Aviation Words — True Airspeed

February 2020 - We all learned this as student pilots, but I, for one, manage to ignore it in routine flights. The use of GPS for over-the-ground speed measurements has largely superceded the need for awareness of true airspeed for longer cross-country flights.

Pilots were all taught that the pitot-based airspeed indicator system is only accurate at standard air pressure and temperature. Unless you're flying at below sea level (generally not a good idea in most parts of the world), inaccuracies increase with altitude, because air pressure goes down and so does outside air temperature.

At higher speeds the compressibility of air must also be considered, since we're not just thinking about how fast the air is flowing but also about how much it is being compressed in the pitot system. The calculation for relatively low speed flight is based on the formula TAS = EAS x sqrt (p0/p), where TAS is true airspeed, EAS is equivalent airspeed, ρ0 is the air density at sea level in the International Standard Atmosphere (15 °C and 1013.25 hectopascals, corresponding to a density of 1.225 kg/m3), and ρ is the density of the air in which the aircraft is flying.

For our purposes, equivalent airspeed is close to indicated airspeed in a well-calibrated system at sea level at standard pressure and temperature. Let's say we're flying at 6,500 feet on a day with standard pressure and temperature at sea level. At this altitude, the air pressure drops from 1013.25 to about 800 hectopascals. The square root of dividing sea level pressure by air pressure at altitude gives us 1.13. In other words, our true airspeed is 13 percent higher than that indicated.

As temperature goes up, the air pressure also goes down, and we start to see similar errors closer to the ground. Remember, it's true airspeed that's giving us lift; even though we might have done our stall testing at or close to standard pressure and temperature using indicated airspeed, those numbers don't stay the same with varying conditions, like summer flying in the mountains.

My little Dynon D10A is a marvellous instrument. I don't think I gave it enough respect when I installed the outside air temperature probe and the pitot system. Using its knowledge of OAT and altitude and a slick little algorithm in its programming, it will happily give me true airspeed. Maybe I'll start paying that indication more attention.

True airspeed indicator on D10A.