TRUE Airspeed: Understanding Its Purpose, Calculation, and Importance for Pilots
One of the most fundamental things we learn about as Private Pilots is airspeed: True, Indicated and Calibrated. Flying around in lower altitudes, we get complacent about True Airspeed, which is modified from Indicated Airspeed (by temperature and pressure). There are lots of other measurements for speed, too, such as ground speed and Mach numbers, but that's a subject for another day.
This brief article will discuss the importance of knowing your True Airspeed and the different ways of using this knowledge.
What is True Airspeed?
When you are learning as a Private Pilot, TAS doesn't vary much from IAS or CAS. This is because IAS is corrected for installation and position errors to obtain calibrated airspeed (CAS). CAS is then further corrected for variations in pressure and density caused by altitude, temperature, and humidity to obtain TAS.
The formula to calculate TAS is TAS = IAS / (1 + (altitude x 0.00002)).
This is all a little complicated but we'll make it easy for you later. But, let's briefly go over how it all works because, you know, science.
At low altitudes, the air density is higher, which means that the same aircraft flying at the same calibrated airspeed (CAS) will have a higher TAS compared to the same aircraft flying at the same CAS at a higher altitude. This is because the denser air provides more molecules for the aircraft to displace, resulting in more lift and less resistance. Therefore, the aircraft can fly faster through the air and achieve a higher TAS.
At higher altitudes, the air density is lower, which means that the same aircraft flying at the same CAS will have a lower TAS compared to the same aircraft flying at the same CAS at a lower altitude. This is because the thinner air provides fewer molecules for the aircraft to displace, resulting in less lift and more resistance. Therefore, the aircraft cannot fly as fast through the air and achieves a lower TAS.
For example, an aircraft flying at a CAS of 200 knots at sea level might have a TAS of around 220 knots, while the same aircraft flying at the same CAS at an altitude of 30,000 feet might have a TAS of around 430 knots. This is because the air density at sea level is much higher than the air density at 30,000 feet, resulting in a much higher TAS at the higher altitude.
Don't forget that temperature is also part of the TAS calculation. At higher altitudes, the temperature is generally colder, which can result in a lower TAS due to the lower air density and increased aerodynamic drag. However, in some cases, particularly in the tropics, the temperature can be warmer at higher altitudes, which can result in a higher TAS due to the decreased aerodynamic drag.
Why is True Airspeed Important?
Why do we go to so much trouble? What is the advantage of knowing your True Airspeed?
Accurate Navigation: TAS is the speed of the aircraft relative to the air mass it is flying through, regardless of the wind. Therefore, it's important for accurate navigation and flight planning, particularly in cross-country and international flights where wind speeds and directions can vary significantly.
Safe Flight Operations: TAS is important for safe flight operations, particularly in high-altitude and high-speed regimes. In these conditions, the aircraft's performance is significantly affected by the air density, which can change rapidly with altitude and temperature. Accurate TAS calculations can help pilots ensure that they are operating their aircraft within its safe operating limits and avoiding dangerous situations such as stall or overspeed. (We'll talk about stalling more later!)
Aircraft Performance: TAS is also important for aircraft performance calculations, including takeoff and landing performance, climb and descent rates, and fuel consumption. Accurate TAS calculations can help pilots optimize their aircraft's performance and improve the overall efficiency of flight operations.
Air Traffic Control: TAS is also used by Air Traffic Control (ATC) for separation of aircraft, particularly in high-density airspace. ATC uses TAS to ensure that aircraft maintain safe distances from each other and avoid mid-air collisions.
How Do You Use TAS Practically?
When you were training for your Private Pilot test, you probably were taught to use a "E6B" calculator. While this is a pretty easy tool to master, modern avionics can display your TAS immediately. Notice the TAS on the Garmin G600 TXi below?
(Hint: It's in the middle of the screen.)
As GPS systems proliferate in our General Aviation Fleet, knowledge like this will be commonplace.
TAS is only accurate in stable atmospheric conditions, meaning if there are significant weather changes, such as a thunderstorm or strong winds, TAS data becomes less reliable. Additionally, the calculation assumes standard atmospheric conditions, which may not always be the case. Therefore, it's critical for pilots to keep a keen eye on aircraft performance and always cross-check airspeed and other flight instruments.
Earlier, we mentioned using True Airspeed as a way to avoid a stall. But, remember wings stall not because of airspeed (molecules moving over the wing) but because of angle of attack. While they are closely related, stalls are not directly caused by your True airspeed.
True airspeed is a fundamental concept for all pilots to understand. It affects many aspects of aviation, from fuel consumption to aircraft limitations. By having a clear understanding of what TAS is, how it's calculated, and how it's used, pilots can ensure safe and efficient flights.