True airspeed, TAS
These indicators are somewhat more expensive but so very handy. The correction for temperature and pressure is called density error. The formula to the left is explained as follows: TAS = EAS √ (ρ0 / ρ), where ρ0 = 1,225 kg/m3 at sea level and ρ is the actual air density at your current altitude.
How is true airspeed calculated?
You can afford to be off a couple of knots. True Airspeed Rule of Thumb: For any altitude flown above 3,000 feet, add 3 knots per 1000′ to get your (rough) true airspeed.
How can I get TAS from IAS?
Mathematically increase your indicated airspeed (IAS) by 2% per thousand feet of altitude to obtain the true airspeed (TAS). For example, the indicated airspeed (IAS) of my Comanche at 8,500 ft. MSL is 170 knots.
