Avgeek Alert: Parsing Aircraft Speeds


As we’ve discussed previously in this blog, velocity is what creates the airflow which in passing across the fuselage wings generates the lift needed to propel a passenger-filled machine weighing dozens of tonnes through the sky. And furthermore, it’s a matter of not one but various speeds, and there are four which are the most important – each using different references and providing different information, and designated by international codes collected by the United NationsOffice of the International Civil Aviation Organization (ICAO). They are as follows:


Indicated Air Speed (IAS)

This is so named because this is the velocity which appears on the cockpit’s airspeed indicator (ASI). The ASI, like the altimeter, is connected to a pitot-static system, whose sensors detect the speed of the aircraft at all times.

As you can imagine, the indicated airspeed is one of the most important values of all those used in a flight, used for example in takeoff, landing, and approach manoeuvres, It essentially measures the relative speed of the aircraft – that is, the speed at which it moves through the volume of air surrounding it.

However, one thing the IAS does not take into account is air density. It takes as a reference the density of the air at sea level, but of course that density is not the same as it is at 8,000 feet, for example. The IAS also doesn’t account for the airspeed indicator’s potential margin of error. So that’s why this next concept is needed.


Calibrated Air Speed (CAS)

This is essentially the “corrected” airspeed. It isn’t that the measuring equipment is incorrect, but there are certain situations in which the sensor system may not receive the adequate air pressure and give a value that is different from the real one. What the CAS does is compensate for these discrepancies and logs a more accurate number.

But is there such a difference between what the ASI measures, that is, the IAS, and the CAS? Well, not really. It is assumed that the difference is negligible and, consequently, that IAS and CAS are equivalent values for navigation purposes.


True Air Speed (TAS)

In essence this is the relative velocity of an aircraft with respect to the volume of air through which it moves. If the CAS corrects for potential variations in the IAS, the TAS corrects the CAS for air density. And then the true speed is the one used in the flight plan.

As we saw before, the density of air varies according to different factors. One of them is pressure (which varies with altitude); another is air temperature – the higher the plane flies, the less dense it is. We also saw that the IAS uses air density at sea level as a reference. This means that two planes at the same speed and at two different altitudes will mark different indicated airspeeds; the IAS of the higher-flying aircraft will be lower because the system receives less dynamic air pressure, and vice versa for the lower-flying one.

So we can deduce that at sea level the TAS is equivalent to the IAS. But as we go higher, the actual speed will be higher than indicated – about two percent for every 1,000 feet of altitude.


Ground Speed (GS)

Finally, there’s yet another important correction factor taken into account when measuring speed: the wind. The GS accounts for this to modify the true speed and yield – now, yes, the real speed at which the aircraft flies.

So how does the wind determine speed? There are basically two main types of wind during flight: the tailwind and the headwind. Just like at ground level, atailwind increases the speed of the moving object and a headwind decreases it.

And why then is it important to refer to the ground speed? Because as it is the true speed of the plane, with all the correction factors incorporated, it’s used to calculate not only the time it will take the aircraft to reach its destination but also how much fuel is needed to fly.

Well, there you have it, the four speeds in aviation. It all sounds rather complicated, but nowadays many of the calculations involved are made automatically by the aircraft systems themselves; the instruments available to the crew on board have evolved a lot, and the cockpits of today have little or nothing to do with those of decades ago due to their digitization.