What Are the Definitions of True Airspeed & Indicated Airspeed?
Indicated airspeed (IAS) is the speed that is shown on the airspeed indicator.
It is not corrected for variations in atmospheric density, instrument errors, or installation errors.
It does not typically vary with altitude or temperature.
True airspeed (TAS) is the actual speed of an aircraft as it travels through the air.
It is corrected for temperature and pressure altitude. Installation of a true speed indicator is not typical but may be found on higher-performance aircraft.
How Do True Airspeed & Indicated Airspeed Differ?
The basis of understanding why TAS and IAS differ comes down to how an airspeed indicator works and what it measures.
An airspeed indicator doesn’t actually measure speed, but it measures pressure instead.
As an aircraft begins to climb, true airspeed becomes greater than the indicated airspeed, resulting in different readings.
This is because fewer and fewer air molecules enter the pitot tube as pressure and temperature decrease with higher altitudes.
So, while an airspeed indicator is accurate at sea level in standard conditions, as soon as you have non-standard temperature or pressure, there is a loss of accuracy, and correction is required.
As a pilot, it’s important to be aware that true airspeed is 2% greater than indicated airspeed for every thousand feet above sea level.
Of course, this is just a rule of thumb and gives a rough estimation, and it would be unwise to always rely on this.
How Can True Airspeed Be Calculated From Indicated Airspeed?
To figure out true airspeed, you need to know what your calibrated airspeed is.
This can be figured out by taking your indicated airspeed and using a chart in Section 5 of your Pilot Operating Handbook.
Accuracy requires knowing the temperature and pressure.
With this information, you can then calculate the air density against calibrated airspeed, and get true airspeed.
An E6B flight computer can also be used to calculate true airspeed, as it accounts for pressure and temperature variation through a built-in airspeed correction scale.
What Are TAS & IAS Used For?
Indicated airspeed is used as the basis for determining aircraft performance.
True airspeed is used for accurate navigation of an aircraft, and is important for the purposes of flight planning and when filing a flight plan.
Without knowing your true airspeed, you would be wildly off accurate fuel burn and time estimates.