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True airspeed (TAS) is defined as the speed an aircraft travels in relation to the air around it. This is a true indication of the speed of the aircraft which generally differs from that indicated by the instruments in the cockpit. This disparity between true and indicated airspeeds is caused by the air pressures encountered at different altitudes. True airspeed is typically calculated manually with a special slide rule or, in the case of larger aircraft, by a flight management computer. Knowing the true airspeed of an aircraft is critically important for accurate navigation and flight planning.
Air pressure decreases gradually with increases in altitude, a phenomenon which sees the air at sea level being more dense than it is, for example, at 5,000 feet. This causes problems in establishing accurate airspeed readings because most airspeed indicators rely on air pressure for their operation. The readings returned by these instruments are known as indicated airspeed (IAS) and differ from true airspeed values which express the aircraft speed in relation to the air mass surrounding it. Airspeed indicators are calibrated to reflect true airspeed at sea level but will slowly begin to return differing readings as the air pressure decreases inversely with altitude.
For example, an aircraft flying at 100 knots at sea level will have airspeed indicator and TAS values of 100 knots. The same aircraft flying at an indicated 100 knots at 15, 000 feet above sea level will actually be flying at approximately 126 knots. Unfortunately, true airspeed is a critical part of most flight-related calculations associated with aircraft performance and navigation; indicated airspeed readings, therefore, cannot be relied on due to these inherent discrepancies. The necessary TAS readings are typically calculated in one of two ways.
The first is the manual method used by the pilots of most light aircraft. This method requires a special manual air data computer or E6-B as it is also known. This is basically a circular slide rule that allows pilots to calculate various flight envelope related factors. Trainee pilots learn to use a manual flight data computer during their training and it becomes an essential part of the flight bag contents for most, even those who end up flying far more complex and sophisticated planes.
The second method of true airspeed calculation is typically only found on larger, commercial aircraft. In these cases, TAS is calculated by the onboard flight management computer. The calculated readings are then used by the computer to execute flight-plan-related calculations such as fuel burn, wind correction, and navigation. The TAS reading is typically displayed for pilot reference on the aircraft's primary or navigational display units.