The airspeed indicator in an aircraft represents dynamic pressure read via the pitot tube. This pressure is interpreted in terms of indicated airspeed. For dynamic pressure we use the following formula:
1/2 rho V-squared (dynamic pressure)
- The term “rho” equals the density of the air.
- “V” is the speed of an aircraft through air (TAS).
The TAS of an aircraft can easily be calculated through a flight computer as shown in the below video.
The Relation between Indicated Airspeed and True Airspeed
Consider an aircraft in flight, climbing from about 1000ft above mean sea level (AMSL) to 15,000ft, maintaining an indicated airspeed of 100 kt. The primary goal of the pilot in this phase is to maintain the required rate of climb and the required IAS.
Since pressure decreases with an increase in altitude, so does density. However, as density decreases, the performance of an aircraft also decreases.
In the above stated formula, if density (rho) is decreasing, then the value of dynamic pressure lessens, thus resulting in a decrease in IAS. However, the pilot must necessarily maintain the IAS during the climb. To compensate the decreasing value of atmospheric density, the value V must increase to satisfy the formula. V is the true airspeed of an aircraft. Hence, the TAS of an aircraft at 15,000ft would be a value greater than its IAS.
In simpler words:
- An aircraft climbs to attain a certain altitude, maintaining its IAS.
- As the airplane climbs, density of the atmosphere decreases i.e. the value of rho decreases.
- To compensate for the decreasing value of rho, the value of V increases.
- V is TAS of an airplane.
Notice how the Boeing 757 enters a rapid climb towards the end of this video. In this instance, as the aircraft climbs to higher altitudes at such a rapid rate of climb, the density decreases at a rate proportional to that of the rate of climb and thus, the difference between indicated airspeed and true airspeed would also increase rapidly.
It is noteworthy that the TAS will not necessarily increase at only higher altitudes. A decrease in density due to any factor, say temperature, will consequent in increased TAS. For instance, increased temperature at sea-level will give a true airspeed higher than the indicated airspeed of an aircraft.
Aeroplane General Knowledge and Aerodynamics, Aviation Theory Centre, 2004.
Instrument Flying Handbook, Federal Aviation Administration, 2007.
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