How do commercial planes know their TAS? Pitot tube,EAS and wind speed, together calculated to to get the TAS?

Question

How do commercial planes know their TAS? Pitot tube,EAS and wind speed, together calculated to to get the TAS?

Answer 1

Some show both IAS and TAS on the airspeed indicator.

If the IAS, CAS, pressure altitude, and temperature are known the TAS can be found easily enough on some circular computers.

Wind speed doesn't have much bearing on TAS.

Answer 2

TAS is a pretty simple calculation for most modern aircraft, rather than repeat it all I will point you to some links but to summarise

TAS - True Airspeed

EAS - Equivalent Airspeed

CAS - Calibrated Airspeed

IAS - Indicated airspeed (what the pilot sees)

Aircraft groundspeed is usually calculated from the navigation system and the difference between TAS and Groundspeed plus using aircraft heading and track  wind speed and direction can be calculated.

TAS equals EAS divided by the square root of the density ratio (air density at the aircraft altitude/air density at sea level) This is the speed of the aircraft relative to the ground if there is no wind. When you think about it if the air is less dense at altitude the aircraft has to travel faster to sense the same pressure, so TAS is greater than EAS

EAS equals CAS corrected for compressibility (only really required at higher altitudes and Mach number

CAS equals IAS corrected for instrument error and position error (position error is the difference between what is measured at the pressure port location and what is freestream and is mainly a factor of  where the static and pitot ports are)

You can also calculate an approximate TAS is if you know the windspeed and direction at your altitude, your heading and your groundspeed.

Hope this helps