The Difference Between Indicated and True Airspeed


During flight, it is imperative that the pilot has the ability to monitor all flight characteristics to ensure that safety and efficiency is maintained throughout an operation. The speed that an aircraft is traveling at is very important, often having an effect on fuel efficiency, structural integrity, and the ability to undertake certain flight maneuvers. Pilots have long relied on the airspeed indicator (ASI) for obtaining airspeed measurements, such readings provided in terms of kilometers per hour, miles per hour, knots, or meters per second. When utilizing instrument readings, it is important to understand the differences between various types of airspeeds, those of which are indicated airspeed, calibrated airspeed, and true airspeed.

In order for the airspeed indicator to measure speed, it relies on both the static and pitot system. Static pressure is a measurement of atmospheric air in a relatively undisturbed location, and captured air is routed into the airspeed indicator case. Meanwhile, ram air from the pitot tube is directed into a diaphragm within the ASI case, causing it to flex. As the two forces act against one another, the diaphragm will expand and contract, causing a needle on the instrument dial to move across a scale. When taking readings directly from the cockpit airspeed indicator, such values are referred to as the indicated airspeed (IAS). While useful, such measurements have not had any correction made for air density variations, installation issues, or instrument errors.

True airspeed (TAS), meanwhile, is considered to be the actual speed of the aircraft as it moves through the air. As indicated airspeed is simply a measurement of pressure rather than speed, such values require correction for the true airspeed to be obtained. Through the correction of the indicated airspeed in regard to installation and instrument errors, the calibrated airspeed (CAS) may be obtained. This reading can be further corrected with airspeed calibration charts to account for both altitudes and nonstandard temperatures, allowing for the pilot to obtain the true airspeed.

The true airspeed of an aircraft is useful for many reasons, typically being the value that is used to determine flight planning. True airspeed ensures that flight times and fuel burning estimates are accurate, preventing confusion, mishaps, and even danger. Generally, true airspeed values increase alongside altitude while air density decreases. For the means of accurately obtaining true airspeed values without major effort, a flight computer such as the E6B may be used. In some instances, an analog airspeed indicator may have a window that computes true airspeed as well.

While the E6B is capable of determining true airspeed values, its effectiveness begins to drop when speeds surpass 100 knots. When such speeds are reached, compressibility becomes a factor as the air in front of the aircraft begins to compress. This results in a loss of calibrated airspeed accuracy, and corrections can often prove difficult for some aircraft. With the Mach number of the aircraft, however, true airspeed can be found as such values factor in compressibility.

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