depicts the normal flap operating range. In the white arc, full flaps can be used (typically for takeoffs and landings). The green arc denotes the aircraft’s normal operating range, while the yellow arc is reserved as a cautionary range, which the pilot should only enter if they are currently flying through calm weather conditions. Lastly, the red arc indicates the maximum allowable airspeed that the aircraft’s frame can support.
Alternate forms of airspeed indicators also exist. Aircraft capable of supersonic flight will also measure their airspeed in Mach numbers, and many newer instruments also use GPS systems to calculate their velocity.
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